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Step 3 of the 9P chain (docs/9p.typ 12.3, docs/registry.typ 9P-8). Nothing here
is about 9P: it lands in the FUSE-served tree and any later transport inherits it.
A script could write into a terminal that already existed and read its rendered
scrollback. It could not START one, RESIZE one or SIGNAL one. Two of those were
already effects the core emits, so `exec` and `winsize` are existing
capabilities acquiring a name; only `sig` is new, and it brings the one new
host method, `push_pty_signal`.
pty/ctl winsize <cols> <rows> | sig INT|TERM|HUP|QUIT|KILL | exec
one verb per line, validate-all then apply-all, EINVAL applies
nothing -- `writeCtl`'s shape and `writeCtl`'s reason
pty/status cols, rows, tty-taken as three %11d fields
pty/data write is input to the process; read is the RAW output stream,
gated on a reader count so a pane nobody reads costs one branch
A pane that is not a terminal has no pty/ at all: the lookup is ENOENT and
readdir does not list it.
`PaneFile` is an enum(u4) and this takes it from 11 values to 15. ONE REMAINS.
That is also why pty/ is a DIRECTORY and not three more flat names -- a
subdirectory costs one value and buys its own namespace, so `ctl` and `data`
did not have to be renamed.
Two things the core does not know, and which are therefore not invented: a
child's EXIT STATUS (a shell's death is `Event.eof`, which removes the pane,
so there is no directory left to read it in) and RAW/COOKED (the core never
sets a termios; the mode belongs to the program on the far side).
Verified live against a daemon: pty/ appears only on the terminal pane; a
`winsize 0 24` and a `sig SIGINT` are refused; a bad verb beside a good one
applies neither; `echo pty-works` written to pty/data runs in the shell and its
output reaches the body; and a blocking read of pty/data returns the raw stream,
OSC 133 marks and all. fs-bench unchanged and still zero allocations.
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`-Dplatform=p4 -Dtarget=riscv32-freestanding` emits a single freestanding OBJECT
exporting a seven-function C ABI, not an executable. The board's toolchain
(../05-zig-p4) owns `_start`, the linker script and the UART driver and links this
in. The seam is bytes rather than types, so neither side can accidentally depend
on the other's internals, and a signature that drifts fails at link time.
The serial line is the whole of the I/O. `src/p4.zig` drives vaxis unchanged over
it: the renderer is a byte writer and `queryTerminalSend` is a byte writer, so the
terminal emulator on the host answers the capability handshake and the firmware
sees a real terminal. Measured going out over the wire on attach: alt screen,
in-band resize, cursor report, kitty keyboard, kitty graphics, DA1.
THREE WORDS EXIST ONLY HERE. `src/board_memory.zig` implements `Peek`, `Poke` and
`Hexdump`, gated on `builtin.os.tag == .freestanding and !isWasm()` - derived from
the TARGET, because they are a property of running with no OS under you rather
than a product option, and because wasm is freestanding too and is exactly what
must be excluded: in a browser an address is an offset into the linear memory this
editor's own heap lives in. Every access goes through `*allowzero volatile`: a
peripheral register is not memory, and address 0 is an ordinary unmapped address
on this bus. One 4 KiB cap per command, set by the console rather than the memory -
an unbounded dump would wedge the only console the board has for eleven hours.
Measured on ESP32-P4 rev v1.3 silicon, driven from a host terminal:
Peek 0x501101a4 0x0e63ce71, then 0xaeaa6919 on a second read - the
RNG register, so the volatile loads are not folded
Poke 0x5011002c 0xdeadbeef LP_STORE0; a later Peek returned 0xdeadbeef
Hexdump 0x5011002c 32 16 bytes a row, hex columns and an ASCII gutter
Peek 0x50110001 `peek: MisalignedAddress` on the message row
That last line is the one that matters. A misaligned 32-bit access traps, and a
trap in firmware is a watchdog reset that takes the session with it, so the check
that turns it into a message is the reason the file is hand-written rather than a
generic reader.
BARE METAL BOOTS AN EMPTY OUTPUT BUFFER. Every other boot layout in `init` makes a
shell, and on this platform that is not a preference but an impossibility: nothing
to fork, no pty to give a terminal pane. Booting one anyway produced precisely what
that describes - a pane whose tag ends in `Filter`, no gutter, no buffer, and every
keystroke vanishing into the Fallback's silent pty. An output buffer is also what
the platform's own words want, since Peek, Poke and Hexdump each fill one.
Sized for the board rather than for a desktop:
* `allocators.zig` gains a p4 tier that is ALL fallback - every capacity is zero,
so each arena spills immediately to the 384 KiB heap the firmware hands over,
and no megabyte-shaped static reservation lands in `.bss`.
* `source_manifest.zig`'s allowlist is EMPTY on p4. The table is ~0.95 MiB of
rodata against a 1.5 MiB flash partition; the firmware's filesystem is the
serial host's, through the Host vtable.
* The grid is clamped and the clamp is measured, not guessed: every cell is paid
for four times (vaxis Screen + InternalScreen, pardes Surface + previous_cells),
so 40x12 fits and 80x24 exhausts the heap during `Pardes.init`.
* `Vaxis.resize` deinits both screens before allocating replacements, so a failed
resize leaves vaxis rendering nothing. The p4 shell keeps the previous geometry
on failure instead of leaving a half-applied one.
Also here: `output_pane_integration_test.zig` had an exhaustive switch over
`Platform` that adding `.p4` left unhandled, which broke `zig build unit-test`
outright - the native test binary is the one consumer no platform build compiles.
346 tests pass again.
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