- Joined: 28 Sep 1999
- Posts: 1197
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VDP I/O stuff
 Posted: Sat Nov 04, 2000 4:18 am
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I did some experiments doing VDP I/O on the Genesis in SMS mode, and here's what I found. (Some of this may only apply to the Genesis, but it's interesting anyways)
- When you write to a VDP register, all following writes go to CRAM, all following reads come from VRAM, and the address register is set up too. For instance, sending a command of $8710 will set the address register to $0710, as well as setting VDP register $07 to $10.
Note that for this reason, Ax Battler does not run correctly on the Genesis. It sends a VRAM write command with bit 15 set, to update the sprite table at 3F00-3FFF. On my machine, no sprites are shown and the palette is changed quite a lot (since that's where all the writes end up going to). I don't know if the SMS supports this behavior, or if it acts like the GG.
- Reading the data port always returns VRAM. So you can't read CRAM, and you'll get valid VRAM data back even after sending a VRAM write, CRAM write, or VDP register set command.
- Writing to the data port does not load the read buffer. In fact, resetting the machine does not affect the read buffer. It is only loaded when you read the data port or send a read command. (the 'default' value on power-up is usually 00 or FF) Meka (ver. 0.53) allows writes to affect the buffer, but that might be fixed by now.
- CRAM and VRAM share the same address. Not terribly exciting, I confirmed this by writing to CRAM several times, then switching to VRAM reads, and the address was modified appropriately. I think some emulators use two addresses (Massage, Master Gear) which is wrong...
If anyone can think of games that disagree with my findings, please let me know so I can look at them.
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