VRAM board for Yamaha CX5MII

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By sd_snatcher

Prophet (3510)

sd_snatcher의 아바타

23-08-2015, 16:20

@Alexey

Isn't the mainbios socketed in your machine?

The MSX1 BIOS is too restricted and hardcoded to 16KB of VRAM. It will be hard work to test 128KB of VRAM properly with this BIOS.

By Wierzbowsky

Guardian (3431)

Wierzbowsky의 아바타

23-08-2015, 16:44

sd_snatcher wrote:

@Alexey
Isn't the mainbios socketed in your machine?
The MSX1 BIOS is too restricted and hardcoded to 16KB of VRAM. It will be hard work to test 128KB of VRAM properly with this BIOS.

Unfortunately not socketed, otherwise I would be already running with MSX2 BIOS. And as I said, I am not going to alter my original MSX1. What I can do is to send the board to someone for testing for the cost of envelope + shipment (4 euro).

By sd_snatcher

Prophet (3510)

sd_snatcher의 아바타

23-08-2015, 16:50

I understand why you don't want to mod your machine.

If you want to, I might patch the NEOS MA-20 BIOS to use the internal I/O ports (98h~9BH) instead of the external I/O ports (88h~8Bh). This way you will be able to use an external BIOS for the tests, without having to do any mods to your machine.

You'll only have to build two ROM cartridges: one for the MainROM, and other for the subrom. You can easily modify existing game cartridges for that.

By Wierzbowsky

Guardian (3431)

Wierzbowsky의 아바타

23-08-2015, 17:04

I have a few 16-64kb cartridge boards where I can put the external ROMs. The problem is that this is a game cartridge so I am not sure it's suitable for system ROMs. And I am not sure whether I can boot without deactivating the original on-board MSX1 ROM. Any hints would be welcome.

By sd_snatcher

Prophet (3510)

sd_snatcher의 아바타

23-08-2015, 17:36

The NEOS MA-20 was designed exactly for that scenario: to boot the external BIOS without having to do any modification on the MSX1 motherboard. Not even deactivating the on-board MSX1 BIOS is required. It seems exactly what you need right now.

Quote:

The problem is that this is a game cartridge so I am not sure it's suitable for system ROMs.

Don't worry. The system ROMs behave exactly as the game cartridge ROMs. The MSX makes no differentiation between the two. The only requirement (as for games) is that the ROM must be mapped to the right address window.

To easily mod the cartridges, you'll have to:

1) Replace the 16KB or 32KB EPROM with a 128KB flash
2) The /CS (pin-20) of the EPROM will be connected to either the msx slot /CS1, /CS2 or /CS12 signals (this is meant to restrict the acces window to frame-1, frame-2 or frame1+2 respectively. Replace that connection to the /SLTSL pin. This will give the CPU access to the full 64KB window.
3) The pin A16 of the Flash must be grounded.
4) Be sure that the /OE (pin-22) of the EPROM is connected to the /RD pin of the MSX slot.

5) If you want to, you may connect the MSX slot /WR pin to the /WE of the flash, to be able to update it directly from the MSX. But unfortunately there's no such software for that yet. Meanwhile I suggest you to use a socket for the flash, so you can easily take it out and rewrite it on an EPROM burner.

If you use a slot expander in your setup (to have a free slot for a diskdrive, i.e.), please remember that the cartridge containing the MainBIOS of the MA-20 *must* sit in the slot x-0. This is a limit imposed by the MA-20 routines that still have to be fixed.

So, do you want me to patch the MA-20 ROMs and try it on your cartridge boards?

By Wierzbowsky

Guardian (3431)

Wierzbowsky의 아바타

23-08-2015, 17:46

Yes, we should try this. I have also a 512kb flash cartridge with Konami mapper designed by Eric. I could boot games that start from address 0000h as well as from 4000h from it. So I can try to do the modifications you mentioned. Thanks for your help!

By sd_snatcher

Prophet (3510)

sd_snatcher의 아바타

23-08-2015, 18:12

OK. I went out to lunch now, so give me an hour and I'll tackle into that. Smile

By sd_snatcher

Prophet (3510)

sd_snatcher의 아바타

23-08-2015, 21:08

Alexey, is at least the Cx5M subROM (aka "80 columns ROM") socketed in your machine?

Because that damn hack nonstandard ROM forces the BIOS back to the slot-0 when its present. Other than that, I just have to rip the RTC routines out.

Does anyone know a trick to easily disable the RTC routines?

By Wierzbowsky

Guardian (3431)

Wierzbowsky의 아바타

23-08-2015, 21:33

I am using Yamaha YIS503II Russian MSX1, not CX5MII. I've seen the patched ROM without RTC for that machine somewhere...

By sd_snatcher

Prophet (3510)

sd_snatcher의 아바타

23-08-2015, 21:47

I found an elegant workaround to disable the damn "80 columns ROM". Now I just need to disable the RTC routines.

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