Opened 5 years ago

Last modified 10 months ago

#30 new completeness

Sega CD Support

Reported by: mikepavone Owned by: mikepavone
Priority: major Milestone: 0.7.0
Component: Genesis/Megadrive Version: 0.5.0
Keywords: Cc:

Description

Add support for the Sega CD/Mega CD add-on.

Change History (3)

comment:1 Changed 22 months ago by mikepavone

  • Milestone changed from 0.6.0 to 0.8.0

comment:2 Changed 21 months ago by Sik

Selfish request ahead :v

Adding support for Sub-CPU, PRG-RAM and WORD-RAM as a starting point would make it possible to run mode 1 software using the Sub-CPU as a glorified coprocessor, even without MCD firmware support. I already have the test program that checks the former two (I can remove the MCD firmware header check if needed), I should probably make another one that also tries to use WORD-RAM.

Mode 1 could be automatically detected by looking for the letter C in the device support field of the ROM header (this is documented in the official docs, Wonder Library does this and krikzz already told me that the Mega Everdrive Pro will support it), the idea is to convince everybody to go with this approach in the future.

I think this first step would need the following (assuming I don't forget anything):

  • Sub-CPU
  • PRG-RAM
  • WORD-RAM
  • Communication ports and Sub-CPU IRQ2
  • The DMA latency issue with WORD-RAM

I have no idea how hard it is to throw in a third CPU even when it's another instance of the 68000 core. I suppose that running off its own master clock could be a problem (Mega CD's master clock is 50MHz, with the Sub-CPU running at CD:MCLK ÷ 4).

LEDs would be nice but not really required. The MCD firmware check in the test ROM can be by-passed by ensuring that "SEGA" shows up at $400100 (Main-CPU), the rest of the firmware is ignored.

comment:3 Changed 10 months ago by mikepavone

  • Milestone changed from 0.8.0 to 0.7.0
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