tty: six virtual terminals, and the plumbing that makes them terminals

/dev/tty1..tty6, Alt+F1..F6 or `chvt N`. Each is an independent terminal --
its own screen, cursor, line discipline, foreground process group and
session. Two shells on two terminals are unrelated processes that share a
keyboard and a screen.

What made it possible is a backing store. The console drew straight into the
framebuffer and scrolled by moving pixels, which works exactly as long as
nothing else wants the screen: there was nowhere to put terminal 2 while
terminal 1 was visible, and nothing to repaint terminal 1 from. Each terminal
owns a grid of cells now and the framebuffer shows whichever grid is active,
so vt_switch() is set-index, repaint, return -- nothing is saved on the way
out because the outgoing grid has been current all along.

The rule the layer is built on: a terminal does not know whether it is
visible. Writes land in the grid, the cursor moves, the grid scrolls, all
unconditionally; drawing is one question asked at the bottom of vt.c. Every
alternative spreads "am I on screen" through the escape parser, and then a
terminal switched away from mid-sequence comes back wrong.

Everything that was a single global is per-terminal now: tty_fg_pgrp (deleted),
termios, the input ring, the escape parser, the UTF-8 decoder, and the cursor
-- which also stops being pixels in gfx.c and becomes cells in the terminal.
TIOCSCTTY was `return 0` and now records proc->ctty, which is what /dev/tty
resolves through; /dev/tty was an alias for /dev/console with the same minor,
and they are different questions.

kbd.c names Alt+F1..F6 as KEY_ALT_F1+n, above 0xFF on purpose -- every byte
value is already a character or a UTF-8 continuation, and a chord colliding
with one would be typed into the terminal instead of switching away from it.
What the chord means is decided in console.c, where the terminals are.

VT_ACTIVATE / VT_WAITACTIVE / VT_GETSTATE / VT_OPENQRY, Linux's numbers.
The argument to ACTIVATE is by value, not a pointer -- same shape as TCFLSH.

Two bugs found by screenshotting the result rather than reading the log:

  - zsh emits 0x0F to leave alt-charset mode, because that is what the `linux`
    terminfo says rmacs is, and the console drew CP437 glyph 0x0F: a sun, in
    the prompt, on every line. A terminal never draws a C0 control character.
  - SGR 7 was unimplemented, so zsh's partial-line marker rendered as a bare
    '#' instead of a reversed block. Reverse video swaps the colours at draw
    time, not at store time, so turning it off restores what was underneath.

Verified by screendump: tty1 and tty2 show different screens, and switching
back repaints tty1's earlier content from its grid. schfs-check passes.

docs/architecture/terminals.md, including what it does not do -- no
scrollback, no reflow, no VT_PROCESS mode for a display server to veto a
switch, which Schcom will need before it owns Alt+F7.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

kernel/fs/devfs.c

@@ -72,7 +72,17 @@ static const struct devnode devnodes[] = {
   { "full",    1, 7,  D_ROOT },
   { "random",  1, 8,  D_ROOT },
   { "urandom", 1, 9,  D_ROOT },
-  { "tty",     1, 1,  D_ROOT },     /* alias for the console */
+  /* /dev/tty is the caller's *controlling* terminal, which is a different
+     thing from the console and used to be the same thing only because there
+     was one terminal. Minor 10 resolves through proc->ctty; see
+     vt_for_minor() in kernel/core/console.c. */
+  { "tty",     1, 10, D_ROOT },
+  { "tty1",    1, 11, D_ROOT },
+  { "tty2",    1, 12, D_ROOT },
+  { "tty3",    1, 13, D_ROOT },
+  { "tty4",    1, 14, D_ROOT },
+  { "tty5",    1, 15, D_ROOT },
+  { "tty6",    1, 16, D_ROOT },
   { "stdin",   1, 1,  D_ROOT },
   { "stdout",  1, 1,  D_ROOT },
   { "stderr",  1, 1,  D_ROOT },