v86: fix windowed DOS box clobbering VBE; 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.
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What
Rebuilds
libv86.jsfromfelixrieseberg/v86@windows95-base, which now carries thevga-defer-vbe-disable-v86fix.Why
Opening any MS-DOS program turned the screen into vertical planar-garbage stripes. Root cause: the windowed DOS VM's
INT 10hreflects to vgabios in V86 mode; vgabios's mode-set begins by writingdispi[4]=0. Win9x's VDD virtualises the standard VGA ports (3B0–3DF) for windowed VMs but passes 1CE/1CF straight through, so the VBE-disable reaches the hardware while the rest of the mode-set is captured into the VM's virtual register file. v86 then dropped out of LFB rendering with the legacy registers still holding the SVGA values.The v86 fix defers clearing
svga_enabledwhendispi[4]is cleared from V86 mode (EFLAGS.VMset) until a legacyattribute_modewrite actually reaches the hardware — which a real passthrough mode-set does on the very next register, but the windowed-VM leak never does.Also
debug-harness.ts: newWIN95_PROBE_DOSBOX=1scenario (opencommand, typedir, optionallyWIN95_PROBE_DOSBOX_ALTENTER=1to fullscreen) — this is the regression test.WIN95_PROBE_VGATRACE=1: opt-in VGA I/O tracer that wrapsio.ports[](not theVGAScreen.portXXX_writemethods, which are captured by-value at registration) and dumps[port, op, value, "eip VMPE cplN"]tuples to/tmp/win95-vgatrace.json. The EIP+CPU-mode column is what pinned the leak on V86-mode vgabios atC000:2C8x.Verified
diroutput visibletools/probe-boot.sh: 2/3 SUCCESS, 1 FAIL_VXDLINK (matches pre-fix baseline)Upstream PR for the v86 side will be opened separately from
felixrieseberg/v86:vga-defer-vbe-disable-v86.