Re: Interesting BIOS offset problem
- From: Herb Johnson <herbrjohnson@xxxxxxxxx>
- Date: 11 May 2007 15:03:16 -0700
On May 10, 4:23 pm, no.s...@xxxxxxxxxxxxxxxxxxxxxxx wrote:
On Thu, 10 May 2007 17:18:16 GMT, David Given <d...@xxxxxxxxxxx> wrote:
However, it's still bad practice to call it[the BIOS table] directly --- the user-level API
states firmly that you all accesses to the OS must be done through the entry
point at 0x0005.
Of course, thats the access point for the OS. There are times when
Direct access is important and it is done CP/M error strings and API
extensions for example....
If you need BIOS access for a specific reason then the jump table is
there for that.
CP/M was not as well crafted as some that hit the scene later but it
is a fairly good software platform and is fairly consistant.
Allison
Indeed, the early development of CP/M was, in part, to move I/O from
the BDOS and into the BIOS, and to access it via the BIOS table. As
hardware become more sophisicated, the BIOS provided a place for
developers (and home gamers) to add support for that hardware, without
any change to the BDOS. Therefore, no sources were needed for the BDOS
and so OS end users could also make changes.
Or, they could use programs which were hardware specific. Applications
like "DU" (disk utility) called the BIOS directly to access track and
sector, to support foreign disk editing even when the BIOS did not
"know" the disk format. Modem programs also called the BIOS, and often
inserted calls to their specifically supported I/O. "Ghost disk"
applications which used page-addressable RAM, patched the BIOS to
support RAM-based file storage.
Critics of CP/M should consider that it was written in the mid-1970's,
for an 8-bit processor, to operate in 32K bytes to 64K bytes. Yet it
supported floppy disks and serial ports, and provided a modest API.
Again, it was end-user adaptable to new hardware: the tools and docs
were provided to assist that. That combination, of adaptability and
tools and docs, set a standard for the next decade through the early
1980's.
Herb Johnson
Herbert R. Johnson, New Jersey USA
<a href="http://www.retrotechnology.com/herbs_stuff/";> web site</a>
<a href="http://www.retrotechnology.net/herbs_stuff/";> domain mirror</
a>
my email address: hjohnson AAT retrotechnology DOTT com
if no reply, try in a few days: herbjohnson ATT comcast DOTT net
"Herb's Stuff": old Mac, SGI, 8-inch floppy drives
S-100 IMSAI Altair computers, docs, by "Dr. S-100"
.
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