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v86: defer V86-mode VBE disable; add DOS-box probe scenario (#359)

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Rebuilds libv86.js from felixrieseberg/v86@windows95-base, which now
carries vga-defer-vbe-disable-v86: when a windowed DOS VM's vgabios
writes dispi[4]=0, Win9x's VDD passes that through (it doesn't know
about ports 1CE/1CF) while virtualising the rest of the mode-set, so
v86 used to drop out of LFB rendering with the legacy registers still
holding SVGA values and the screen turned to planar garbage. The fix
defers the disable until a legacy attribute-mode write actually
reaches the hardware.

debug-harness: WIN95_PROBE_DOSBOX=1 opens command from Run, types
dir, optionally Alt+Enters (WIN95_PROBE_DOSBOX_ALTENTER=1).
WIN95_PROBE_VGATRACE=1 wraps the VGA io.ports[] entries (not the
VGAScreen methods, which are captured by-value at registration) and
dumps [port, op, value, eip+VM/PE/CPL] tuples to
/tmp/win95-vgatrace.json — that EIP/mode column is what pinned the
leak on V86-mode vgabios at C000:2C8x.
This commit is contained in:
Felix Rieseberg
2026-04-11 17:32:07 -07:00
committed by GitHub
parent 85c44513cb
commit 766497bd5d
4 changed files with 143 additions and 16 deletions
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8
.claude/skills/probe-win95/SKILL.md
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@@ -37,7 +37,13 @@ WIN95_SMB_SHARE="$HOME/Downloads" \
```
`WIN95_PROBE_SCRIPT='HOST/HOST'` types `\\HOST\HOST` into Start → Run on
desktop. `/` → `\` substitution (env var / shell quoting, pragmatism). The
desktop. `WIN95_PROBE_DOSBOX=1` instead opens `command`, types `dir`,
and (with `WIN95_PROBE_DOSBOX_ALTENTER=1`) toggles fullscreen — this is
the regression scenario for the windowed-DOS-box VBE leak.
`WIN95_PROBE_VGATRACE=1` wraps the VGA I/O ports at the `io.ports[]`
layer and writes `[port, op, value, "eip VMPE cplN"]` tuples to
`/tmp/win95-vgatrace.json` every tick (heavy — can hit 1M entries during
boot). `/` → `\` substitution (env var / shell quoting, pragmatism). The
harness drives it via XT scancodes — Win95 doesn't have Win+R (Win98+
only), so the sequence is Esc, Esc, Ctrl+Esc, R, backslashes + text,
Enter.

9
.claude/skills/update-v86/SKILL.md
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@@ -23,7 +23,7 @@ fallbacks, no fetching from copy.sh.
## The fork branch
v86 should be checked out on **`felixrieseberg/v86:windows95-base`**.
That branch merges three feature branches, each upstreamable on its own:
That branch merges four feature branches, each upstreamable on its own:
- **`electron-renderer-fs-loader`** (PR #1540) — `src/lib.js` uses
`require("fs")` instead of `await import("node:fs/promises")`. Dynamic
@@ -40,6 +40,13 @@ That branch merges three feature branches, each upstreamable on its own:
(VBADOS VBMOUSE) can read absolute cursor position and track the host
cursor 1:1 without pointer lock. Consumes the `mouse-absolute` bus
event that `MouseAdapter` already emits.
- **`vga-defer-vbe-disable-v86`** — `src/vga.js` defers `dispi[4]=0`
written from V86 mode until a legacy attribute-mode write reaches the
hardware. Win9x's VDD virtualises ports 3B0–3DF for a windowed DOS VM
but not 1CE/1CF, so vgabios's VBE-disable leaks through while the rest
of its mode-set is captured into the VM's virtual register file —
without this the screen turns to planar garbage the moment you open a
DOS box.
## Prerequisites