28 Mar
2008
28 Mar
'08
3:06 a.m.
On Mar 27, 2008, at 7:53 PM, Jim Leonard wrote:
Eric Smith wrote:
1. Because you can.
2. Because it can save a bit of RAM. Remember in the early days of
the PC, saving bytes was important for a number of programs. They had
to fit in "typical" machines at the time. In many cases this was on a
16K machine, every byte counted and not all of the functions in the
BIOS were exported by INT xx entry points. In many ways IBM made it
easy because they published the complete assembly listing of the BIOS
ROM.
While I was at IBM (Boca Raton, FL), there were rules on the locations
of certain BIOS routines (ie their entry points needed to be at a
certain address in the ROM). This continued into the 1990's.
TTFN - Guy
Suppose you're trying to market a PC clone.
How successful will
you be
if some piece of popular software for the PC won't run on your clone,
because that software depends on direct calls into the ROM, or direct
access to tables in the ROM (e.g., the character generator)? Back in
those days, *lots* of software did wacky things like that.
Yes, but *why*? I am getting to be what I consider a pretty decent
assembler programmer, specifically on the 5150/5160, and I honestly
can't see *why* I would *ever* want to specifically jump directly
into the middle of the BIOS. I *call* BIOS routines, obviously, but
what possible benefit or purpose could there be to jumping directly
into ROM?