View Full Version : Xerox 820-II & big problem with **non-standard** bootdisks

nige the hippy
November 11th, 2006, 03:35 PM
Oh lordy I'm at it again.....

Picked up a Xerox 820-II for 1.99 (minus keyboard & drives) but so what, keyboard is passive, and drive connector on the back looks to be hook-uppable to. got it up and running, (PSU smoked, then memory faulty) but guess what, no CP/M disks, and guess what too... apparently the disks are non-standard:-

Single Density on the first track, then Double Density for the rest, and only 35 tracks from what I can find out

It's a repeat of the Sirius 1 thing! I had

Can anyone tell me whether it's true about the sd/dd thing, where i can get a disk/image from, and should I now just dump it in the back of my shed for the spiders to eat???

yours eternally hopelessly,

nige the hippy
November 11th, 2006, 03:51 PM

While I had the thing apart (and i also had the Eprom burner out after copying DongFeng's BIOS (Has it worked?)), I copied the roms, so if anyone needs them I have intel hex images.

I also have hex images of the Wren Executive's roms.

Any Idea where I should stick them?


Terry Yager
November 11th, 2006, 04:45 PM
There is no such thing as a 'standard' CP/M disk format, but the SD boottrack, DD disk thing was pretty common. Earlier versions (1.3, 1.4) didn't support double density, so the bootloader (before the BIOS is loaded from disk) had to be in a format understood by the system.


Terry Yager
November 11th, 2006, 05:23 PM
Gee, that's about as clear as mud. Lemme try again...

The IBM-PC and compatibles keep thier BIOS info stored in ROM which is within the OS's memory space, but in the CP/M world, the BIOS (including disk parameters) is stored on the disk, and must be read-in by a small program called the boot-loader. Most hardware manufacturers would place the loader into ROM, which was bank-switched out as soon as the loader began running. The loader just reads track zero, and begins executing the code found there, i.e. CP/M, including the custom BIOS (CBIOS). The OS would not be 'aware' of double-density until the 'disk parameter block' is found inside of CP/M (in the infamous 'page zero'), after the OS is running (CP/M assumes 128 bytes per sector)...

Hmmmn...this isn't getting any clearer, is it?

Majik sufficiently degraded is indistinguishable from technology (sorry, Mr. Clarke).


November 11th, 2006, 05:49 PM
No, that was clear. Not as bad as the IBM 1130, it booted from a single card. Card has 12 holes in each column and computer was a 16 bit word machine. So.... the first 1/3 of the card was spent patching the proper bits into later instructions to get them to work. Obviously, there was 4 bits missing for each instruction as it was read in. It was a self modifying program ... oooooo "I'll be bahck"

Terry Yager
November 11th, 2006, 06:47 PM
Yeah, but it still sounds kinda circular, which ain't A Bad Thing, if ya remember that 'boot' is short for 'bootstrap', which refers to 'pulling oneself up by one's own bootstraps'. The whole concept involves circular thinking...


nige the hippy
November 12th, 2006, 04:29 AM
Explaination is wonderfully clear....

I understand the thingy about the particular machine's bios being within the CP/M system, rather than on rom.

The prob i have is ... Even if I find a disk image of the boot disk, according to various authorities elsewhere on the web, it's difficult to mix densities on most pc 5.25 disk writing programs (true or not?)

I gather from you, that with the right bios, it might be possible to maintain single density throughout the disk. although that still doesn't help, cos I haven't found any system disk images, never mind tweaked ones.

Terry Yager
November 12th, 2006, 06:18 AM
True, some PC disk controllers do not support FM encoding, but many do, ya just hafta find the right one. As for a copying program, mebbe Mike B. can help. I'm not sure if his ImageDisk program can work with mixed densities or not.
I don't have any Xerox images, but they shouldn't be hard to find. Several members here have 820s, I'm sure.


December 8th, 2006, 01:21 PM
I have these disks, PM me if you're still looking so I can make a copy for you