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; ina 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
BLOS 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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