The console's attribute has always meant something to the screen and
nothing to the serial line: its low nibble picks one of sixteen ink and
paper pairs, and only videoPutCell ever read it. So the fault screen's
red was red in the window and grey down the wire, and Examples/colours
printed " ordinary highlighted " with nothing to tell them apart.
It is said in ANSI now, on the same terms the cursor is said in: a
register write only marks it and the next character sends it, so setting
a scheme and printing nothing says nothing, and setting the same scheme
twice costs one sequence rather than two. Only the scheme nibble crosses
- the page bits say which tiles a cell draws from, which is a fact about
the screen's own art.
THE ORDER WAS ALREADY RIGHT, which is worth saying because it looks like
a borrowing and is not. Both sets enumerate a three-bit colour, red green
blue counted in binary: one is red in both, three is yellow in both, six
is cyan in both. The same arithmetic done twice, forty years apart. The
one place they differ is slot 0, and that difference is forced - this
screen is ink on black, so ink cannot be black, and slot 0 is grey where
ANSI's is black.
Every sequence begins with a reset, so going from bank 8 to bank 1 does
not write red on the grey paper bank 8 left behind.
Scheme 0 is a bare reset rather than grey on black, and a terminal is
assumed to start plain - so a machine that never asks for a colour says
nothing at all, and one that does put the terminal back on its way out.
Two ways out, because the two endings have different rules: stopping on
purpose goes through stdio, since atexit runs BEFORE the buffer is
flushed and a reset written to the file descriptor would arrive in front
of the text it is meant to follow. Dying on a signal writes the four
bytes directly and accepts that the buffer may be lost.
colourTest walks all sixteen and then halts WITH ONE STILL SET, so the
recording shows the reset after the halt line rather than before it.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
THE CURSOR DID NOT BLINK, and the reason is worth stating: it blinks on the machine's clock,
and the machine's clock had stopped. A console waiting on a key stops the CPU, so no cycles
passed, so the phase never moved - and the one moment somebody is looking at a cursor is the
moment they are being asked to type.
Waiting is now charged as IDLE CYCLES, which is what they were built for: a machine stopped
on a device is not using memory, the same distinction WAIT makes, arrived at from the other
direction. And the devices are told as it happens rather than when the instruction finally
finishes, because a display controller does not stop blinking because the processor is
waiting on a keyboard, any more than a disk stops turning.
A keyboard file can now say NOTHING happened. A zero is a byte no keyboard sends, so it is
free to mean "a moment went by with nobody typing" - which is the commonest thing behind a
window and the only thing a file otherwise could not express. That unlocked the whole waiting
path: three checks that the cursor is lit, then dark half a second later, then lit again,
which is what blinking is.
And Programs/Examples/colours.asm, because the palette had nowhere a newcomer could read it.
It prints the sixteen pairs, prints each one again turned inside out, and then CHANGES ONE by
writing three bytes into the palette - so the difference between using the colours a machine
wakes up with and choosing your own is visible in one program. Its header explains what a
cell is, what the attribute nibble does, why palette entries are four bytes rather than
three, and why video memory has to be reached through the controller.
The manual now says where the palette lives and points at it.
SplitLint found a redundant RSTA in the example, which was worth acting on rather than
suppressing: the zero was already in A from the mode write two lines up, and saying so in a
comment teaches that SETD does not touch A, which is a thing worth knowing.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
AnachronautandClaude Opus 5