28 Mar
2008
28 Mar
'08
3:50 a.m.
Guy Sotomayor wrote:
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 can appreciate this philosophy, I still can't see how it would
be beneficial on the 5150/5160. For example, let's say I want to move
the cursor to a new location, say (10,15). Typical way is something
like this:
mov ah,2 ;set video mode
mov bh,0 ;video page 0
mov dh,15 ;col 15
mov dl,10 ;row 10
int 10h ;do it
That's 10 bytes.
The INT call pushes the flags, CS, and IP onto the stack, jumps to the
routine, the routine executes, and then *in the ROM routine itself*
there is an IRET, which POPs IP, CS, and flags and then continues from
that point. It is the IRET that is causing me trouble on how to figure
out what better way to do this, because once you hit the IRET you had
better have the IP, CS, and flags on the stack because they're going to
get restored whether you want them to or not. And you can't just patch
in a JMP into the ROM code to return, because it's ROM.
So the only scenario I can think of is something like, maybe you only
want to move the cursor downward, on the same page, and you don't want
to set those extra input registers. Let's also assume that you know the
exact location in ROM where you want to JMP so that the BIOS routine can
do only what you want it to. You would still have to do something like
this:
mov bh,0 ;set video page 0
mov dl,10 ;move cursor down only
CALL 1234:5678 (go directly to your magic function)
That's 9 bytes. So you have saved ONE byte, yes, but that is for an
extremely specific optimized case. The CALL is necessary because of the
IRET that is coming at the end of the function.
If that was a more common practice than I realize, then I guess I stand
corrected. But it just doesn't seem worth it. When I'm *that*
pressured for space, I use code overlays (if the language allows it) or
something else, like using non-visible video RAM for storage. (Yes,
that's disgusting, but I'm not going to turn down 12K of free RAM just
because I'm only using one video page! Remember, even a monochrome
card, with it's single 80x25 video page, has 96 bytes of unused ram...)
--
Jim Leonard (trixter at oldskool.org) http://www.oldskool.org/
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A child borne of the home computer wars: http://trixter.wordpress.com/