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Mega Drive Quirks

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Revision as of 00:32, 28 December 2010 by GerbilSoft (talk | contribs)

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: <asm>clr.w $C00000</asm> 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 isn't supported by either the MD1 or MD2 hardware. However, the Majesco Genesis 3 does support TAS, which breaks a few games that incorrectly use the TAS instruction.

  • Emulators that do not implement TAS: Probably all.
  • Emulators that do implement TAS: Unknown
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