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A ROM Mystery

falter

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Jan 22, 2011
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When I was working on my Mindsets, I zeroed in on bad ROMs as a possible suspect for why my Video Production System and Professional Video Graphics System wouldn't post properly. In dumping the ROMs of all three of my machines, including the working one, I determined that the contents of the LO ROMs of the VPS and PVGS matched the contents of the working machine's EPROM (note: the former two machines used 23128 ROMs and the working machine used 27128 EPROMs).

By burning the 1.07 HI EPROM contents to new 27128s and placing them in the afflicted machines, they posted correctly. I'm confident the VPS ROMs were just ROM versions of the 1.07 BIOS found in EPROM on the original Mindset. But I'm curious. could the PVGS have had a different HI ROM and still kept the LO one the same as the original machine? Would that be possible? It's just that the PVGS code, although in spots it looked similar with just wrong bits set, it wasn't consistently wrong the way the VPS ROM was. I only wonder because the PVGS was changed a bit from the other two machines - it had things like analog RGB output instead of digital. So I wondered if they tweaked the ROM at all and if running 1.07 is incorrect.

For example, on the correct ROM, from address 00000 the first three bytes are B8 00 28. On the 'bad' PVGS ROM, they are A8, 00, 08.

If I look at the difference between B8 and A8, a single bit is different:

B8 = 10111000
A8 = 10101000

Likewise with the third byte, which should be 28, but is 08 in the PVGS ROM:

28 = 00101000
08 = 00001000

If I go down to address 00040, it should be EB, but in the PVGS ROM is A3:

EB = 11101011
A3 = 10100011

It seems to me that basically certain bits that should be 1s are 0s. If I could see a clear pattern I'd feel more confident the ROM in question should resemble the 1.07 EPROM, but because the bits that aren't set to 1 are in different places from address to address, I'm not sure.

I've attached a screen cap of the first 10 or so addresses of each.
 

Attachments

  • mindsetbios107hi.png
    mindsetbios107hi.png
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  • mindset pvgs hi rom.png
    mindset pvgs hi rom.png
    67.6 KB · Views: 2
If the "LO" ROM was exactly the same on all machines, then it's highly unlikely the "HI" ROM was different. Because that would mean the only changes were all at odd addresses. The chances of that happening just coincidentally are very very low.
 
Remind me, has these machines got a 16-bit processor or an 8-bit processor?

If 8-bit, then it is possible. The two ROMs could contain different content.

If 16-bit (as Plasma has stated) this would be unlikely.

However, if only a small area is affected, the ROMs could contain tables of data and it is conceivable (but highly improbable) that only the High Byte could be affected.

Dave
 
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