Sega Mega Drive/Quirks

This is a list of several MD hardware quirks that don't exactly fit in any other categories.

CLR on VDP data port

If the VDP is set to VRAM/CRAM/VSRAM Write, using the clr instruction on the VDP data port will result in a system crash: clr.w $C00000 This is because the clr instruction is treated as a Read-Modify-Write instruction on the MC68000 and MC68008 CPUs. The CPU first attempts to read the value located at $C00000, but the VDP's in data write mode, so the system hangs. This was fixed in the MC68010 CPU, which does not read the address before writing 0.

• Emulators that implement this quirk: Unknown

TAS instruction support

The MC68000 has an instruction, TAS (test-and-set), that is used for synchronizing multiple MC68000s in a multi-CPU system. The TAS instruction uses a different bus cycle than other instructions, and is effectively treated as a no-op for memory writes by both the MD1 or MD2 hardware. However, the Majesco Genesis 3 does support TAS, which breaks a few games that incorrectly use the TAS instruction. (The TAS instruction ends up setting the high bit of the destination memory operand.)

• Emulators that implement TAS as no-op for memory access: Gens, Gens/GS, Gens/GS II, probably all others
• Emulators that implement TAS as write to memory: Unknown

Games that use the TAS instruction:

• Gargoyles by Buena Vista Interactive
• Ex-Mutants by Sega