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SplitBit-Emulator/Programs/CosmOS/Source/text.asm
T
Anachronaut 9c144469b4 Take the SplitLint findings that are one operation, leave the rest 
Twenty four more sites, and the interesting part is which ones were left
alone. A rule emerged while reading them and it held all the way through:
apply where the repetition is INSIDE one operation, skip where the author's
own structure says it is a new thought, and never where two equal values
mean different things.

Taken:

- Five registers reassigned to a value they already held, where both are
  the same quantity: two masks in one expression in Snake, two spaces
  printed by the monitor, both halves of block zero in waitTest, and a RSTA
  in Pour that the very next instruction overwrote.
- Eighteen SETDs that reload a pointer inside one operation - a store back
  into the variable just read, or an INCD stepping to the second byte of a
  two byte value. Those read correctly without the reload.
- sbfsNext, which branched to the label on the line below it.

Left, with reasons that are the useful part of this:

- Eight registers where the same number means two different things. CosmOS
  and the loader set A to 1 for a blit command and then to 1 again for a
  bank number; Asm compares a type against 3 and then a status against 3.
  Removing those couples one quantity to another that is equal by accident
  and would part company silently.
- Ten RSTAs that open the RSTA/RSTB/CCF/ADD "return zero" block. The
  redundancy is what makes that idiom self contained; taking it out makes
  the return value depend on the line above.
- Eleven SETDs that begin an arm of a comparison chain. Each arm loads,
  compares and branches, and they get reordered - the repetition is the
  reason a new arm can be dropped in anywhere.
- Twenty five SETDs separated from their pointer by a blank line or a
  comment, which is the author saying a new thought starts here.
- Two CCFs before arithmetic, which this codebase writes unconditionally.
- Three redundant branches in test programs whose recorded output includes
  addresses, where three fewer bytes moves what the test demonstrates.

Nine recorded outputs moved and every one is a size in a listing or, for
Life, five more generations inside the same cycle budget. Behaviour is
unchanged everywhere: cosmosSnake and cosmosEdit pass byte for byte while
Snake loses eight bytes and Edit twelve.

CosmOS is 10,902 bytes of program against 10,937, and the native assembler
12,173 against 12,183. The CosmOS README's size for Edit moved twice in one
sitting, and this morning's check caught it both times - which it could not
have done before that claim was reworded to name what it was about.
2026-08-26 18:02:35 -04:00

305 lines
6.5 KiB
NASM
Raw Blame History

; text.asm
; Picking a line of typing apart.
;
; A shell reads a line and has to decide what was asked for. That is two jobs: cutting the
; first word off the line, and telling whether a word is the one being looked for. There
; is nothing else here, because there is nothing else a command line needs yet.
;
; Written by Anachronaut
#Program
; DP0 names a line ending in a zero byte. Cuts the first word off it, in place, by writing
; a zero byte over the space that ends the word. DP0 is unchanged, because a RET puts it
; back, so afterwards DP0 names just the first word.
;
; Where the rest of the line begins goes in TextRest, with any spaces between skipped. A
; line with only one word on it leaves TextRest naming that line's zero byte, which reads
; as an empty argument rather than as a missing one, and is the same thing here.
textSplit:
LDA.0
BRA textSplitHere ; The line ended, so the whole of it was one word.
INIB 0x20
CCF
SUB
BRQ textSplitCut
INCD.0
BRI textSplit
textSplitCut:
RSTA
STA.0 ; The space becomes the end of the word.
INCD.0
textSplitSkip:
LDA.0
BRA textSplitHere
INIB 0x20
CCF
SUB
BNQ textSplitHere ; Something that is not a space: the rest starts here.
INCD.0
BRI textSplitSkip
textSplitHere:
SETD.1 TextRest
STD.0.1
RET
; DP0 and DP1 name strings ending in zero bytes. Q is zero if they are the same.
;
; The two ending together is what makes them the same. Comparing until one of them ends
; would call "dir" and "dirty" the same word, which is the kind of thing a shell gets
; wrong once and confusingly.
textSame:
LDA.0
LDB.1
CCF
SUB
BNQ textDiffer
LDA.0
BRA textAlike ; Equal, and both of them zero: they ended together.
INCD.0
INCD.1
BRI textSame
textAlike:
RSTA
RSTB
CCF
ADD ; Q is zero: the same.
RET
textDiffer:
RSTA
INIB 0d1
CCF
ADD ; Q is one: not the same.
RET
; DP0 names text. Reads hexadecimal digits off the front of it into TextValue, most
; significant byte first. Q is zero if there was at least one digit to read.
;
; Digits past the fourth push the earlier ones off the top rather than being refused,
; which is what typing over an address does on every monitor there has ever been.
textHexWord:
RSTA
SETD.1 TextValue
STA.1
INCD.1
STA.1
SETD.1 TextDigits
STA.1
textHexLoop:
LDA.0
CALL textHexDigit
MVQA
INIB 0xFF
CCF
SUB
BRQ textHexEnd ; Not a digit, so the number stopped before it.
CALL textHexShift
SETD.1 TextDigits
LDA.1
INCA
STA.1
INCD.0
BRI textHexLoop
textHexEnd:
SETD.1 TextDigits
LDA.1
BRA textHexNothing
RSTA
RSTB
CCF
ADD ; Q is zero: there was a number.
RET
textHexNothing:
RSTA
INIB 0d1
CCF
ADD ; Q is one: there was not.
RET
; A holds the digit just read. Moves TextValue up by one place and puts the digit in the
; hole that leaves.
;
; A and B are a circular shift register sixteen bits long, so rotating them left four
; times multiplies the pair by sixteen. What fell off the top of the high byte comes round
; into the bottom of the low one, which is exactly the nybble the new digit wants, so it
; is masked away first.
textHexShift:
SETD.1 TextDigit
STA.1
SETD.0 TextValue
LDA.0
INCD.0
LDB.0
SHL SHL SHL SHL
SETD.0 TextValue
STA.0 ; The high byte is finished.
PSHB
POPA
INIB 0xF0
AND
MVQA
SETD.1 TextDigit
LDB.1
OR
SETD.0 TextValue
INCD.0
STQ.0
RET
; A holds a character. Q is what it is worth as a hexadecimal digit, or 0xFF if it is not
; one. Upper and lower case both count, because nobody wants to be told which they meant.
;
; Everything below works from the distance above '0', which is why the letters are tested
; at seventeen and thirty two rather than at anything recognisable.
textHexDigit:
INIB 0x30
CCF
SUB
BRC textHexNo ; Below '0'.
MVQA
INIB 0d10
CCF
SUB
BNC textHexUpper ; Ten or more above '0', so not 0 to 9.
RSTB
CCF
ADD ; Q is the digit itself.
RET
textHexUpper:
INIB 0d17
CCF
SUB
BRC textHexNo ; Between '9' and 'A'.
MVQA
INIB 0d6
CCF
SUB
BNC textHexLower ; Past 'F'.
INIB 0d10
CCF
ADD
RET
textHexLower:
INIB 0d32
CCF
SUB
BRC textHexNo ; Between 'F' and 'a'.
MVQA
INIB 0d6
CCF
SUB
BNC textHexNo ; Past 'f'.
INIB 0d10
CCF
ADD
RET
textHexNo:
RSTA
INIB 0xFF
CCF
ADD
RET
; ---- A number written in decimal ----
;
; DP0 names it. Q is the value, and TextDigits says how many digits were read, which is
; zero when there was no number there at all. Stops at the first thing that is not a digit.
;
; Decimal rather than hex, and one byte rather than two, because this is for the numbers a
; person types at a program: a line number, a count, a how many. Nobody counts lines in
; hex, and nobody types a line number above 255 on a machine this size. textHexWord is
; still the one for an address, where hex is what everybody means.
;
; Ten times the running total is worked out as eight of it plus two of it, because nothing
; on this machine multiplies. Anything past 255 wraps, which is what the same sum does
; everywhere else here.
textNumber:
RSTA
SETD.1 TextValue
STA.1
SETD.1 TextDigits
STA.1
textNumberLoop:
LDA.0
BRA textNumberDone
; Below '0' or above '9' ends it.
INIB 0d48
CCF
SUB
BRC textNumberDone ; It borrowed, so the character was below '0'.
MVQA
INIB 0d10
CCF
SUB
BNC textNumberDone ; It did not borrow, so it was ten or more past '0'.
PSHA ; The digit, while the total is multiplied.
SETD.1 TextValue
LDA.1
LDB.1
CCF
ADD ; Twice.
MVQA
MVQB
PSHA ; Twice, kept: ten is eight and two.
CCF
ADD ; Four times.
MVQA
MVQB
CCF
ADD ; Eight times.
MVQA
POPB
CCF
ADD ; Ten times.
MVQA
POPB
CCF
ADD ; And the digit.
STQ.1
SETD.1 TextDigits
LDA.1
INCA
STA.1
INCD.0
BRI textNumberLoop
textNumberDone:
SETD.1 TextValue
LDA.1
RSTB
CCF
ADD ; Q is the value, the way a routine hands a byte back.
RET
#Data
; Where the rest of the line begins, after textSplit has taken a word off the front.
TextRest:
0x00 0x00
; What textHexWord read, and what it needs while reading it.
TextValue:
0x00 0x00
TextDigits:
0x00
TextDigit:
0x00
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