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7e82b64d471865129aeff7b2f3336d8adefb2653
SplitBit-Emulator/Programs/CosmOS/Apps/Lander.asm
T
Anachronaut 7e82b64d47 A dock that slides into line, and settles squarely on the port 
Two things, and the second is the one that was actually wrong.

The lander is slid into place at half a pixel a frame rather than put
there. A dock is allowed eight pixels out in either direction, so
snapping moved it a whole tile in a single frame - a jump, at the very
moment the player was being told they had been careful. The worst gap
closes in about half a second, which reads as the two of them settling
together.

And it settles SQUARELY now. It used to come to rest four pixels out
however carefully it was flown, because the lander is drawn from half a
screen less half a tile - which is what centres an eight pixel lander on
the middle - while the station was drawn from half a screen exactly, so
its left edge sat where the lander's centre was. Both come off the same
origin now, and a gap of nothing puts one exactly above the other.

stationGap is factored out along the way. Three callers wanted it: the
one that draws the station, the one that decides whether it can be docked
with, and now the one that slides the lander in under it. The sideways
ease goes through that wrapped gap rather than the raw positions, because
a dock made either side of the moon's seam has a raw difference of most
of a moon.

---- And the fixtures moved to frame 400 ----

A dock waits to be told its message has been read, and reading a state
file costs a disk read, so a placed run starts a good deal later than a
plain one. The acknowledgement was at frame 100 and stopped working the
day the state file arrived: the program had not reached the dock yet and
the press went by unheard, which shows up as every sprite missing and
reads like a drawing bug. Everything after it is sampled well clear of
both ends.

Two checks, each seen to fail on its own break - the alignment settles at
4,-8 without the shared origin, and the slide reads 0,-8 the whole way
without the easing. Which of the two axes each one actually watches is
written down beside them, because it is not the one you would guess.
2026-09-04 11:23:54 -04:00

3315 lines
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NASM
Raw Blame History

; Lunar Porter. Rung one: it flies.
;
; A lander over a moon that wraps. Thrust, gravity, and a surface that comes back round if you
; keep going one way - there is no edge to fall off and no wall to hit, because the map's
; column origin is a ring in hardware and 128 cells of it is 1024 pixels of moon.
;
; ---- What is not here yet ----
;
; Landing, crashing, fuel, cargo, bases. This rung exists to answer whether it FEELS right,
; because everything after it is bookkeeping by comparison and none of it is worth building
; on a lander that is no fun to fly.
;
; ---- Sixteenths of a pixel ----
;
; Position and velocity are sixteen bit, in sixteenths of a pixel. That is the unit that makes
; the whole thing work with adds alone: gravity is a small number added to a velocity, and a
; velocity is a number added to a position, and there is no multiply or divide anywhere.
;
; The moon is 1024 pixels round, which is 16,384 sixteenths, which is 2^14 - so GOING ALL THE
; WAY ROUND IS AN AND WITH 0x3FFF. Not a comparison, not a subtraction, and never wrong at the
; seam. Picking the units so the wrap is a mask is most of the reason this is short.
;
; Written by Anachronaut
#Include services.asm
#Program
#Base 0x5000
start:
; The atlas holds the tiles and the sprite table; the screen holds the map.
INIA 0d4
OUTA 0xE3
INIA 0x30
OUTA 0xE2
INIA 0x03
OUTA 0xE8
INIA 0d5
OUTA 0xE3
INIA 0x3A
OUTA 0xE2
INIA 0x03
OUTA 0xE8
; The screen back afterwards. This one really does need it: it redefines tiles, fills every
; cell of the map and changes the mode, and screenGive is what puts the MODE back - the
; floor the system gives every program does not.
SWI osTakeScreen
CALL loadState ; Before the view, because a state may say which view to be in.
CALL setView ; The block every gauge below reads its screen numbers out of.
SETD.0 Wide
LDA.0
OUTA 0x31 ; Forty columns to start in. B swaps to eighty and back.
; ---- And the view put back to the top of the map ----
;
; THE SHELL SCROLLS. Its row origin is wherever the last command left it, and the map is a
; ring 128 rows tall that the screen shows 25 of - so a moon drawn into rows nought to 24
; while the screen is looking at row forty is a moon nobody can see. It came out as terrain
; that was missing, or half there, depending on how far down the prompt had got.
;
; Nothing here is being tidy: this is the difference between the rows this program WRITES
; and the rows the screen READS, and only one of them is under its control.
;
; The column origin and the fine offsets are set every frame by follow, so they need no
; help. The row origin is set once, here, because nothing scrolls vertically after this.
SETD.0 ScrollRow
LDA.0
OUTA 0x34
RSTA
OUTA 0x38 ; No fraction of a cell downwards either.
CALL mulReady ; The quarter square table, once. The orbit below needs a product.
CALL putTiles
CALL makeMoon
CALL carvePads
CALL drawMoon
CALL putLander
CALL putGauge
CALL putOrbital
INIA 0x01
OUTA 0x02 ; Key mode.
; ---- Is there a controller ----
;
; Asked once, and about all four. If there is one anywhere, the console's arrow keys are
; ignored: under a window the same keypress reaches BOTH - the pad as a level and the
; console as a byte - and a thruster that fired twice for one press would be a mystery to
; anybody tuning it.
INA 0x64
SETD.1 HasPad
STA.1 ; ANY bit, so any of the four counts as having one.
everyFrame:
CALL waitFrame
CALL readPad
CALL readControls
CALL zoomButton
CALL fall
CALL move
CALL moveStation
CALL follow
CALL touchdown
CALL dock
CALL showLander
CALL showStation
CALL showDrift
CALL showFall
CALL showFuel
CALL showHeight
SETD.0 Flying
LDA.0
BNA everyFrame
RSTA
OUTA 0x02
SWI osExit
; ---- Tiles ----
;
; The ground is a solid block, which is a Fill and needs no art. The lander has a shape and
; comes out of the Data Segment. Both are well above the 135 glyphs the character generator
; copies back, so neither costs the shell a letter.
putTiles:
INIA 0d4
OUTA 0xE3
INIA 0x32
OUTA 0xE4
RSTA
OUTA 0xE5 ; Tile 200 begins at 0x3200.
INIA 0x01
OUTA 0xE2
RSTA
OUTA 0xE6
INIA 0x40
OUTA 0xE7
INIA 0x02
OUTA 0xE8 ; Fill: 64 pixels of index one.
INIA 0x01
OUTA 0xE0
SETD.1 LanderArtAt
SETD.0 LanderArt
STD.0.1
LDA.1
OUTA 0xE1
INCD.1
LDA.1
OUTA 0xE2
INIA 0d4
OUTA 0xE3
INIA 0x32
OUTA 0xE4
INIA 0x40
OUTA 0xE5 ; Tile 201, one tile on from 200.
RSTA
OUTA 0xE6
INIA 0x40
OUTA 0xE7
INIA 0x01
OUTA 0xE8 ; Blit.
OUTA 0xE0 ; Bank one again, A still holding the one the blit above wanted.
SETD.1 StationArtAt
SETD.0 StationArt
STD.0.1
LDA.1
OUTA 0xE1
INCD.1
LDA.1
OUTA 0xE2
INIA 0d4
OUTA 0xE3
INIA 0x32
OUTA 0xE4
INIA 0x80
OUTA 0xE5 ; Tile 202, one tile on again.
RSTA
OUTA 0xE6
INIA 0x40
OUTA 0xE7
INIA 0x01
OUTA 0xE8 ; Blit.
RET
; ---- A moon, one column at a time ----
;
; A random walk over 128 columns, clamped so there is always sky above and ground below. The
; numbers come from an eight bit shift register with feedback, which is the cheapest thing
; that is not obviously a pattern: a byte, shifted right, exclusive-ored with a constant when
; the bit that fell off was set.
;
; The SEED IS FIXED, so the moon is the same moon every time. That is worth more than variety
; while the physics is being tuned - a landing that was too hard yesterday should be too hard
; today - and a seed is one byte to change later.
makeMoon:
INIA 0d18
SETD.1 Height
STA.1
SETD.2 Terrain
RSTA
SETD.1 Column
STA.1
makeColumn:
SETD.1 Height
LDA.1
STA.2 ; This column's height, as it stands.
INCD.2
CALL nextRandom
MVQA ; The answer is in Q, because Q is what survives a RET. Without
; this the AND below tests whatever A happened to hold - which
; was the height itself, so the moon oscillated by one row and
; looked flat.
INIB 0x01
AND
BNQ makeUp
; Down a row, unless that is already as low as the ground goes.
SETD.1 Height
LDA.1
INIB 0d22
CCF
SUB
BRQ makeNext ; Equal, so it is already at the floor.
LDA.1
INCA
STA.1
BRI makeNext
makeUp:
; And up a row, unless that is as high as it goes.
SETD.1 Height
LDA.1
INIB 0d10
CCF
SUB
BRQ makeNext
LDA.1
DECA
STA.1
makeNext:
; ---- The counter is in memory, and has to be ----
;
; B is the obvious place for a loop counter and it is the wrong one here: every comparison
; below is an INIB, so the count was overwritten by whichever bound was last tested and the
; loop reset itself for ever. This machine has two registers and a dozen uses for them; a
; counter that has to survive arithmetic lives in memory.
SETD.1 Column
LDA.1
INCA
STA.1
INIB 0d128
CCF
SUB
BNQ makeColumn
RET
; ---- Somewhere to put it down ----
;
; A random walk does not leave flat ground, and a lander wants some. So four pads are CARVED
; after the moon is made rather than looked for in it: searching can fail, and a fallback that
; carves anyway is the carving plus a search nobody needed.
;
; Each is four columns levelled to the height the first of them happened to have, so the pads
; sit in the landscape rather than on a shelf above it - the moon decides where they are, and
; this only decides that they are flat.
;
; The columns are marked in an array of their own. Asking "is this a pad" while drawing has to
; be one lookup: four comparisons per column per row is 12,800 of them for a screen.
carvePads:
SETD.3 PadTable
INIA 0d4
SETD.1 PadsLeft
STA.1
carveOne:
; The height this pad is levelled to, which is whatever its first column already was.
LDA.3
SETD.1 Terrain
DPUA.1
LDA.1
SETD.1 PadHeight
STA.1
RSTA
SETD.1 PadWide
STA.1
carveColumn:
; This column of the pad: levelled, and marked as somewhere to land.
LDA.3
SETD.1 PadWide
LDB.1
CCF
ADD
MVQA ; The pad's first column plus how far along this is.
PSHA
SETD.1 Terrain
DPUA.1
SETD.0 PadHeight
LDA.0
STA.1
POPA
SETD.1 IsPad
DPUA.1
; ---- WHICH base, and not merely that there is one ----
;
; The number plus one, so nought still means no pad and the array is still one lookup. A
; flag would have to be followed by "and which of the four", which is the same walk done
; twice for an answer that was already in hand.
INIA 0d4
SETD.0 PadsLeft
LDB.0
CCF
SUB
MVQA ; Four less what is left to carve, which counts up from nought.
INCA
STA.1
SETD.1 PadWide
LDA.1
INCA
STA.1
INIB 0d8 ; ---- Eight columns, not four ----
;
; Four was 32 pixels of pad in a moon 1024 round, which is a
; target somebody flying by feel misses over and over. Eight is
; still small and is somewhere a person can aim at.
CCF
SUB
BNQ carveColumn
INCD.3
SETD.1 PadsLeft
LDA.1
DECA
STA.1
BNA carveOne
RET
; The shift register. Q comes back the new value, because Q is what survives a RET.
nextRandom:
SETD.1 Seed
LDA.1
INIB 0x01
AND
PSHQ ; The bit that is about to fall off.
LDA.1
RSTB
SHR ; A:B right one, and B was nought, so A is the byte halved.
POPB
BNB randomTap
STA.1 ; DP1 is still Seed, from reading it above.
RSTB
CCF
ADD ; Into Q, which is where a subroutine answers.
RET
randomTap:
INIB 0xB8
XOR
SETD.1 Seed
STQ.1
RET
; ---- The moon, drawn ----
;
; Rows outside and columns inside, which is the only affordable way round: cells along a row
; are next to each other, so the address is named once and the controller's Data port steps
; through 128 of them. Cells down a COLUMN are a page apart, and doing it that way would mean
; naming an address for every one of the 3,200 cells.
; ---- Drawn from twenty four rows above the world, not from the world's top ----
;
; The eighty column screen is fifty rows and the flyable band is thirty: the ceiling is eight
; rows above the origin and the deepest valley is twenty two below it. So the extra twenty
; rows a zoom buys have to go SOMEWHERE, and the first version gave them all to rock - it
; drew fifty rows down from the origin, which put the same sky on the screen as before with
; twice as much moon under it. Measured: 47 per cent moon zoomed in and 71 per cent zoomed
; out, which is a zoom that shows you less of what you are flying in.
;
; They go above instead. The rows run from minus twenty four to twenty five, which is the
; whole band with the ground near the bottom of it, and negative rows are sky by definition
; because nothing has a surface up there. In forty columns the ones above the origin are not
; on the screen at all, and drawing them anyway costs a startup and clears whatever the shell
; left in them.
drawMoon:
INIA 0xE8 ; Minus twenty four.
SETD.1 Row
STA.1
drawRow:
INIA 0d5
OUTA 0xE3
SETD.1 Row
LDA.1
INIB 0x7F
AND
MVQA ; Round the ring of 128, so minus twenty four is row 104.
INIB 0x40
CCF
ADD
OUTQ 0xE4 ; The map starts at 0x4000 and a row is a page.
RSTA ; A held the row; DestLow has to be nought.
OUTA 0xE5
SETD.2 Terrain
SETD.1 Column
STA.1 ; A is still the nought that went to DestLow.
drawCell:
; Above the world's origin, where no column has a surface, so there is nothing to compare.
SETD.1 Row
LDA.1
INIB 0x80
AND
BNQ drawSky
; ---- Ground at the surface row and everything under it ----
;
; ROW MINUS HEIGHT and not the other way round, which matters at the surface itself: the
; two are equal there, equal does not borrow, and the row a column's surface is on has to
; be ground rather than the last of the sky.
LDA.1
LDB.2
CCF
SUB
BRC drawSky ; Borrowed: this row is above the surface, so it is sky.
BRQ drawSurface ; Equal: this row IS the surface, which may be somewhere to land.
drawGround:
INIA 0xC8
OUTA 0xE9 ; Tile 200, the ground block.
RSTA
OUTA 0xE9 ; Attribute nought: grey on black, which is a fine moon.
BRI drawNextCell
; ---- The top of a pad, in a colour that says so ----
;
; The same solid tile in a different scheme, which costs no art at all: an attribute is a
; nibble added to every index in the tile, so one block is a grey moon and a cyan landing pad
; depending on the byte beside it.
drawSurface:
SETD.1 IsPad
SETD.0 Column
LDA.0
DPUA.1
LDA.1
BRA drawGround ; Not a pad, so it is ordinary moon.
DECA ; Back to the base's own number.
SETD.1 PadColour
DPUA.1
INIA 0xC8
OUTA 0xE9 ; The solid block again.
LDA.1
OUTA 0xE9 ; And this base's scheme, which is the whole of its name.
BRI drawNextCell
drawSky:
RSTA ; A is still this column's height, from the comparison above.
OUTA 0xE9
OUTA 0xE9 ; Tile nought is the space, and no colour is needed for nothing.
drawNextCell:
INCD.2
SETD.1 Column
LDA.1
INCA
STA.1
INIB 0d128
CCF
SUB
BNQ drawCell
SETD.1 Row
LDA.1
INCA
STA.1
INIB 0d26
CCF
SUB
BNQ drawRow ; Fifty rows, from minus twenty four up to and including 25.
RET
; ---- The lander, as a sprite ----
;
; Sprite nought, and it never moves horizontally: the world scrolls under it and it stays at
; the middle of the screen. That is one byte a frame instead of two and it is also what makes
; a moon that wraps invisible to the player - there is no moment where the lander jumps.
putLander:
INIA 0d4
OUTA 0xE3
INIA 0xC0
OUTA 0xE4
RSTA
OUTA 0xE5
INIA 0xC9
OUTA 0xE9 ; Tile 201.
INIA 0x03
OUTA 0xE9 ; Attribute three, so it is yellow against a grey moon.
SETD.0 ShipX
LDA.0
OUTA 0xE9
INCD.0
LDA.0
OUTA 0xE9 ; X: the middle of the screen, less half a tile. Never moves.
OUTA 0xE9
OUTA 0xE9 ; Y, which every frame overwrites.
INIA 0x11
OUTA 0xE9 ; One tile by one.
RSTA
OUTA 0xE9 ; No flags, natural size, no depth.
RET
; ---- The gauge, which lives where the moon cannot scroll it away ----
;
; The window is a layer at a SCREEN position rather than a position in the world, so a bar
; drawn in it stays where it is put however far the moon turns underneath. One row, at the
; top, and the label written once because it never changes.
;
; The letters are ordinary tiles. The character generator starts at the space, so glyph n is
; character n plus thirty two - which makes F, U, E and L into 38, 53, 37 and 44.
putGauge:
INIA 0x02
OUTA 0x3D ; Two rows: the gauge, and whatever there is to say.
RSTA
OUTA 0x3E ; At the top of the screen.
INIA 0d5
OUTA 0xE3
INIA 0xC0
OUTA 0xE4
RSTA
OUTA 0xE5 ; The window begins at 0xC000 of the screen bank.
INIA 0d38
OUTA 0xE9
RSTA
OUTA 0xE9 ; F
INIA 0d53
OUTA 0xE9
RSTA
OUTA 0xE9 ; U
INIA 0d37
OUTA 0xE9
RSTA
OUTA 0xE9 ; E
INIA 0d44
OUTA 0xE9
RSTA
OUTA 0xE9 ; L
RET
; ---- And how much of it is left ----
;
; A byte of fuel makes a bar of up to 31 cells, which is the byte over eight: a shift, because
; there is no divide. Thirty five cells of room after the label, so a full tank does not quite
; fill the row and there is somewhere for it to be seen to stop.
;
; Redrawn whole every frame. Thirty five cells is seventy bytes out of one port with the
; address named once, which is cheaper than working out which of them changed.
showFuel:
RSTA
SETD.1 Fuel
LDB.1
CALL eighth ; Q is the fuel over eight, in the low half where it belongs.
MVQA
SETD.1 GaugeLeft
STA.1
INIA 0d5
OUTA 0xE3
INIA 0xC0
OUTA 0xE4
INIA 0x0A
OUTA 0xE5 ; Cell five of the window row, just past the label.
RSTA
SETD.1 GaugeAt
STA.1
gaugeCell:
; Solid while there is bar left to draw, blank after it.
SETD.1 GaugeLeft
LDA.1
BRA gaugeEmpty
DECA
STA.1
INIA 0xC8
OUTA 0xE9 ; The solid block, which is the ground's tile.
INIA 0x02
OUTA 0xE9 ; Scheme two, so the bar is not the colour of the moon.
BRI gaugeNext
gaugeEmpty:
OUTA 0xE9
OUTA 0xE9 ; A is nought here: no tile and no colour.
gaugeNext:
SETD.1 GaugeAt
LDA.1
INCA
STA.1
INIB 0d35
CCF
SUB
BNQ gaugeCell
RET
; ---- What the pad is holding ----
;
; One read, every button at once, and it does not go away when it is looked at. THIS IS THE
; THING THE CONSOLE CANNOT DO: a key that is down and staying down sends nothing, so a
; thruster driven by the console can only be pumped and never leaned on.
; ---- ANY of the four, not pad nought ----
;
; One person flies this, and which socket they plugged into is not a thing they should have
; to know. A controller does not always land on nought - the front end hands out the numbers
; the host gave it - so a game that reads only the first one works on some machines and
; silently does nothing on others, which is the worst of both.
;
; Four reads and three ORs. A port is an immediate byte inside the instruction that names it,
; so it cannot be computed and the four are written out.
readPad:
INA 0x60
SETD.1 Held
STA.1
INA 0x61
LDB.1
OR
STQ.1
INA 0x62
LDB.1
OR
STQ.1
INA 0x63
LDB.1
OR
STQ.1
RET
; ---- The console, which is still worth reading ----
;
; For q, always, because a pad has no letter for it. And for the arrows on a machine with no
; controller, where one press is one burn and that is the best that can be done - it reads as
; pumping the engine, which is a thing this machine makes true rather than a thing this
; program chose.
zoomKeyed:
CALL toggleView
RET
readControls:
INA 0x01
INIB 0x01 ; READY
AND
BRQ noKey
INA 0x00
SETD.1 KeyHeld
STA.1
INIB 0x71 ; q, whatever else is plugged in.
CCF
SUB
BRQ quit
; ---- And z swaps the zoom, pad or no pad ----
;
; ABOVE the test for a controller, unlike the arrows. Which way the lander is flown is a
; question the arrows have to ask, because a pad answers it better; how much of the moon is
; on the screen is not - somebody flying on a controller may still have a keyboard in front
; of them, and somebody without one would otherwise have no way to zoom at all.
;
; No edge to worry about here. The console delivers a key ONCE, which is the difference
; between a key and a button and the reason the pad below needs remembering and this does
; not.
LDA.1
INIB 0x7A ; z.
CCF
SUB
BRQ zoomKeyed
SETD.0 HasPad
LDA.0
BNA noKey ; There is a pad, so the arrows are its business and not this.
LDA.1 ; DP1 is still KeyHeld, from storing the key above.
INIB 0x80 ; Up
CCF
SUB
BRQ burnUp
LDA.1
INIB 0x81 ; Down
CCF
SUB
BRQ burnDown
LDA.1
INIB 0x82 ; Left
CCF
SUB
BRQ burnLeft
LDA.1
INIB 0x83 ; Right
CCF
SUB
BRQ burnRight
noKey:
RET
quit:
RSTA
SETD.1 Flying
STA.1
RET
burnUp:
SETD.0 SpeedDown
SETD.1 ThrustUp
CALL addWord
RET
burnDown:
; The same rule the pad's retro thruster has: nothing to push away from on the ground.
SETD.0 Landed
LDA.0
BNA burnDownDone
SETD.0 SpeedDown
SETD.1 ThrustDown
CALL addWord
burnDownDone:
RET
burnLeft:
SETD.0 SpeedAcross
SETD.1 ThrustLeft
CALL addWord
RET
burnRight:
SETD.0 SpeedAcross
SETD.1 ThrustRight
CALL addWord
RET
; ---- Gravity, which is the reason this is a game ----
;
; NOT EVERY FRAME. One sixteenth of a pixel per frame per frame is the smallest step this
; arithmetic can take and it is still far too much - it crossed the screen in a second and
; flew like a gas giant. So it is applied one frame in FallEvery, which divides the pull by
; that much and costs a byte and a compare.
;
; The alternative was a finer unit for velocity than for position, which means a shift every
; time one is added to the other, twice a frame, for ever. A counter is cheaper and it is one
; byte to change while the feel is being found.
fall:
SETD.1 FallTick
LDA.1
DECA
STA.1
BNA fallDone
SETD.1 FallEvery
LDA.1
SETD.1 FallTick
STA.1
; Gravity does not pull on a lander that is sitting on the ground or holding onto something.
SETD.0 Docked
LDA.0
BNA fallResting
SETD.0 Landed
LDA.0
BNA fallResting
CALL exchange
; ---- Orbit, which is gravity MINUS the swing outwards ----
;
; The first version of this only ever SUBTRACTED from gravity, and that is why it never
; behaved like an orbit. Cancelling some of the pull can stop a fall from getting worse; it
; can never turn one round, because nothing in it ever pushes up. A lander a little too slow
; sank for ever and a lander a little too fast rose for ever, and both of them were right.
;
; What actually happens is a difference between two things. Gravity pulls in and going round
; throws out, and which of them wins depends on the sideways speed:
;
; below orbital speed the pull wins and the lander falls
; at orbital speed they cancel and the lander circles
; above orbital speed the swing wins and the lander climbs
;
; That sign change is the whole mechanic. Falling buys sideways speed (see the exchange
; above), so a fall carries the lander past orbital speed and the swing turns it into a
; climb: a periapse. Climbing spends that speed back, drops it below orbital, and the pull
; turns the climb into a fall: an apoapse. Round and round, which is what an orbit is.
;
; ---- A gradient out of integer arithmetic ----
;
; The step is one sixteenth of a pixel a tick and there is nothing between that and nothing,
; so the strength cannot be scaled down directly. Instead the distance from orbital speed is
; added into a byte every tick and the step is taken only when that byte carries - so the
; fraction of ticks that act is the distance over 256, smoothly, with no divide.
;
; Times four, because the speed alone put orbit at a lateral speed no lander would ever
; reach. At four it lands on 64 sixteenths - four pixels a frame, which crosses the moon in
; about three seconds and takes ten seconds of holding a thruster to build. Something worked
; up to rather than stumbled into.
SETD.0 SpeedAcross
CALL magnitude
SETD.0 DriftHigh
LDA.0
BNA fallOutFull ; A whole high byte of it: far past orbital, and thrown outwards.
SETD.0 DriftLow
LDA.0
INIB 0d64
CCF
SUB
BRC fallInward ; Borrowed, so it is short of orbital speed and the pull wins.
; ---- Above orbital speed, so the swing wins ----
MVQA ; Q is how far past, from the comparison just above.
CCF
SUB ; B is still 64, from that same comparison.
BNC fallOutFull
RSTB
SHL
SHL ; Times four; A is the high half, so the answer comes out in A.
BRI fallOutStep
fallOutFull:
INIA 0xFF
fallOutStep:
SETD.1 OrbitStep
STA.1
INIA 0x01
SETD.1 Outward
STA.1
BRI fallSwing
; ---- Below orbital speed, so the pull wins ----
fallInward:
MVQA ; Q is the shortfall, less its sign.
NOTA
MVQA
INCA
INIB 0d64
CCF
SUB
BNC fallInFull
RSTB
SHL
SHL
BRI fallInStep
fallInFull:
INIA 0xFF ; Standing still, which is the whole of gravity and no swing.
fallInStep:
SETD.1 OrbitStep
STA.1
RSTA
SETD.1 Outward
STA.1
fallSwing:
SETD.0 OrbitStep
LDA.0
SETD.1 OrbitAt
LDB.1
CCF
ADD
STQ.1
BNC fallResting ; No carry, so this tick is one of the ones that does nothing.
SETD.0 Outward
LDA.0
BNA fallPushed
SETD.0 SpeedDown
SETD.1 Gravity
CALL addWord
BRI fallResting
fallPushed:
SETD.0 SpeedDown
SETD.1 Lift
CALL addWord
fallResting:
; ---- And whatever is being leaned on, on the same tick ----
;
; A held thruster fires here rather than every frame, for the reason gravity does: a
; sixteenth of a pixel is the smallest step this arithmetic takes, and applied sixty times a
; second it is an enormous acceleration. On the tick, thrust and gravity are two numbers
; whose RATIO is the whole feel of the thing, and both are one byte to change.
;
; Position still moves every frame. Only the acceleration is stepped, which nothing can see.
SETD.0 Held
LDA.0
INIB 0x08 ; Up
AND
BRQ fallNotUp
CALL takeFuel
BRQ fallNotUp ; Nothing in the tank, so nothing out of the engine.
SETD.0 Landed
LDA.0
BRA fallAlreadyUp
RSTA
STA.0 ; Lifting off, which is the only way to stop being landed.
SETD.0 EmptyText
CALL sayInWindow ; And whatever the base said goes with it.
fallAlreadyUp:
SETD.0 SpeedDown
SETD.1 PadUp
CALL addWord
fallNotUp:
; ---- And down, at half the strength of up ----
;
; Arresting a rise took a sideways burn and a wait for the orbit to come back round, which is
; how a rendezvous is really flown and is a lot to ask of somebody who has not flown one
; before. A retro thruster makes the same correction directly.
;
; HALF THE STRENGTH, deliberately. Up is two sixteenths a tick and this is one, which is the
; same as gravity - so it can stop a climb and it can hurry a descent, and it can never turn
; a landing approach into a crash faster than simply letting go would. The cheap way out of
; a mistake stays the expensive one.
SETD.0 Landed
LDA.0
BNA fallNotDown ; Sitting on the ground, where there is no down to go.
SETD.0 Held
LDA.0
INIB 0x04 ; Down
AND
BRQ fallNotDown
CALL takeFuel
BRQ fallNotDown ; Nothing in the tank, so nothing out of the engine.
SETD.0 SpeedDown
SETD.1 PadDown
CALL addWord
fallNotDown:
SETD.0 Held
LDA.0
INIB 0x02 ; Left
AND
BRQ fallNotLeft
CALL takeFuel
BRQ fallNotLeft
SETD.0 SpeedAcross
SETD.1 PadLeft
CALL addWord
fallNotLeft:
SETD.0 Held
LDA.0
INIB 0x01 ; Right
AND
BRQ fallDone
CALL takeFuel
BRQ fallDone
SETD.0 SpeedAcross
SETD.1 PadRight
CALL addWord
fallDone:
RET
; ---- Falling buys sideways speed; climbing spends it ----
;
; What the first orbit was missing. Cancelling gravity by speed lets a fast lander stop
; falling and gives it no way back: a little too slow and it sank for ever, faster and faster,
; because ALTITUDE DID NOTHING. Nothing turned height into speed or speed into height.
;
; A real orbit is that exchange. Falling towards periapse trades height for speed, which buys
; more relief, which turns the fall into a climb; climbing towards apoapse pays it back and
; the climb becomes a fall. That cycle is the whole character of an orbit.
;
; ---- And the rate is the PRODUCT, which is why this needed a multiply ----
;
; The first attempt traded at a rate set by the vertical speed alone, and it drained a climb
; all the way to nought: a lander that climbed long enough had no sideways speed left to buy
; relief with, and sank. Gating it above a minimum made it worse - the climb spent its way
; down to the gate and locked itself out, frozen there for ever.
;
; The honest rate is proportional to BOTH speeds, and that is what fixes it: as the sideways
; speed falls towards nought the trade stops by itself. The product is its own gate, and no
; threshold is needed or wanted.
exchange:
SETD.0 SpeedAcross
CALL magnitude
SETD.0 DriftHigh
LDA.0
BNA exchangeQuick ; A whole high byte of sideways speed: take the largest step.
SETD.0 DriftLow
LDA.0
SETD.1 MulA
STA.1
SETD.0 DriftLeftward
LDA.0
SETD.1 AcrossWasLeft
STA.1 ; Which way it is going, before the next magnitude overwrites it.
SETD.0 SpeedDown
CALL magnitude
SETD.0 DriftHigh
LDA.0
BNA exchangeQuick
SETD.0 DriftLow
LDA.0
SETD.1 MulB
STA.1
CALL mul8
; ---- Half the product ----
;
; The product reaches 65,025 and the accumulator is a byte, so what is wanted is a window
; into the middle of it. The high half alone was tried first and it was both too weak and
; too coarse: a lander at 80 sideways falling at 2 has a product of 160, whose high half is
; nought, so the trade did not happen at all until the fall was already fast. A dead patch
; right where the turn is supposed to begin.
;
; Half keeps the low bits, so the trade starts as soon as there is any fall at all, and it
; is eight times stronger besides. That is what closes the loop inside the height of the
; screen rather than somewhere off the top of it: the whole sky here is about 145 pixels
; between the ground and the ceiling, and a quarter - tried in between - still ran the
; lander into the roof before the turn came round.
SETD.0 MulHigh
LDA.0
INIB 0d02
CCF
SUB
BNC exchangeQuick ; 512 or more, which saturates the step whatever the low half.
RSTB
SHL
SHL
SHL
SHL
SHL
SHL
SHL ; Times a hundred and twenty eight, which comes out in A.
SETD.1 ExchangeStep
STA.1
SETD.0 MulLow
LDA.0
RSTB
SHR ; And the low half halved, which is the rest of the window.
LDB.1 ; DP1 still names the step, from the store just above.
CCF
ADD ; At most 128 plus 127, so this cannot carry.
MVQA
BRI exchangeAdd
exchangeQuick:
INIA 0xFF
exchangeAdd:
SETD.1 ExchangeAt
LDB.1
CCF
ADD
STQ.1
BNC exchangeDone ; No carry, so there is nothing to trade this tick.
; ---- Which way the step goes ----
;
; Falling grows the sideways speed and climbing shrinks it, which is a step AWAY from nought
; or TOWARDS it depending on which way the lander is already going. Both signs are the top
; bit a magnitude left behind, so the exclusive or of them is the whole decision: the same
; means grow, different means shrink.
SETD.0 DriftLeftward
LDA.0
SETD.1 AcrossWasLeft
LDB.1
XOR
BNQ exchangeSlower
SETD.0 SpeedAcross
SETD.1 StepUp
CALL addWord
RET
exchangeSlower:
SETD.0 SpeedAcross
SETD.1 StepDown
CALL addWord
exchangeDone:
RET
; One sideways, towards standing still, whichever way it is going.
spendAcross:
SETD.0 SpeedAcross
CALL magnitude
SETD.0 DriftLeftward
LDA.0
BNA spendAcrossLeft
SETD.0 SpeedAcross
SETD.1 StepDown
CALL addWord
RET
spendAcrossLeft:
SETD.0 SpeedAcross
SETD.1 StepUp
CALL addWord
RET
; ---- One unit, if there is one ----
;
; Q comes back nought when the tank is empty, and the thruster that asked does not fire. There
; is no message and no ending: a lander with no fuel is still flying, it just cannot do
; anything about where. What happens next is gravity, and gravity is patient.
;
; Charged PER THRUSTER PER TICK, so holding two at once costs two - which is the honest price
; and makes a drift you corrected expensive in a way a drift you avoided is not.
takeFuel:
SETD.1 Fuel
LDA.1
BRA takeFuelNone
DECA
STA.1
INIA 0x01
RSTB
CCF
ADD ; Q is one, which the caller reads as yes.
RET
takeFuelNone:
RSTB
CCF
ADD ; A is nought here, so Q is too, which is no.
RET
; ---- Where it is now ----
;
; Across first, and the wrap is an AND rather than a comparison: the moon is 2^14 sixteenths
; round, so falling off one side is the top bits going away.
move:
SETD.0 AcrossLow
SETD.1 SpeedAcross
CALL addWord
SETD.0 AcrossHigh
LDA.0
INIB 0x3F
AND
STQ.0
SETD.0 DownLow
SETD.1 SpeedDown
CALL addWord
; ---- The world has no lid, but a sixteen bit height does ----
;
; Climbing makes the height count down past nought and round to 65535, and a lander that
; kept going came back through the bottom and hit the ground FROM ABOVE. Two thousand pixels
; of climb is a long way and entirely reachable with a full tank.
;
; ---- And it is 192 pixels up, not 64, because the screen grew ----
;
; Sixty four was the whole of the sky a forty column screen had over the world's origin, so
; it was never a fact about the world - it was a fact about the view. The wide one starts
; twenty four rows higher, and a lander stopped at the old line was stopped a long way short
; of the top of its own picture for no reason it could see.
;
; Pinned rather than ended. Leaving upward is RECOVERABLE - gravity is always available and
; a lander with fuel can always come back - so stopping the run there would punish a state
; the player can fly out of. What kills you out here is running dry a long way from the
; ground, which is a death somebody flew into rather than one a boundary handed them.
;
; TWO DIFFERENT LINES, and they have to be. The warning covers being AT the ceiling or above
; it, or a pinned lander reads as back inside the world the next frame and the warning is
; written and wiped sixty times a second. The pin covers being STRICTLY above it, or the
; height is dragged back every frame and the lander can never descend at all.
SETD.0 DownHigh
LDA.0
INIB 0x80
AND
BRQ moveInside ; The height is positive, so it is somewhere below the top.
LDA.0 ; DP0 is still DownHigh, from the sign test.
INIB 0xF5
CCF
SUB
BRC moveAdrift ; Borrowed: at 0xF400 or above it, 192 pixels over the world's top.
moveInside:
; Back under the ceiling, so the warning goes if there was one.
SETD.0 Adrift
LDA.0
BRA moveDone
RSTA
STA.0
SETD.0 EmptyText
CALL sayInWindow
moveDone:
RET
moveAdrift:
; Strictly above? Then the height is pinned and the climb is spent. Exactly at the ceiling
; is left alone, so gravity has somewhere to move it to.
SETD.0 DownHigh
LDA.0
INIB 0xF4
CCF
SUB
BNC moveAdriftSaid ; No borrow, so it is at the ceiling and not past it.
RSTA
SETD.0 DownLow
STA.0
INIA 0xF4
SETD.0 DownHigh
STA.0
; ---- Only the climb is spent, never the fall ----
;
; Zeroing the speed outright pinned the lander here for ever: gravity adds to it once a
; tick, and a clamp that ran every frame wiped the pull nine times out of ten before it
; could become downward motion. A ceiling nobody can leave is worse than the wrap it
; replaced.
SETD.0 SpeedDownHigh
LDA.0
INIB 0x80
AND
BRQ moveAdriftSaid ; Positive: already coming down, so leave it be.
RSTA
SETD.0 SpeedDown
STA.0
SETD.0 SpeedDownHigh
STA.0
; ---- And the sideways speed that bought the climb ----
;
; Without this the ceiling was a trap with no way out. A lander held here has its climb
; wiped every time, so the exchange below sees no fall and no climb, trades nothing, and the
; sideways speed that is pushing it up NEVER CHANGES: above orbital speed the swing pushes
; up, the pin wipes it, and round again for ever. Measured: pinned at the top with 117
; sideways, unmoving, for the whole of a four minute flight.
;
; So the pin spends one sideways as well, which is only reached on the ticks the lander is
; actually trying to climb - about one frame in ten. Six seconds of scraping along the roof
; brings it back under orbital speed and it falls away on its own. Pushing against the top
; of the sky costs the speed that got you there.
CALL spendAcross
moveAdriftSaid:
; Said once rather than sixty times a second.
SETD.0 Adrift
LDA.0
BNA moveDone
INIA 0x01
STA.0
SETD.0 AdriftText
CALL sayInWindow
RET
; ---- The view, which follows it ----
;
; The lander is at the middle of the screen, so the view starts half a screen behind it. In
; PIXELS: the position is sixteenths, so it is shifted down four first.
follow:
SETD.0 AcrossLow
CALL toPixels
; Half a screen back, and round the moon if that went below nought.
SETD.0 PixelLow
SETD.1 HalfScreen
CALL subWord
SETD.0 PixelHigh
LDA.0
INIB 0x03
AND
STQ.0 ; 1024 pixels round, so three bits of high byte.
; The whole cells are the column origin and the remainder is the fine offset.
SETD.0 PixelLow
LDA.0
INIB 0x07
AND
OUTQ 0x37 ; Fine X.
SETD.0 PixelHigh
LDA.0
SETD.0 PixelLow
LDB.0
CALL eighth
OUTQ 0x36 ; The column origin. IN Q, because RET puts A back.
RET
; ---- The lander, put where it now is ----
; ---- Round it goes, one orbital speed a frame ----
moveStation:
SETD.0 StationLow
SETD.1 StationSpeed
CALL addWord
SETD.0 StationHigh
LDA.0
INIB 0x3F
AND
STQ.0 ; The same wrap the lander has: 2^14 sixteenths round.
RET
; ---- Where that puts it on the screen ----
;
; The lander is always at the middle, so the station's column is the middle plus HOW FAR ROUND
; IT IS FROM THE LANDER. That difference is taken in the moon's own arithmetic and wrapped to
; fourteen bits, which measures it the long way round whenever the station is behind: over
; half a moon ahead IS under half a moon behind, so anything past the half way point is turned
; into a negative offset instead.
;
; Off the edge needs no test. A sprite's X is signed and sixteen bits, so a station three
; hundred pixels to the left is asked for at minus a hundred and forty and the device declines
; to draw it, which is a comparison this does not have to make.
; ---- How far round the moon the station is from the lander, and which way ----
;
; Wrapped to fourteen bits like everything else here, which measures the long way round
; whenever the station is behind - so anything past the half way point is turned into a
; distance BACKWARDS before anybody uses it. Three callers want this now: the one that draws
; the station, the one that decides whether it can be docked with, and the one that slides the
; lander in under it afterwards.
;
; Half a moon is 8,192 sixteenths, which is 512 pixels - and that fits in the twelve signed
; bits toPixelsSigned hands back, so the drawing side can take this straight.
stationGap:
SETD.0 StationLow
LDA.0
SETD.1 AcrossLow
LDB.1
CCF
SUB
SETD.1 GapLow
STQ.1
SETD.0 StationHigh
LDA.0
SETD.1 AcrossHigh
LDB.1
SUB ; And the borrow the half below left.
MVQA
INIB 0x3F
AND
SETD.1 GapHigh
STQ.1
LDA.1
INIB 0x20
AND
BRQ stationGapDone ; Under half a moon, so it is ahead and the gap stands.
SETD.0 GapHigh
LDA.0
INIB 0x40
CCF
SUB
STQ.0 ; Less a whole moon, which makes it a distance backwards.
stationGapDone:
RET
showStation:
CALL stationGap
SETD.0 GapLow
CALL toPixelsSigned
; ---- From the lander's own column, not from the middle ----
;
; ShipX is half a screen LESS HALF A TILE, because that is what centres an eight pixel lander
; on the middle. The station was drawn from the middle itself, so its left edge sat where the
; lander's centre was and a perfectly aligned dock still looked four pixels out. Same origin
; for both, and a gap of nothing puts one exactly above the other.
SETD.0 PixelLow
LDA.0
SETD.1 ShipX
LDB.1
CCF
ADD
SETD.1 StationAt
STQ.1
SETD.0 PixelHigh
LDA.0
SETD.1 ShipX
INCD.1
LDB.1
ADD
SETD.1 StationAt
INCD.1
STQ.1
; And its row, which moves with the view exactly as the lander's does.
SETD.0 StationDownLow
CALL toPixelsSigned
SETD.0 PixelLow
LDA.0
SETD.1 ShipYAdd
LDB.1
CCF
ADD
SETD.1 StationTop
STQ.1
SETD.0 PixelHigh
LDA.0
SETD.1 ShipYAdd
INCD.1
LDB.1
ADD
SETD.1 StationTop
INCD.1
STQ.1
INIA 0d4
OUTA 0xE3
INIA 0xC0
OUTA 0xE4
INIA 0x60
OUTA 0xE5 ; Sprite six begins ninety six bytes in.
INIA 0xCA
OUTA 0xE9 ; Tile 202.
INIA 0x04
OUTA 0xE9 ; ---- Attribute four, blue ----
;
; Not taste. White is what the ceiling warning is counted in, cyan
; is what a landing pad is counted in, magenta is the instruments
; and yellow is the lander. Blue is the one ink no check measures,
; and picking a measured one has broken a test twice already.
SETD.0 StationAt
LDA.0
OUTA 0xE9
INCD.0
LDA.0
OUTA 0xE9 ; X.
SETD.0 StationTop
LDA.0
OUTA 0xE9
INCD.0
LDA.0
OUTA 0xE9 ; Y.
INIA 0x11
OUTA 0xE9 ; One tile by one.
RSTA
OUTA 0xE9 ; No flags.
OUTA 0xE9
OUTA 0xE9
OUTA 0xE9
OUTA 0xE9 ; No target size, which HERE means its natural one.
OUTA 0xE9 ; And no depth.
RET
; ---- A world position in pixels, with its sign kept ----
;
; toPixels shifts a sixteen bit position down four and masks what is left to twelve bits, so a
; position above the world's origin comes back as a large POSITIVE number rather than a small
; negative one. That was harmless while the top of the screen WAS the origin: anything up
; there was off the picture either way. Now that the wide view starts twenty four rows higher
; it is on the picture, and it has to be on the right part of it.
toPixelsSigned:
CALL toPixels
SETD.0 PixelHigh
LDA.0
INIB 0x08
AND
BRQ toPixelsSignedDone
LDA.0
INIB 0xF0
OR
STQ.0 ; The twelfth bit was the sign, so the top nibble is too.
toPixelsSignedDone:
RET
showLander:
SETD.0 DownLow
CALL toPixelsSigned
INIA 0d4
OUTA 0xE3
INIA 0xC0
OUTA 0xE4
INIA 0x04
OUTA 0xE5 ; Y is bytes four and five of the entry.
SETD.0 PixelLow
LDA.0
SETD.1 ShipYAdd
LDB.1
CCF
ADD
OUTQ 0xE9
SETD.0 PixelHigh
LDA.0
INCD.1
LDB.1
ADD ; And the carry, which is the other half of the shift.
OUTQ 0xE9
RET
; ---- Green if it would survive, red if it would not ----
;
; The magnitude is in DriftLow and DriftHigh and DP1 names the limit. Q comes back the
; attribute to draw with, which turns a bar from a number into an ANSWER: a person aiming at a
; pad does not want to know their speed, they want to know whether they can put it down.
barColour:
SETD.0 DriftHigh
LDA.0
BNA barColourFast ; A whole high byte of speed is past any limit worth having.
SETD.0 DriftLow
LDA.0
LDB.1
CCF
SUB
BNC barColourFast ; No borrow, so it is at or over the limit.
INIA 0x02
RSTB
CCF
ADD ; Scheme two, green, which is within tolerance.
RET
barColourFast:
INIA 0x01
RSTB
CCF
ADD ; Scheme one, red, which is not.
RET
; ---- How fast sideways, as a bar ----
;
; A moon has no air, so a drift never stops by itself and stopping one means cancelling the
; velocity EXACTLY. That is not hard to do; it is hard to do BLIND, which is what it was - a
; number nothing on the screen said anything about.
;
; So sprite one is a bar whose width is the drift and which disappears at nought. It grows
; right from the middle of the screen for a rightward drift and left for a leftward one, so
; which way is as plain as how fast, and "stopped" is the one state with nothing drawn.
;
; The whole of it is the sprite's target width, which the device stretches one tile into. The
; program does no drawing at all: it works out one number a frame and writes it.
; ---- How big a signed pair is, without its sign ----
;
; DP0 names a two byte value, low half first. Leaves the size in DriftLow and DriftHigh and
; the sign in DriftLeftward, which is nought for a positive one.
;
; A two's complement pair is inverted and stepped, and the step's carry is what runs into the
; high half - the same carry that makes addWord work, used one instruction at a time because
; there is nothing to add.
magnitude:
LDA.0
SETD.1 DriftLow
STA.1
INCD.0
LDA.0
SETD.1 DriftHigh
STA.1
INIB 0x80
AND
SETD.1 DriftLeftward
STQ.1 ; The top bit of the high half, which is the sign.
BRQ magnitudeDone
SETD.0 DriftLow
LDA.0
NOTA
MVQA
INCA
STA.0
SETD.0 DriftHigh
LDA.0
NOTA
MVQA
RSTB
ADD ; Plus the carry the step above left, which is the other half.
STQ.0
magnitudeDone:
RET
showDrift:
SETD.0 SpeedAcross
CALL magnitude
SETD.1 GentleAcross
CALL barColour
SETD.1 DriftColour
STQ.1
; ---- Clamped, because a bar wider than the screen says nothing a full one does not ----
;
; And because a target width is sixteen bits: an unclamped one would be asking the device to
; draw a bar sixty thousand pixels wide, which it clips, but only after being asked.
SETD.0 DriftHigh
LDA.0
BNA driftWide ; Anything in the high half is already past the limit.
SETD.0 DriftLow
LDA.0
INIB 0d120
CCF
SUB
BRC driftReady ; Borrowed, so it is under the limit and stands.
driftWide:
INIA 0d120
SETD.0 DriftLow
STA.0
driftReady:
; ---- Two zeros, meaning two different things ----
;
; A TARGET WIDTH OF NOUGHT IS THE NATURAL WIDTH, not an empty sprite. So a bar with no drift
; in it came out eight pixels wide - one whole tile, sitting at the middle of the screen,
; saying "stopped" in the same shape it says "drifting slightly". What draws nothing is a
; SIZE of nought, which is the other zero and the other byte.
SETD.0 DriftLow
LDA.0
BRA driftEmpty
INIA 0x11 ; One tile by one, however wide it ends up drawn.
BRI driftSized2
driftEmpty:
RSTA ; No tiles at all, which is the off switch.
driftSized2:
SETD.1 DriftSize
STA.1
; ---- Where it starts, in sixteen bits ----
;
; The middle for a rightward drift and that much back for a leftward one, and it is a WORD
; now because the middle of an eighty column screen is 320 and will not go in a byte.
SETD.0 MiddleX
LDA.0
SETD.1 DriftAt
STA.1
INCD.0
LDA.0
INCD.1
STA.1
SETD.0 DriftLeftward
LDA.0
BRA driftAt ; Nought is rightward, so the bar starts at the middle.
SETD.0 DriftAt
LDA.0
SETD.1 DriftLow
LDB.1
CCF
SUB
SETD.1 DriftAt
STQ.1
INCD.0
LDA.0
RSTB
SUB ; And the borrow, which is the other half of the step back.
STQ.0
driftAt:
INIA 0d4
OUTA 0xE3
INIA 0xC0
OUTA 0xE4
INIA 0x10
OUTA 0xE5 ; Sprite one begins sixteen bytes in.
INIA 0xC8
OUTA 0xE9 ; The ground block, which is a solid tile.
SETD.0 DriftColour
LDA.0
OUTA 0xE9 ; Green while this drift could be landed with, red while not.
SETD.0 DriftAt
LDA.0
OUTA 0xE9
INCD.0
LDA.0
OUTA 0xE9 ; X.
SETD.0 BarY
LDA.0
OUTA 0xE9
INCD.0
LDA.0
OUTA 0xE9 ; Y, near the bottom and clear of the ground most places.
SETD.0 DriftSize
LDA.0
OUTA 0xE9 ; One tile by one, or nothing at all when there is no drift.
RSTA
OUTA 0xE9 ; No flags.
SETD.0 DriftLow
LDA.0
OUTA 0xE9
RSTA
OUTA 0xE9 ; The width, which is the drift. NOUGHT DRAWS NOTHING.
INIA 0d3
OUTA 0xE9
RSTA
OUTA 0xE9 ; Three pixels tall.
OUTA 0xE9 ; And no depth.
RET
; ---- Has it arrived, and how hard ----
;
; The terrain is an array in Data Memory and not something read back out of the map, which is
; the whole reason this is cheap: the ground under the lander is one index into 128 bytes,
; where asking the screen would mean a transfer through the controller every frame.
;
; The column is the world position divided by eight, masked to the moon's 128. The surface is
; that column's row times eight, and the lander's feet are its top plus its eight pixels.
touchdown:
SETD.0 AcrossLow
CALL toPixels
SETD.0 PixelHigh
LDA.0
SETD.0 PixelLow
LDB.0
CALL eighth
MVQA
INIB 0x7F
AND ; The moon is 128 columns, so seven bits of it.
MVQA
SETD.1 LandColumn
STA.1 ; Kept, because the verdict below wants it too.
SETD.1 Terrain
DPUA.1 ; Terrain plus the column, which is the row its surface is on.
LDA.1
; Times eight, into pixels. A is the high half of the shift register and B the low, so the
; row goes in B and three turns LEFT multiply it - and a row is at most 24, so 192 fits in
; the low half and the high one stays empty.
RSTB
CCF
ADD
MVQB ; The row, in the low half.
RSTA
SHL
SHL
SHL
RSTA
CCF
ADD
SETD.1 SurfaceAt
STQ.1
; The feet: where the lander's top is, plus the eight pixels of it.
SETD.0 DownLow
CALL toPixels
SETD.0 PixelHigh
LDA.0
BNA touchdownDone ; Above the screen entirely, so nowhere near the ground.
SETD.0 PixelLow
LDA.0
INIB 0d8
CCF
ADD
MVQA
SETD.1 SurfaceAt
LDB.1
CCF
SUB
BRC touchdownDone ; Borrowed: the feet are still above the surface.
; Already sitting on it, so there is nothing new to say. Without this the arrival happens
; again every frame the lander rests, which is a base handing out cargo sixty times a
; second.
SETD.0 Landed
LDA.0
BNA touchdownDone
; ---- Arrived. Now, how ----
;
; Both speeds, and both have to be gentle. A landing that was soft downwards and sliding
; sideways is a lander on its side, which is the interesting half of the difficulty: the
; drift bar is the thing that was blind about it until this rung.
SETD.0 SpeedDown
CALL magnitude
SETD.0 DriftHigh
LDA.0
BNA touchdownCrash ; Anything in the high half is far too fast to argue about.
SETD.0 DriftLow
LDA.0
SETD.1 GentleDown
LDB.1
CCF
SUB
BNC touchdownCrash ; No borrow, so it is at or past the limit.
SETD.0 SpeedAcross
CALL magnitude
SETD.0 DriftHigh
LDA.0
BNA touchdownCrash
SETD.0 DriftLow
LDA.0
SETD.1 GentleAcross
LDB.1
CCF
SUB
BNC touchdownCrash
; ---- Down safely, which is not the end of anything ----
;
; The view settles, the arrival is dealt with, and the lander sits where it is until
; somebody opens the throttle again. A landing used to stop the program, which is fine for a
; game about landing once and wrong for one about carrying things between four places.
RSTA
OUTA 0x37
OUTA 0x38
CALL arrive
CALL waitKey
; Sitting still, exactly on the surface. Both speeds go, because a lander that kept a
; hundredth of a pixel a frame would drift off its pad while nobody was looking.
RSTA
SETD.0 SpeedDown
STA.0
INCD.0
STA.0
SETD.0 SpeedAcross
STA.0
INCD.0
STA.0
CALL restOnSurface
INIA 0x01
SETD.0 Landed
STA.0
RET
touchdownCrash:
SETD.0 CrashedText
touchdownStop:
; ---- The view settled before anything is said ----
;
; The console draws into the map, and the map is what is being scrolled - so a message
; printed while the view is three pixels into a cell comes out three pixels off the top,
; with as much of its first row missing as the cell above it has lost.
;
; Nothing is being aligned for tidiness: the flying is over, so the fractional part of the
; view has no more work to do, and putting it back is what makes the whole message visible.
; A HUD that had to stay readable WHILE the map moved is a different problem - see the
; CosmOS README - and this is not it.
RSTA
OUTA 0x37
OUTA 0x38
SWI osPrintString
INIA 0x0A
OUTA 0x00
; ---- And a look at what happened ----
;
; The verdict was printed and the program then left immediately, taking the screen with it -
; so the one thing anybody wanted to see, the lander sitting on the ground it had just
; arrived at, was replaced by a shell prompt before it could be looked at.
CALL waitKey
RSTA
SETD.1 Flying
STA.1 ; Which ends the loop, and the program tidies up as it always did.
touchdownDone:
RET
; ---- Docking, which is landing on something that is moving ----
;
; The same shape as touchdown and for the same reason: close enough, and slow enough, or it is
; a wreck. What makes it a different skill is that "slow enough" is measured against the
; STATION rather than against nothing. Its speed is orbital speed, which is the number the
; marks on the drift bar point at - so the instrument for this was already on the screen
; before there was anything to dock with.
;
; The tolerances are the ground's own, deliberately, so there is one number to argue with if
; this turns out to be too kind.
dock:
SETD.0 Docked
LDA.0
BNA dockRiding
; Close enough, going round, which is the same gap the station is drawn from.
CALL stationGap
SETD.0 GapLow
CALL magnitude
SETD.0 DriftHigh
LDA.0
BNA dockDone
SETD.0 DriftLow
LDA.0
INIB 0d128
CCF
SUB
BNC dockDone ; A whole tile away or more, so there is nothing to touch.
; ---- And close enough, up and down ----
SETD.0 StationDownLow
LDA.0
SETD.1 DownLow
LDB.1
CCF
SUB
SETD.1 DockRiseLow
STQ.1
SETD.0 StationDownHigh
LDA.0
SETD.1 DownHigh
LDB.1
SUB
SETD.1 DockRiseHigh
STQ.1
SETD.0 DockRiseLow
CALL magnitude
SETD.0 DriftHigh
LDA.0
BNA dockDone
SETD.0 DriftLow
LDA.0
INIB 0d128
CCF
SUB
BNC dockDone
; ---- Touching. Now, how fast, and RELATIVE TO WHAT ----
;
; The station is going at orbital speed, so a lander going at orbital speed is standing still
; beside it however fast the ground below says they are both travelling. That difference is
; the whole of the manoeuvre.
SETD.0 SpeedAcross
LDA.0
INIB 0d64
CCF
SUB
SETD.1 DockVxLow
STQ.1
INCD.0
LDA.0
RSTB
SUB ; And the borrow, which is the other half of the difference.
SETD.1 DockVxHigh
STQ.1
SETD.0 DockVxLow
CALL magnitude
SETD.0 DriftHigh
LDA.0
BNA dockStruck
SETD.0 DriftLow
LDA.0
SETD.1 GentleAcross
LDB.1
CCF
SUB
BNC dockStruck
; Downwards, where the station has no speed at all, so the difference is simply the fall.
SETD.0 SpeedDown
CALL magnitude
SETD.0 DriftHigh
LDA.0
BNA dockStruck
SETD.0 DriftLow
LDA.0
SETD.1 GentleDown
LDB.1
CCF
SUB
BNC dockStruck
; ---- Aboard ----
INIA 0x01
SETD.0 Docked
STA.0
CALL payFuel
SETD.0 DockedText
CALL sayInWindow
CALL waitKey
dockDone:
RET
dockStruck:
SETD.0 StruckText
BRI touchdownStop ; Which settles the view, says it, and ends the run.
; ---- Riding, until something is leaned on ----
;
; Held to the station rather than merely paid by it, because that is what docking IS. The
; speeds are the station's, so the pair drift round together, and the lander sits one tile
; under it so both are still visible.
;
; ANY thruster lets go. It has to be checked before the speeds are put back, or a burn would
; be wiped by the snap on the very frame it was made and the lander could never leave.
dockRiding:
SETD.0 Held
LDA.0
INIB 0x0F
AND
BNQ dockLetGo
; ---- Slid into place, not snapped to it ----
;
; A dock is allowed at up to eight pixels out in either direction, so putting the lander
; exactly under the station the instant it takes hold moved it a whole tile in one frame -
; a jump, right at the moment the player was being told they had been careful. Half a pixel
; a frame closes the worst of it in about half a second, which reads as the two of them
; settling together.
;
; The sideways one goes through the WRAPPED gap rather than the raw positions, because a
; dock made either side of the moon's seam has a raw difference of most of a moon.
CALL stationGap
CALL easeAcross
SETD.0 StationDownLow
LDA.0
INIB 0d128
CCF
ADD
SETD.1 DockTargetLow
STQ.1
SETD.0 StationDownHigh
LDA.0
RSTB
ADD ; One tile below it, carry included.
SETD.1 DockTargetHigh
STQ.1
SETD.0 DownLow
SETD.1 DockTargetLow
CALL easeToward
INIA 0d64
SETD.0 SpeedAcross
STA.0
RSTA
INCD.0
STA.0 ; Orbital speed exactly, which is what it is riding.
SETD.0 SpeedDown
STA.0
INCD.0
STA.0
RET
dockLetGo:
RSTA
SETD.0 Docked
STA.0
RET
; ---- Half a pixel of the way there, or the whole of it if that is nearer ----
;
; DP0 names the word to move, DP1 the place it should end up. The gap is taken as a signed
; sixteen bit number, so "close enough to finish" is two cases and not one: a small gap
; forwards has a high byte of nought, and a small gap backwards has one of 0xFF.
easeToward:
LDA.1
LDB.0
CCF
SUB
SETD.2 EaseLow
STQ.2
INCD.0
INCD.1
LDA.1
LDB.0
SUB ; And the borrow, which is the other half of the gap.
SETD.2 EaseHigh
STQ.2
DECD.0
DECD.1 ; Both pointers back at the low bytes they came in on.
LDA.2 ; DP2 still names the high half of the gap.
BNA easeBack ; Not nought, so the gap runs backwards or is a long way off.
SETD.2 EaseLow
LDA.2
INIB 0d8
CCF
SUB
BRC easeArrive ; Borrowed: under a step, so put it exactly there.
SETD.1 EaseForward
CALL addWord
RET
easeBack:
SETD.2 EaseLow
LDA.2
INIB 0d248
CCF
SUB
BNC easeArrive ; No borrow: within a step of nought, going the other way.
SETD.1 EaseBackward
CALL addWord
RET
easeArrive:
LDA.1
STA.0
INCD.0
INCD.1
LDA.1
STA.0
RET
; ---- The same, for a position that has to go round a moon to get there ----
;
; The gap is already worked out and wrapped, in GapLow. Adding to a position and then wrapping
; it is the whole difference from the routine above: on a ring the target is "whatever closes
; that gap", not a place, because the place can be on the other side of the seam.
easeAcross:
SETD.0 GapHigh
LDA.0
BNA easeAcrossBack ; Not nought, so the gap runs backwards or is a long way off.
SETD.0 GapLow
LDA.0
INIB 0d8
CCF
SUB
BRC easeAcrossWhole ; Borrowed: under a step, so close the whole of it at once.
SETD.0 AcrossLow
SETD.1 EaseForward
CALL addWord
BRI easeAcrossWrap
easeAcrossBack:
SETD.0 GapLow
LDA.0
INIB 0d248
CCF
SUB
BNC easeAcrossWhole
SETD.0 AcrossLow
SETD.1 EaseBackward
CALL addWord
BRI easeAcrossWrap
easeAcrossWhole:
SETD.0 AcrossLow
SETD.1 GapLow
CALL addWord
easeAcrossWrap:
SETD.0 AcrossHigh
LDA.0
INIB 0x3F
AND
STQ.0 ; Round the moon, the way every other step along it is.
RET
; ---- A key, or a button ----
;
; Somebody flying on a controller should not have to reach for the keyboard to say "yes, I
; read that". A or Start does it, and so does any key - asked for rather than waited on, so
; one already pressed counts.
waitKey:
CALL readPad
SETD.0 Held
LDA.0
INIB 0x50 ; A, or Start.
AND
BNQ waitKeyDone
INA 0x01
INIB 0x01 ; READY
AND
BRQ waitKey
INA 0x00
waitKeyDone:
RET
; ---- Put down exactly on the ground ----
;
; The feet are the top plus eight, so the top is the surface less eight - times sixteen, which
; is what the position is measured in. Three turns left of the shift register with the pixels
; in the low half, and the answer needs both halves because a screen is taller than 255
; sixteenths.
restOnSurface:
CALL groundLevel
SETD.0 GroundLow
LDA.0
SETD.1 DownLow
STA.1
SETD.0 GroundHigh
LDA.0
SETD.1 DownHigh
STA.1
RET
; ---- How far above the ground, as a bar up the left edge ----
;
; The orbit put the lander OFF THE TOP OF THE SCREEN, and an instrument panel that only works
; while the ground is in sight is no use to a game whose best flying happens above it. Both
; other bars are rates: they say how fast, and neither of them says where.
;
; Height above the surface underneath rather than above some fixed line, so it reads NOUGHT
; the moment the lander is down - a bar that still showed a third of itself while sitting on a
; pad would be an altimeter nobody could trust. It moves as the moon does underneath, which is
; not noise: passing over a mountain really does leave less room below.
;
; Half a pixel of bar to a pixel of sky. The flyable band is about 256 pixels from the ceiling
; to the lowest ground and the screen is 200 tall, so a bar drawn pixel for pixel would run
; off the top of the very screen it is trying to describe.
showHeight:
CALL groundLevel
SETD.0 GroundLow
LDA.0
SETD.1 DownLow
LDB.1
CCF
SUB
SETD.0 HeightLow
STQ.0
SETD.0 GroundHigh
LDA.0
SETD.1 DownHigh
LDB.1
SUB ; And the borrow the half below left, which is the other half.
SETD.0 HeightHigh
STQ.0
LDA.0 ; DP0 is still HeightHigh, from the store above.
INIB 0x80
AND
BNQ showHeightNone ; Negative: the feet are under the surface, so there is no sky.
LDA.0 ; The AND left A alone, so it still holds the high half.
INIB 0d32
CCF
SUB
BNC showHeightFull ; 8,192 sixteenths or more, which is the whole bar and then some.
LDA.0 ; DP0 still names the high half, and the compare did not touch A.
SETD.0 HeightLow
LDB.0
SHL
SHL ; ---- Over sixty four, which comes out in A ----
;
; A is the HIGH half of the shift register, so two turns LEFT of
; the whole sixteen bits leaves bits six to thirteen in A - which
; is the height over sixty four, in one instruction each and no
; division anywhere.
;
; Over sixty four rather than thirty two because the ceiling moved:
; the band is 368 pixels now, and half a pixel of bar to a pixel of
; sky would stand 184 tall and run off the top of the forty column
; screen it is drawn beside.
BRI showHeightReady
showHeightFull:
INIA 0d127
BRI showHeightReady
showHeightNone:
RSTA
showHeightReady:
SETD.1 HeightBar
STA.1
; ---- Grown upwards, from a fixed foot ----
;
; A sprite is placed by its TOP, so a bar that grows up is one whose top moves as its height
; changes. Both come out of the same number and the foot stays put.
SETD.0 HeightFoot
LDA.0
LDB.1
CCF
SUB
SETD.1 HeightAt
STQ.1
INCD.0
LDA.0
RSTB
SUB ; And the borrow, which is the other half of the foot.
INCD.1
STQ.1
; Nought is the off switch, and it is the SIZE that switches it: a target height of nought
; is the natural height, which would be a tile sitting on the ground pretending to be sky.
SETD.0 HeightBar
LDA.0
BRA showHeightEmpty
INIA 0x11
BRI showHeightSized
showHeightEmpty:
RSTA
showHeightSized:
SETD.1 HeightSize
STA.1
INIA 0d4
OUTA 0xE3
INIA 0xC0
OUTA 0xE4
INIA 0x30
OUTA 0xE5 ; Sprite three begins forty eight bytes in.
INIA 0xC8
OUTA 0xE9 ; The solid block.
INIA 0x05
OUTA 0xE9 ; ---- Scheme five, magenta ----
;
; Red and green are taken and they MEAN something here: how fast,
; and whether it can be landed with. Neither is a thing an altitude
; is, so magenta is the panel's other job - where the lander is
; rather than how it is going - and the marks below share it.
;
; Cyan was the first choice and it is the colour of a landing pad.
; The pads are checked as whole cells of it, so a three pixel bar
; in the same colour would have made that check count the panel.
INIA 0d8
OUTA 0xE9
RSTA
OUTA 0xE9 ; X, hard against the left edge and clear of the moon's middle.
SETD.0 HeightAt
LDA.0
OUTA 0xE9
INCD.0
LDA.0
OUTA 0xE9 ; Y.
SETD.0 HeightSize
LDA.0
OUTA 0xE9
RSTA
OUTA 0xE9 ; No flags.
INIA 0d3
OUTA 0xE9
RSTA
OUTA 0xE9 ; Three pixels wide.
SETD.0 HeightBar
LDA.0
OUTA 0xE9
RSTA
OUTA 0xE9 ; The height, which is the altitude. NOUGHT DRAWS NOTHING.
OUTA 0xE9 ; And no depth, A being nought already.
RET
; ---- A state, placed rather than flown to ----
;
; Sixteen bytes read straight over the numbers the program would otherwise start with. If the
; file is not there nothing happens and the defaults stand, so a disk without one is the game
; as it has always been.
;
; ---- Which exists because some things cannot be flown to ----
;
; A successful dock needs the lander alongside the station and matched, and NINETY SIX pad
; files failed to get there - not for want of trying, but because climbing spends sideways
; speed, so a lander cannot rise while matched and arrives slower than orbital every time.
; Reaching it wants two burns and a phase, which is a real manoeuvre rather than something a
; straight line of button presses stumbles into. The same was true of the orbit, whose check
; had to be deleted for measuring the boot time instead of the physics, and of the ride, and
; of the strike, whose window is nineteen frames wide. Four things queued behind one facility.
;
; BINARY, because that is what a file is on this machine. A tool to build one from readable
; text is a small job for another day; until then the tests write the bytes, which is plain
; enough while the fields are named.
loadState:
SETD.0 StateName
SETD.1 StateBuffer
SWI osFileRead
BNQ loadStateNone ; Not there, or would not read, and either way the defaults stand.
SETD.0 StateAcross
SETD.1 AcrossLow
CALL copyWord
SETD.0 StateDown
SETD.1 DownLow
CALL copyWord
SETD.0 StateSpeedAcross
SETD.1 SpeedAcross
CALL copyWord
SETD.0 StateSpeedDown
SETD.1 SpeedDown
CALL copyWord
SETD.0 StateStation
SETD.1 StationLow
CALL copyWord
SETD.0 StateStationDown
SETD.1 StationDownLow
CALL copyWord
SETD.0 StateFuel
LDA.0
SETD.1 Fuel
STA.1
SETD.0 StateWide
LDA.0
SETD.1 Wide
STA.1
SETD.0 StateLanded
LDA.0
SETD.1 Landed
STA.1
loadStateNone:
RET
; Two bytes, from where DP0 says to where DP1 does.
copyWord:
LDA.0
STA.1
INCD.0
INCD.1
LDA.0
STA.1
RET
; ---- Two zoom levels, which the device turns out to have already had ----
;
; Forty columns and eighty are the same map, the same 8x8 cells and the same engine; only how
; many of them fit on the screen differs. The map is 128 by 128 either way and the moon is
; exactly 128 columns of it, so forty shows under a third of the moon and eighty shows nearly
; two thirds - and the front end scales whatever it is handed up to the same window, so 320 by
; 200 at four times and 640 by 400 at twice fill the same glass.
;
; THAT IS A ZOOM, and nothing in the video device had to change to get it. What has to change
; is every screen coordinate in this program, because the middle of the screen is 160 on one
; and 320 on the other. They all live in one block.
setView:
SETD.0 ViewNarrow
SETD.1 Wide
LDA.1
BRA setViewFrom
SETD.0 ViewWide
setViewFrom:
SETD.1 View
INIA 0d17
SETD.2 ViewLeft
STA.2
setViewByte:
LDA.0
STA.1
INCD.0
INCD.1
SETD.2 ViewLeft
LDA.2
DECA
STA.2
BNA setViewByte
RET
; ---- Out for the orbit, in for the landing ----
;
; The moon is redrawn because it is filled as many rows deep as the mode shows, and a moon
; drawn 25 deep on a screen showing 50 is a moon floating over nothing. The fixed sprites go
; again because their places moved with the middle.
toggleView:
SETD.0 Wide
LDA.0
INIB 0x01
XOR
STQ.0
CALL setView
SETD.0 Wide
LDA.0
OUTA 0x31
SETD.0 ScrollRow
LDA.0
OUTA 0x34 ; The wide view starts higher up, which is where the sky is.
CALL drawMoon
CALL putLander
CALL putGauge
CALL putOrbital
RET
; ---- One press, one swap ----
;
; A pad is LEVEL and not an event: it says what is held NOW, sixty times a second. A view that
; swapped whenever B was down would swap sixty times a second, which is not a zoom, it is a
; strobe. So the swap happens on the frame the button goes down, and the frame it comes up is
; remembered for the comparison rather than acted on.
zoomButton:
SETD.0 Held
LDA.0
INIB 0x20 ; B.
AND
MVQA
SETD.1 ZoomWas
LDB.1
STA.1 ; Now becomes what "last frame" means next time round.
CCF
SUB
BRQ zoomDone ; The same as last frame, so nothing has been pressed or let go.
LDA.1 ; DP1 still names it, and it now holds this frame's answer.
BRA zoomDone ; It changed to NOT held, which is the release and not the press.
CALL toggleView
zoomDone:
RET
; ---- Where orbital speed is, marked on the drift bar ----
;
; The bar says how fast sideways and the physics says 64 sixteenths is the speed at which the
; swing outwards cancels the pull. Without a mark on it that number is FOLKLORE: a pilot can
; feel that somewhere around here the falling stops, and has no way to see where. With one,
; the bar reaching the mark IS circular orbit, under it is falling and past it is climbing,
; and the whole mechanic becomes something read at a glance instead of guessed at.
;
; One each way, because a moon that wraps can be gone round in either direction.
;
; Placed so the BAR'S EDGE MEETS THE MARK at exactly orbital speed. The rightward bar starts
; at the middle and runs 64 pixels to 223, so its mark begins at 224; the leftward one ends at
; the middle and starts at 96, so its mark is the two pixels before that. Adjacent rather than
; overlapping, or a bar at orbital speed would hide the thing it is being measured against.
;
; Written once. They never move, and a sprite table is a memory rather than a display list.
putOrbital:
; The leftward mark: sixty four back from the middle, and two more for its own width, so the
; bar's leading edge meets it rather than the bar covering it up.
SETD.0 MiddleX
LDA.0
INIB 0d66
CCF
SUB
SETD.1 MarkAt
STQ.1
INCD.0
LDA.0
RSTB
SUB ; And the borrow, because the middle can be 320.
INCD.1
STQ.1
INIA 0d4
OUTA 0xE3
INIA 0xC0
OUTA 0xE4
INIA 0x40
OUTA 0xE5 ; Sprite four begins sixty four bytes in.
CALL putMark
; And the rightward one, sixty four on from it.
SETD.0 MiddleX
LDA.0
INIB 0d64
CCF
ADD
SETD.1 MarkAt
STQ.1
INCD.0
LDA.0
RSTB
ADD
INCD.1
STQ.1
INIA 0d4
OUTA 0xE3
INIA 0xC0
OUTA 0xE4
INIA 0x50
OUTA 0xE5 ; And sprite five, sixteen bytes after it.
CALL putMark
RET
; One mark, at the column in MarkAt, straddling the drift bar so the bar meets it end on.
putMark:
INIA 0xC8
OUTA 0xE9 ; The solid block, which is what every bar here is made of.
INIA 0x05
OUTA 0xE9 ; Magenta, the same as the height bar: both say where, not how fast.
; White was the first choice and white is what TEXT is, so the
; ceiling check - which counts white to find its warning - started
; counting these instead and read a warning that was never up.
SETD.1 MarkAt
LDA.1
OUTA 0xE9
INCD.1
LDA.1
OUTA 0xE9 ; X.
SETD.0 BarY
LDA.0
INIB 0d4
CCF
SUB
OUTQ 0xE9
INCD.0
LDA.0
RSTB
SUB
OUTQ 0xE9 ; Y, four pixels above the bar it brackets.
INIA 0x11
OUTA 0xE9 ; One tile by one, however small it ends up drawn.
RSTA
OUTA 0xE9 ; No flags.
INIA 0d2
OUTA 0xE9
RSTA
OUTA 0xE9 ; Two pixels wide.
INIA 0d11
OUTA 0xE9
RSTA
OUTA 0xE9 ; And eleven tall, so it shows above and below a three pixel bar.
OUTA 0xE9 ; No depth, A being nought already.
RET
; ---- Where a lander standing on the ground below would have its top ----
;
; The same sum, wanted in two places: putting one down on the surface, and measuring how far
; above the surface one is. In sixteenths, because that is what a height is measured in.
groundLevel:
SETD.0 SurfaceAt
LDA.0
INIB 0d8
CCF
SUB
MVQB
RSTA
SHL
SHL
SHL
SHL ; Four turns, which is times sixteen.
SETD.0 GroundHigh
STA.0
RSTA
CCF
ADD
MVQA
SETD.0 GroundLow
STA.0
RET
; ---- A line of text, into the window ----
;
; DP0 names a string. It goes into window row one and the rest of the row is BLANKED, so a
; short message never leaves the tail of a long one behind it.
;
; Into the window rather than out of the console, which is the whole of two bugs at once. The
; console draws into the map, so a message printed while flying was a message the lander then
; flew over - and printing scrolls, so every one of them moved the whole world up a row.
; The window is at a screen position and forty cells wide, and neither is true of it.
;
; The letters are ordinary tiles: the character generator starts at the space, so glyph n is
; character n less thirty two.
sayInWindow:
INIA 0d5
OUTA 0xE3
INIA 0xC1
OUTA 0xE4
RSTA
OUTA 0xE5 ; Window row one begins at 0xC100.
SETD.1 SayAt
STA.1
sayCell:
LDA.0
BRA sayBlank ; The string has ended, so the rest of the row is nothing.
INIB 0d32
CCF
SUB
OUTQ 0xE9
INIA 0x07
OUTA 0xE9 ; Scheme seven, white, which is nothing else on this screen.
INCD.0
BRI sayNext
sayBlank:
OUTA 0xE9
OUTA 0xE9 ; A is nought here, and DP0 stays put so it stays nought.
sayNext:
SETD.1 SayAt
LDA.1
INCA
STA.1
SETD.2 RowCells
LDB.2
CCF
SUB
BNQ sayCell
RET
; ---- What a base does when a lander arrives ----
;
; Empty and at a base: it hands over cargo for the base ACROSS THE MOON, two along, so the
; pairs are cyan with red and green with blue. Carrying and at the right base: it takes the
; cargo and pays in fuel. Carrying and at the wrong one: nothing, which is the whole reason
; the destination is on the screen.
arrive:
SETD.1 IsPad
SETD.0 LandColumn
LDA.0
DPUA.1
LDA.1
BRA arriveNowhere
DECA
SETD.1 AtBase
STA.1
SETD.0 Carrying
LDA.0
BRA arriveLoad
; Carrying something. Is this where it goes?
DECA
LDB.1 ; DP1 is still AtBase, from storing it above.
CCF
SUB
BNQ arriveWrong
RSTA
STA.0 ; And DP0 is still Carrying, from reading it.
CALL payFuel
SETD.0 DeliveredText
CALL sayInWindow
RET
arriveLoad:
; Two along, round a moon of four bases, which is the far side of it.
SETD.1 AtBase
LDA.1
INIB 0x02
CCF
ADD
MVQA
INIB 0x03
AND
MVQA
INCA ; Plus one, so nought can go on meaning nothing.
SETD.0 Carrying
STA.0
SETD.0 LoadedText
CALL sayInWindow
RET
arriveWrong:
SETD.0 WrongText
CALL sayInWindow
RET
arriveNowhere:
SETD.0 LandedText
CALL sayInWindow
RET
; ---- Paid ----
;
; Eighty units, and a tank that will not take more than it holds. The carry from a byte's
; worth of adding is what says it went past, which is the same flag a sixteen bit sum uses to
; run one half into the other.
payFuel:
SETD.0 Fuel
LDA.0
INIB 0d80
CCF
ADD
BRC payFuelFull
STQ.0
RET
payFuelFull:
INIA 0xFF
STA.0
RET
; ---- How fast downwards, as a bar down the side ----
;
; The same idea stood on its end. Sprite two, three pixels wide, growing DOWN from the middle
; of the screen while falling and UP while climbing - so which way is as plain as how fast,
; and a lander holding its height has no bar at all.
;
; Two bars and two limits is the whole instrument panel: when both are green the lander can be
; put down, and that is a question a person can answer at a glance instead of by counting.
showFall:
SETD.0 SpeedDown
CALL magnitude
SETD.1 GentleDown
CALL barColour
SETD.1 FallColour
STQ.1
; Clamped, and to less than the drift bar because there is half a screen either way.
SETD.0 DriftHigh
LDA.0
BNA fallBarWide
SETD.0 DriftLow
LDA.0
INIB 0d90
CCF
SUB
BRC fallBarReady
fallBarWide:
INIA 0d90
SETD.0 DriftLow
STA.0
fallBarReady:
; No tiles at all when there is nothing to show, which is the off switch: a target size of
; nought is the NATURAL size, not an empty sprite.
SETD.0 DriftLow
LDA.0
BRA fallBarEmpty
INIA 0x11
BRI fallBarSized
fallBarEmpty:
RSTA
fallBarSized:
SETD.1 FallSize
STA.1
; Where it starts: the middle going down, and that much back going up.
SETD.0 MiddleY
LDA.0
SETD.0 DriftLeftward
LDB.0
BRB fallBarAt ; Nought is downward, so it starts at the middle.
SETD.0 DriftLow
LDB.0
CCF
SUB
MVQA
fallBarAt:
SETD.1 FallAt
STA.1
INIA 0d4
OUTA 0xE3
INIA 0xC0
OUTA 0xE4
INIA 0x20
OUTA 0xE5 ; Sprite two begins thirty two bytes in.
INIA 0xC8
OUTA 0xE9 ; The solid block.
SETD.0 FallColour
LDA.0
OUTA 0xE9
SETD.0 FallX
LDA.0
OUTA 0xE9
INCD.0
LDA.0
OUTA 0xE9 ; X, in two bytes, because the screen is wider than one.
SETD.0 FallAt
LDA.0
OUTA 0xE9
RSTA
OUTA 0xE9 ; Y.
SETD.0 FallSize
LDA.0
OUTA 0xE9
RSTA
OUTA 0xE9 ; No flags.
INIA 0x03
OUTA 0xE9
RSTA
OUTA 0xE9 ; Three pixels wide.
SETD.0 DriftLow
LDA.0
OUTA 0xE9
RSTA
OUTA 0xE9 ; And as tall as the lander is fast.
OUTA 0xE9 ; No depth.
RET
; ---- Sums ----
;
; DP0 names the two byte value being changed, low half first, and DP1 the one being added to
; it. The carry runs from one half to the other, which is what ADD taking the carry flag is
; for, and CCF at the top is what stops the last sum leaking into this one.
addWord:
CCF
LDA.0
LDB.1
ADD
STQ.0
INCD.0
INCD.1
LDA.0
LDB.1
ADD
STQ.0
RET
subWord:
CCF
LDA.0
LDB.1
SUB
STQ.0
INCD.0
INCD.1
LDA.0
LDB.1
SUB
STQ.0
RET
; ---- Sixteenths into pixels ----
;
; DP0 names a two byte value, low half first. A and B are one sixteen bit register and it
; ROTATES rather than shifts, so four turns to the right bring the bottom four bits round into
; the top - which is why the high byte is masked afterwards and the low byte is not.
toPixels:
LDB.0
INCD.0
LDA.0
SHR
SHR
SHR
SHR
SETD.1 PixelHigh
STA.1 ; Kept whole for a moment, because B has to come out of B first.
RSTA
CCF
ADD ; Nothing plus B is B, in Q, which can be moved.
MVQA
SETD.1 PixelLow
STA.1
SETD.1 PixelHigh
LDA.1
INIB 0x0F
AND
STQ.1
RET
; ---- A pixel column, from a pixel ----
;
; A IS THE HIGH HALF and B the low, which is the way round the shift register is - it was
; written the other way first and the view scrolled by 256 cells for every one it should have.
; Three turns right is a divide by eight, and a moon 1024 pixels round is 128 columns, so the
; whole answer is in the low half and the high half is the bits that rotated out of it.
;
; ---- And the answer comes back in Q ----
;
; It came back in A first, which is a lie a subroutine cannot tell: RET puts A back the way it
; found it, so the caller wrote out the high byte of the position it had passed in. The fine
; register was computed inline and was right, the coarse register was not, and the picture
; scrolled smoothly within a cell and never advanced one. Q is the only register that crosses
; a RET, which is why every answer in this program comes back in it.
;
; B cannot be read at all. Adding nothing to it puts it in Q, which is where this wanted to
; be anyway.
eighth:
SHR
SHR
SHR
RSTA
CCF
ADD
RET
waitFrame:
INA 0x30
INIB 0x01
AND
BRQ waitFrame
RET
#Data
#Base 0x3000
; Sixteenths of a pixel, low half first, because that is the order the sums above walk in.
; ---- Which is over a base, because that is where a day starts ----
;
; 0x14A0 sixteenths is 330 pixels, which is column 41 - inside the pad carved at 40. Starting
; in the middle of nowhere meant a straight descent landed in the middle of nowhere, which is
; a fine thing to be able to do and a poor thing to have to.
AcrossLow:
0xA0
AcrossHigh:
0x14
DownLow:
0x00
DownHigh:
0x02 ; A little way down from the top.
SpeedAcross:
0x00 0x00
SpeedDown:
0x00
SpeedDownHigh:
0x00
; ---- The four numbers the feel lives in ----
;
; Adjacent on purpose, because tuning them is what the first rung is for.
Gravity:
0x01 0x00
FallEvery:
0d10 ; ---- Gravity one frame in ten ----
;
; Every frame was Jupiter and one in six was still touchy: the
; ratio between thrust and gravity is the FEEL, and how often the
; tick comes round is how fast that feel arrives. Slower ticks are
; the same lander with more time to think in.
FallTick:
0d1
; A press on a machine with no pad, which has to be a whole burn because it happens once.
ThrustUp:
0xF8 0xFF ; Eight sixteenths upwards, which is minus eight.
ThrustDown:
0x04 0x00 ; Four down, which is half of the eight that lifts.
ThrustLeft:
0xFC 0xFF ; Four to the left.
ThrustRight:
0x04 0x00
; ---- And a held one, which happens on every tick it is held for ----
;
; TWO AGAINST GRAVITY'S ONE, which makes climbing and falling the same speed: hold it and you
; rise as fast as letting go drops you. Three was the first try and it left the moon in about
; a second of holding. The ratio between these and Gravity is the whole feel of the thing and
; it is one byte each.
PadUp:
0xFE 0xFF ; Minus two.
; ---- Sideways, at half the vertical ----
;
; ONE, not two. There is no air on a moon, so nothing slows a drift but the opposite thruster
; and stopping means cancelling the velocity exactly. At two a tick the smallest correction
; was twice as big as it needed to be and overshooting was the normal outcome.
PadDown:
0x01 0x00 ; Plus one: half of up, and exactly gravity's own step.
PadLeft:
0xFF 0xFF ; Minus one.
PadRight:
0x01 0x00
Held:
0x00
HasPad:
0x00
DriftLow:
0x00
DriftHigh:
0x00
DriftLeftward:
0x00
DriftAt:
0x00 0x00
DriftSize:
0x00
DriftColour:
0x00
FallColour:
0x00
FallSize:
0x00
FallAt:
0x00
; ---- The tank ----
;
; One byte, and a byte is enough: over eight it is a bar of up to 31 cells, and at one unit a
; thruster a tick it is about forty seconds of holding the engine open. Sixteen bits would be
; more arithmetic for a number nobody reads to the unit.
Fuel:
0xFF
GaugeLeft:
0x00
GaugeAt:
0x00
SurfaceAt:
0x00
; ---- What counts as gentle ----
;
; In sixteenths of a pixel a frame, so twelve is three quarters of a pixel a frame and eight
; is half of one. Sideways is the tighter of the two on purpose: a lander that arrives
; straight down at three quarters of a pixel is a landing, and one sliding at the same speed
; is a lander on its side.
GentleDown:
0d12
GentleAcross:
0d8
; ---- The numbers that change when the screen does ----
;
; Copied over from whichever table below matches the mode, so a gauge reads a variable and
; never has to know which screen it is drawing on. The order here and in both tables is the
; same order and has to stay that way: setView copies the block straight across.
View:
HalfScreen:
0xA0 0x00
ShipX:
0x9C 0x00
MiddleX:
0xA0 0x00
BarY:
0xBC 0x00
FallX:
0x2C 0x01
MiddleY:
0d100
HeightFoot:
0xB4 0x00
RowCells:
0d40
ScrollRow:
0d0
ShipYAdd:
0x00 0x00
ViewLeft:
0x00 ; How much of the block is still to copy.
; Forty columns: 320 by 200, and the moon 25 rows deep because that is all there is to see.
ViewNarrow:
0xA0 0x00 ; Half a screen, which is where the lander is held.
0x9C 0x00 ; And the lander itself, half a tile back from it.
0xA0 0x00 ; The middle, where both bars start.
0xBC 0x00 ; The drift bar's row, twelve pixels off the bottom.
0x2C 0x01 ; The fall bar's column, twenty pixels in from the right.
0d100 ; Half the height, which is where the fall bar starts.
0xB4 0x00 ; The height bar's foot.
0d40 ; Cells to a window row.
0d0 ; The map row the top of the screen is, and
0x00 0x00 ; how far the lander's own Y has to move with it.
; Eighty columns: 640 by 400, the same cells at half the size, so twice as much moon either
; way. Every screen number doubles except the ones measured from an edge, which are the edge
; less the same margin - a panel should sit the same distance in whichever screen it is on.
ViewWide:
0x40 0x01 ; 320.
0x3C 0x01 ; 316.
0x40 0x01 ; 320.
0x84 0x01 ; 388, which is twelve off the bottom of four hundred.
0x6C 0x02 ; 620, twenty in from the right of six hundred and forty.
0d200
0x7C 0x01 ; 380.
0d80
0d104 ; Twenty four rows ABOVE the world's origin, which is the point.
0xC0 0x00 ; So the lander's Y moves 192 pixels down to match.
; Which of the two is up. Nought is forty columns, which is what the program starts in.
Wide:
0x00
; ---- The sixteen bytes a state file holds, in the order it holds them ----
;
; Named rather than numbered, so whatever writes one and this that reads it are describing the
; same fields. Low byte first, the way every other word here is.
StateName:
"/lander.state"
StateBuffer:
StateAcross:
0x00 0x00 ; Where round the moon, in sixteenths.
StateDown:
0x00 0x00 ; And how far down, which is negative above the origin.
StateSpeedAcross:
0x00 0x00
StateSpeedDown:
0x00 0x00
StateStation:
0x00 0x00 ; Where the station is, so a rendezvous can start alongside one.
StateStationDown:
0x00 0x00
StateFuel:
0x00
StateWide:
0x00 ; Which screen to start in: some of this is only visible wide.
StateLanded:
0x00
StateSpare:
0x00 ; Room, because sixteen is a rounder number than fifteen.
; ---- And a whole block of room after them ----
;
; osFileRead lands a file in Data Memory and a file is stored in blocks of 256, so reading
; sixteen bytes into sixteen bytes of room writes over whatever follows. It did: the first
; version of this put the buffer in front of the view tables and the program read its state,
; wiped the numbers every gauge draws from, and left immediately - "finished" and back to the
; prompt, with nothing on the screen to say why.
#Reserve 0d240
; What B was doing last frame, because a pad says what is held and not what was pressed.
ZoomWas:
0x00
PixelLow:
0x00
PixelHigh:
0x00
Height:
0x00
Row:
0x00
Column:
0x00
Seed:
0x5D
KeyHeld:
0x00
Flying:
0x01
Terrain:
#Reserve 0d128
; One byte a column: whether a lander touching down there is at a base. Written by the carving
; and read while drawing and while landing, which is the whole reason it is an array rather
; than four comparisons done again every time somebody asks.
IsPad:
#Reserve 0d128
PadTable:
0d8 0d40 0d72 0d104 ; Four bases, evenly round a moon 128 columns about.
; ---- Their names, which are colours ----
;
; A base is told from a base by the scheme its pad is drawn in, so "the cyan one" is a thing a
; person can say and a thing the machine already knows. Yellow is missing on purpose: it is
; the lander, and a base the same colour as the thing landing on it would be a poor joke.
;
; Cargo goes to the base ACROSS THE MOON, two along, so the pairs are cyan with red and green
; with blue. That is what makes the wrapping surface a route rather than scenery.
PadColour:
0d2 0d6 0d4 0d1 ; Green, cyan, blue, red.
PadNames:
0x00 ; Room kept: the messages name colours in words below.
PadsLeft:
0x00
PadWide:
0x00
PadHeight:
0x00
LandColumn:
0x00
AtBase:
0x00
; Where the orbit sum has got to. The sideways speed goes in here every tick and the carry out
; of it is a tick of gravity that never happened.
OrbitAt:
0x00
; How hard the swing is pulling or pushing this tick, and which way. And one step upwards,
; which is the only thing here that gravity did not already need.
OrbitStep:
0x00
; Ground level under the lander, the height above it, and what the bar makes of that.
GroundLow:
0x00
GroundHigh:
0x00
HeightLow:
0x00
HeightHigh:
0x00
HeightBar:
0x00
HeightAt:
0x00 0x00
HeightSize:
0x00
MarkAt:
0x00 0x00
Outward:
0x00
Lift:
0xFF 0xFF
; Where the height-for-speed trade has got to, and which way the lander was going when the
; product was worked out. One step of sideways speed, either way.
ExchangeAt:
0x00
ExchangeStep:
0x00
AcrossWasLeft:
0x00
StepUp:
0x01 0x00
StepDown:
0xFF 0xFF
SayAt:
0x00
; ---- What is aboard, and whether the feet are down ----
;
; Carrying is the destination base plus one, so nought means an empty hold and the number is
; still one byte. Landed is what stops a base handing out cargo sixty times a second while a
; lander sits on it.
Carrying:
0x00
Landed:
0x00
; Holding onto the station, which is the same kind of rest as sitting on the ground.
Docked:
0x00
; Where the lander is sliding to, and how far it has to go to get there.
DockTargetLow:
0x00
DockTargetHigh:
0x00
EaseLow:
0x00
EaseHigh:
0x00
EaseForward:
0x08 0x00 ; Half a pixel a frame, which settles the worst gap in about half
EaseBackward:
0xF8 0xFF ; a second - slow enough to see and fast enough not to wait for.
DockGapLow:
0x00
DockGapHigh:
0x00
DockRiseLow:
0x00
DockRiseHigh:
0x00
DockVxLow:
0x00
DockVxHigh:
0x00
; Whether the lander is pinned against the ceiling, so the warning is said once rather than
; sixty times a second and taken away when it comes back down.
Adrift:
0x00
; ---- Forty cells is the row, so forty characters is the limit ----
LandedText:
"Nowhere in particular. Nothing here."
LoadedText:
"Loaded. Take it across the moon."
DeliveredText:
"Delivered. Eighty units of fuel."
WrongText:
"Not the base this cargo is for."
AdriftText:
"Leaving the moon. Nothing up here."
EmptyText:
0x00
CrashedText:
"Crashed."
DockedText:
"Docked. Eighty units of fuel."
StruckText:
"Wrecked against the station."
; ---- The station, which is a body and not a special case ----
;
; A circular orbit is exactly what a thing at orbital speed DOES under the rules already here:
; the pull and the swing cancel at 64 sixteenths, at any height, because gravity never falls
; off and the moon wraps. So the station needs no physics of its own - a position, a constant,
; and the same wrap the lander uses.
;
; Four rows above the world's origin, which is off the top of a forty column screen and
; comfortably inside a wide one. That is deliberate: it makes the station a reason to use the
; zoom rather than something that can be flown to without it.
;
; It starts half a moon away, so it has to be found. At four pixels a frame a lap is 256
; frames, a little over four seconds, so it will not stay lost for long.
StationLow:
0xA0
StationHigh:
0x34
StationDownLow:
0x00
StationDownHigh:
0xFA ; ---- Minus 1,536 sixteenths, which is twelve rows up ----
;
; About two thirds of the way from the ground to the ceiling. It
; was four rows up when four rows was most of the sky there was,
; which put it exactly where anything climbing away from the
; surface had to pass - and being run down there is not a hazard,
; it is a toll. Twelve rows up is clear of work near the ground and
; still a dozen rows under the ceiling.
StationSpeed:
0x40 0x00 ; Orbital speed, and the whole of its behaviour.
; How far round the moon the station is from the lander, and where that puts it on the screen.
GapLow:
0x00
GapHigh:
0x00
StationAt:
0x00 0x00
StationTop:
0x00 0x00
StationArt:
0x00 0x00 0x01 0x01 0x01 0x01 0x00 0x00
0x00 0x00 0x00 0x01 0x01 0x00 0x00 0x00
0x01 0x01 0x00 0x01 0x01 0x00 0x01 0x01
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x01 0x01 0x00 0x01 0x01 0x00 0x01 0x01
0x00 0x00 0x00 0x01 0x01 0x00 0x00 0x00
0x00 0x00 0x01 0x01 0x01 0x01 0x00 0x00
StationArtAt:
#Reserve 0d2
LanderArt:
0x00 0x00 0x01 0x01 0x01 0x01 0x00 0x00
0x00 0x01 0x01 0x01 0x01 0x01 0x01 0x00
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x01 0x01 0x01 0x01 0x01 0x01 0x01 0x01
0x00 0x01 0x01 0x01 0x01 0x01 0x01 0x00
0x00 0x01 0x00 0x00 0x00 0x00 0x01 0x00
0x01 0x01 0x00 0x00 0x00 0x00 0x01 0x01
0x01 0x00 0x00 0x00 0x00 0x00 0x00 0x01
LanderArtAt:
#Reserve 0d2
#Include math.asm
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