My XT adventure

[ruud]

Hallo allemaal,

Here in the Netherlands we have a saying: "Change of food makes one eat". So I turned my attention to the PC-XT and its compatibles. From 1985 (!) on I had to idea to "create" my own BIOS. The idea was to create one that wrote data to my debugger card so I could use it to repair brooken boards. But the project faded away for two reasons:
- The company bought the Logimer, a commercial product that did exactly the same. Advantage: it came with ROMs for the AT as well.
- More interesting computers like the AT, 386 etc. showed up.

Of course I'm not creating a BIOS from scratch, I disassembled (more or less) various ones and took the best parts of each. Of course I had a look the the Commodore PCs but here I ran into a bummer: the PC-1 and PC10/20-III have some completely incompatible hardware including the keyboards. I checked this by exchanging the BIOS with the one of a real IBM PC-XT; both boards use 32 KB-EPROMs. In both cases the boards didn't accept neither the C= keyboard nor the IBM keyboard :(

The keyboards of the older PC's are IBM compatible. And I'm gratefull for that. Two days ago my original 27 year old IBM keyboard brooke down. I know I have some XT-KB's laying around but where exactly ??? Then I found these older PC10 ones and, huray, they worked :)

I still work on a XT compatible I assembled myself in 1989 for a brewery and given back some years ago. Its 20 MB still works fine but sometimes I get write errors. But MFM-drives are very rare. This triggered the idea to use an IDE drive. I already found out I can use a 16-bits FDD/HDD/Multi-IO card from a AT/386/486 !!! The only thing that doesn't completely work is the 16-bits hard disk interface: only 8-bits work (quite obvious). But that shouldn't be problem, IDE drives work fine with 8 bits as well. Disadvantage: you loose half of the drives capacity. But I'm happy with only 10 of the 20 GB as well. Problem: I have to write my own driver for this type of drive.

DOS 3.3 only supports 32 MB partitions. To make a good use of the hard disk, DOS 5 or higher is needed. But they "eat" more memory. No problem for a 386+ machine with their UMB memory. So I decided to add another 64 KB memory to the board in the 0Dxxxxh area using an original IBM memory expansion kit. I used the same trick with an AT in the beginning of 90's: a 386 was still to expensive and my 16 MHz 286 + co-processor was fast enough. And AFAIK I still have the original drivers :)

So far my XT advanture....

A question: does anyone know a newsgroup or forum daeling with old PCs? TIA!