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

Makefile

@@ -25,7 +25,7 @@ KERN_CORE = $(addprefix $(BUILD)/kernel/core/, \
 
 KERN_DEV  = $(addprefix $(BUILD)/kernel/dev/, \
               bio.o cdev.o device.o disk.o fb.o ide.o input.o irq.o kbd.o mouse.o \
-              pci.o text.o ttf.o uart.o gfx.o ramdisk.o procfs.o)
+              pci.o text.o ttf.o uart.o gfx.o vt.o ramdisk.o procfs.o)
 
 # Drivers for hardware that has to be *found* rather than assumed. Anything
 # discovered by walking a bus belongs here; the fixed-address legacy devices
@@ -107,7 +107,7 @@ KERN_NOENTRY = $(filter-out $(BUILD)/kernel/arch/x86_64/entry.o, $(KERNOBJS))
 # Third-party software does NOT go here — see ports/.
 # -----------------------------------------------------------------------
 ULIB    = $(addprefix $(BUILD)/lib/, start.o glue.o usys.o)
-CMDS    = init schinit fbinfo shmtest socktest inputtest nettest reflink schattr ping
+CMDS    = init schinit fbinfo shmtest socktest inputtest nettest reflink schattr ping chvt
 CMDBINS = $(addprefix $(BUILD)/cmd/, $(CMDS))
 
 # -----------------------------------------------------------------------