Backspace, whatever the terminal calls it

CosmOS's line editor looks for 0x08, which is what Voyager's keyboard
sends. A POSIX terminal sends its own erase character instead, and on
most of them that is 0x7F.

It stayed hidden while the terminal was doing the editing: canonical
mode consumes the erase character itself and hands over a finished
line. Key mode turns ICANON off, which is the point of it, so from the
day the shell started editing its own line - 7360374 - Backspace worked
in Voyager and did nothing at all in the console-only emulator. The
Programming Manual already claimed the console normalised Backspace on a
terminal the way it normalises the arrow keys; the code did not.

The erase character is read from the terminal's own VERASE rather than
assumed to be 0x7F, because some terminals are set to 0x08 and a person
who has moved their erase key has said where it is. Forward Delete is
untouched and stays 0x86: two keys, two values.

Only when standard input really is a terminal. A file or a pipe holding
0x7F holds a byte somebody wrote, not a key somebody pressed.

Three checks in terminal.sh under a pseudo-terminal whose VERASE is set
on the slave side: erase at 0x7F arrives as Backspace, erase at 0x08
still does, and a 0x7F read from a file is left alone. The last one is
what fails on the obvious wrong fix of translating 0x7F unconditionally,
which was verified with break.sh along with the fix's removal.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

SplitBit Programming Manual.md

@@ -307,6 +307,8 @@ Backspace is 0x08 and always has been. It is a different key from Delete and doe
 
 **The console normalises, which is what it has always done.** Behind a window it turns the key somebody pressed into a byte; on a terminal it turns `ESC [ A` and its neighbours into the same byte. That is the same act it performs on Return and Backspace, and it is why a program does not have to know which of the two it is talking to. The translation happens only when there really is a terminal: a file or a pipe holds exactly the bytes somebody put in it, and a program reading one gets those bytes untouched - which is also how a test presses an arrow key.
 
+**Backspace is part of that, and the terminal says what it is.** A terminal has an erase character of its own, and while it is doing the editing it consumes that character itself and hands over a finished line. Key mode stops it editing, so from then on the byte arrives as it was sent - usually 0x7F, sometimes 0x08, and whatever the person at the keyboard has configured. The console asks the terminal which one it is rather than assuming, and delivers 0x08. Forward Delete stays 0x86: they are two keys and they stay two keys.
+
 **These arrive in key mode only.** Line mode delivers characters, and a program in line mode is being handed a line that something else has already finished editing, so a key meaning "move the cursor left" arrived too late to mean anything. The console drops them there. This is what a terminal does too: it has always given a program in line mode backspace and line kill, and has never given it arrow keys.
 
 **But asking the status port in line mode does not throw one away.** A key line mode will not deliver is not the same as a key that is gone, because the mode can change: a program that looks at the status port and then asks for key mode - which is exactly what a system does before it reads a line - would otherwise find that the first key it was reaching for had been swallowed by the looking. So the console holds it and delivers it as soon as something is willing to take it. Reading the data port in line mode does discard it, and must: that read is the delivery, and a key held there would be met again forever.