VRAM board for Yamaha CX5MII

Funny thing is that when I try to vpoke to 4000h, I get "Illegal function call". So I suspect that the RAM size may be hardcoded in BIOS.

MSX Basic 1.0 and the MSX1 BIOS doesn’t know how to access > 16K (the upper three bits need to be specified in register 14). Nevertheless, strange that TESTRAM can’t find it, one would imagine it bypasses the BIOS and checks directly.

There must be some software which uses screen 5 by accessing the VDP directly and without checking the MSX version, I just don’t know which. I know how to verify but it involves writing some assembly code… I don’t have a screen 5 project ready to run.

From Basic you can try to select screen 5 manually and cycle through the 4 pages:

10 VDP(0)=VDP(0) AND &HF1 OR &H6
20 VDP(1)=VDP(1) AND &HE7
30 VDP(2)=&H1F + 0 * 32
40 I$=INPUT$(1)
50 VDP(2)=&H1F + 0 * 32
60 I$=INPUT$(1)
70 VDP(2)=&H1F + 0 * 32
80 I$=INPUT$(1)
90 VDP(2)=&H1F + 0 * 32
100 I$=INPUT$(1)
110 SCREEN 0

You can compare the results with 16K and 128K VRAM… On a CX5MII with 16K VRAM, I see the same picture on all 4 pages, and the bottom 1/3rd of the screen shows uninitialised data which changes every frame. If the VRAM expansion works I would expect a solid colour, probably either white or black, and a different picture for the second page.

But yeah, no way to fill the VRAM above 16K from MSX Basic 1.0.

The first screenshot is what I got on my CX5MII (16K) too, and the pattern at the bottom is moving.

You mean, no changes between 16K and 64K right? That’s strange… I can’t explain it. I was expecting the same result as for 128K on pages 0-1.

I notice the uninitialised memory in the 16K is white whereas the 64K and 128K have horizontal lines, I bet that is due to different initialisation state of the memory brand. For the 128K, it looks like it works, indeed.

I should really order those components and build it as well…

Hello, Alexey,

Do you actually use MSX1 or MSX2 bios in your YIS503/II? You can quickly upgrade to MSX2 by simply putting the MSX2 extension in one of the main slots temporarily and replacing the main 32K ROM.

Have you tried some other DRAM types, slower, than the one you have on the photo?

@Manuel: I use the original MSX1 BIOS and I doubt it's possible to put MSX2 BIOS to cartridge and boot from it without deactivating the EEPROM with the original MSX1 BIOS

@Grauw: I noticed the typo myself and tried the modifed program with different memory configurations. The pages are not switched for some reason, there's always the same picture. Funny thing is that if you stop the program, you can see the listing of it in screen 5 in pixel form.

@Maxis: I tried DRAM from OKI with 100ns, same result. I don't have any slower DRAM. Also I don't want to modify my MSX1, I just want to test whether the RAM expansion works. Seems that the computer sees the memory, but can't use it due to whatever restrictions. I hope Grauw upgrades his CX5MII and can properly test the module.

Thanks for your feedback. Any other ideas are welcome.