80-Bus News

BIOS in EPROM

In an issue some time ago Richard Beal was discussing the considerable hassle of patching a new BIOS into the MOVCPM.COM file for relocating CP/M.

I have just finished my own custom BIOS (for a Nascom 2 and two 8″ Shugart drives) and have adopted another system for relocating CP/M which does not involve relocating the BIOS.

My BIOS sits in EPROMs; in a 56k system these start from E000 where it is entered by a reset jump. It contains its own routines for loading the CCP and BDOS from disk, to the desired place in RAM. A ‘56k’ CCP & BDOS would be loaded near the top of RAM, whereas a ‘32k’ CCP and BDOS would be loaded about half-way up. Having done this, the BIOS then initialises the jump table just after the BDOS and transfers control to the CCP. This has the advantages that the BIOS can be in EPROMs since it is always in the same place, one can use anybody’s system disk to run in your system, regardless of which BIOS is on the disk, there is no restriction on the size of the bootstrap loader because there isn’t one (normally it has to fit on the first sector of the disk and has to be short) and full disk read error recovery routines can be implemented in it. One therefore has a better chance of loading the system tracks off a slightly suspect disk; this is important since without disks one cannot do anything except some trivial debugging using SIMON, or reverting the whole Nascom back to NAS-SYS3 and ZEAP and writing some test routines to find what is wrong. I have found no need for SIMON and a 6k BIOS size limit is handy if you want to modify CONOUT to drive one of the flashy graphics/alphanumeric cards; some need a lot of software for writing text.

I have used the standard MOVCPM to produce a ‘56k’ system (with the MDS-800 BIOS on it which is not used) and my BIOS starts at E000. It can extend up to F800, a total of 6k which is enough for most requirements. Even then there is a big gap for stack/data and with a slightly smaller BIOS (same size as Mr. Beal’s I think) a 60k system can be configured. With non-Nascom hardware a 64k system is possible, using the IVC card.

The only disadvantages I can think of is that EPROMs are bit more difficult to patch that the MOVCPM is when in RAM.

I feel that people who implement CP/M on their own hardware will be interested in this approach, since they will simply buy a CP/M 2.2 disk in their chosen format, configure it for the biggest RAM they can get and are not trying to produce a licensed and commercially saleable system.

Incidentally, I have been told by Digital Research that the [v] option should be ‘avoided’ when copying large files. It does indeed produce spurious ‘Verify Error’ messages and aborts, but my disk read/write routines do not report any errors. The appearance of this problem is consistent for a given file. Does anyone have any clues why this should be ?

Yours sincerely, P. Holy, Worthing.

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