Project for botting code from expansion port

[patroclus02]

I've been working on a rewritable ROM board that connects to expansion port to boot code from there at $000000 when CART isn't grounded, of course.

I finally finisehd it, but it doesn't work. I connected two flash roms in parallel (8 data bits from one, 8 bits from the other), using the same adreess bus and strobes. I use /CAS_0 for /OE and /ROM for /CE.

I went through all joints twice.. I see no errors.

I tested a rom made in C (64Kb size), that has the starting code, as my Genesis has BIOS that checks that thing. Nothing either.

It really confuses me, why a ROM at $000000 does not boot from expansion port when /CART isn't grounded... I suppose it should work for the info I have and read here. All address lines are held high (I think, as I only used a multimeter), so the strobes. It's like freeze.

Any ideas?

 

[ExCyber]

I thought there might be an enable (analagous to /CART) on the expansion port, but /ROM and /ASEL look like outputs judging from what they're grouped with on the schematic. The only thing that looks like an input is /DISK, which is routed to the VDP alongside the /JAP and NTSC signals, and probably at a minimum sets a corresponding bit in the version register. The name doesn't make a lot of sense for enabling ROM mapping given that there seem to be dedicated strobes for talking to an FDC, but I guess you might want to give it a shot (though I'd pull it down with a resistor just to be safe).

 

[patroclus02]

Well, I took a few captures with a logic analyzer, and it actually does a few ROM acces (/ROM and /CAS_0 asserted), and they seem to be at low addresses like 0,1,2,3,1,2,3,4,5,6..

After a few reads, and some more out of ROM area, all lines go high and Genesis freezess... strange

 

[ExCyber]

Are you sure your TMSS code is correct? In particular, does it check the version register and do the "SEGA" write?

 

[patroclus02]

I just tested a 256 Kb commercial game. It works. Strange.

I say strange, because I first test a demo coded in XGCC. So, as it uses the sega.s file, the initialization and TMSS code shpuld be correct.

Right?

 

[ExCyber]

The TMSS write code in sega.s looks right as far as I can tell. Does your compiled ROM have "SEGA" (I think " SEGA" works also) at offset 0x100?

 

[patroclus02]

Yes, SEGA at offset $100. I might try another demo compiled in XGCC, maybe the one I try just doesn't work?? In emulator runs well. Who knows.

 

[LocalH]

Feel free to test this ROM with your setup. I haven't personally tested it on a TMSS-capable Genesis, but it works flawlessly on my Genesis 1 without TMSS, and I used Sega's official TMSS-handling code.

 

[patroclus02]

Thank you, I will try see if it work. By the way, is it written in ASM?

 

[LocalH]

Yes, 100%. Other than the TMSS code, it's all mine (although my post-TMSS initialization is heavily based on that found in Sonic 1, but written by me).