8088 clone BIOS swap, and writing 27C512 instead of 27C256

[mR_Slug]

I've got this XT Golden Clone board, esactly the same as this one:
http://minuszerodegrees.net/xt_clone_bios/X'Golden Turbo-Board of revision 1.jpg
(large flie 10MB)

There was a physically broken chip on it at U82 (HD74S74P) that I've replaced with a DM74S74N.

I went to write a BIOS image (Turbo_XT_3.10_2764.bin) from here:
http://minuszerodegrees.net/xt_clone_bios/xt_clone_bios.htm

And It wont boot, so I tried the supersoft ROMs, and then I realised I only have 27C512 ROMs.

Is there a way to use 27C512 in place of a 27C256? Alternativly, If I pulled the BIOS out of a 64-256K
IBM XT board, is it just U18 I need?

 

[eeguru]

Technically it should work as long as Pin 1/VPP is not floating and grounded. To be safe though, double the 32KB image up in the 27C512 image so that it occupies both the lower and upper half. That way Pin 1 which is A15 on a 27C512 becomes a don't care pin.

In general you can always use a larger JEDEC size as long as you replicate the image up to it's capacity so upper address lines are DNCs.

 

[mR_Slug]

Thanks. It worked SS diagnostics came up immediately. Everything plugged into the board passed.

So I went to try the "Turbo_XT_3.10_2764.bin" and the "SID BIOS version v4.1.zip". These are both 8KB, The SS was 32KB. I did the same thing, copy the ROM so it starts at 0000h and 8000h. Didn't work with either. Do I have to copy the 8KB ROM to 0000h, 2000h, 4000h, 8000h.....

Does every time you half the size, another next highest Address line need to be a DNC? So for a 64KB, ROM:
32KB: A15 DNC
16KB: A15, A14 DNC
08KB: A15, A14, A13, DNC

 

[eeguru]

Yes, you have to 'tile' up the smaller image so it occupies every smaller image block size. So if you have an 8K image going into a 27C256, just cat 4 copies of it together and burn it.

 

[mR_Slug]

IT LIVES!

Thanks.

I've been looking through the various BIOS images. There doesn't seem to be a common way the images are terminated, like with an EOF. Which leads me to, how does the motherboard determine the difference between an 8KB copied twice, and a 16KB chip, where it would need an additional address line?

 

[eeguru]

Most systems with ROM sockets are fixed sized for things like system BIOS, ROM BASIC, etc. Obviously the PCB has to have address lines for the max size it supports. If it supports placing smaller parts in larger sockets, there needs to be a size indicator in software. For example BIOS Option ROMs have a length encoded in the 3rd byte (# of 512 byte blocks).

Glad you got things working