msx-vdp-audit

name	msx-vdp-audit
description	Validate MSX video emulation for the TMS9918A/TMS9928A/TMS9929A VDP family in EMULATORS, including ports

MSX VDP Audit

Core Workflow

Use this skill to audit whether EMULATORS models the MSX1 VDP family as a port-driven video chip with separate VRAM and TMS9918A-compatible side effects.

1. Read references/vdp.md before making claims or edits.
2. Inspect the relevant implementation paths first:
   • include/MSX/VDP.hpp, src/MSX/VDP.cpp
   • include/MSX/Screen.hpp, src/MSX/Screen.cpp
   • include/MSX/PortManager.hpp, src/MSX/PortManager.cpp
   • include/MSX/Commands.hpp, src/MSX/Commands.cpp
   • include/TEXASINSTRUMENTS/TMS99xxFamily/TMS99xxFamily.hpp, src/TEXASINSTRUMENTS/TMS99xxFamily/TMS99xxFamily.cpp
   • include/TEXASINSTRUMENTS/TMS99xxFamily/TMS99xxFamilyRegisters.hpp, src/TEXASINSTRUMENTS/TMS99xxFamily/TMS99xxFamilyRegisters.cpp
3. When changing C++ code, also use $emulators-framework-cpp-style.
4. Confirm the intended chip/model first. TMS9918A/TMS9928A/TMS9929A behavior is not the same as V9938/V9958 behavior.
5. Treat port protocol, register side effects, VRAM table addressing, and status reads as correctness requirements, not renderer details.

Audit Priorities

Prioritize correctness in this order:

• Port #98/#99 protocol, including two-byte control writes, address latch, register writes, status reads, and VRAM auto-increment.
• Status register flags and clear-on-read behavior for VBlank interrupt, fifth-sprite overflow, and sprite collision.
• Register R0-R7 write masks, read/write exposure, and mode-bit combinations.
• Pattern name, color, pattern generator, sprite attribute, and sprite pattern table base calculations, including Graphics II masking quirks.
• Text, Graphics I, Graphics II, and Multicolor mode rendering rules.
• Sprite evaluation: Y coordinate offset, terminator, size/magnification, priority, 4 sprites per scanline, overflow index, collision flag.
• PAL/NTSC frame timing and VDP INT wiring to the Z80/BIOS.

Validation Hints

Useful local anchors:

• emulators/MSXPrograms/KEY/GRAPHICS1.key
• emulators/MSXPrograms/KEY/SPRITES1.key
• emulators/MSXEmulator*/MSX.fmt
• emulators/MSXEmulatorC/TexasInstruments.hlp
• VDP monitor commands in src/MSX/Commands.cpp

For visual bugs, reduce to a small VRAM/register setup before comparing whole games. For port bugs, write Z80 tests that touch #98/#99 directly and also through BIOS routines such as WRTVDP, RDVRM, WRTVRM, SETRD, and SETWRT.