Subj : NASM segment registers- again
To   : Robin Sheppard
From : Jasen Betts
Date : Sun Oct 21 2001 11:11 pm

Hi Robin.

21-Oct-01 04:21:44, Robin Sheppard wrote to All


 RS> This is a re-post of a message I posted recently.  I didn't get
 RS> any replies, so was wondering if it even got out.  On the
 RS> assumption that it probably didn't, I'm posting it again. -------

it did. I don't have a good answer but I'll try.

 RS> Does anyone know how to load segment registers at runtime using
 RS> NASM, when generating COFF output?

no.

 RS> Specifically, I'm trying to port some 16-bit timer-handler code I
 RS> wrote so it'll run under DJGPP.  The assembly code has three
 RS> functions, _hooktimer, _unhooktimer, and newtimer.  The first two
 RS> are global symbols, so my C code can use them to (you guessed it)
 RS> hook and unhook the timer.  The newtimer procedure is the actual
 RS> interrupt handler, and all it does currently is send an
 RS> end-of-interrupt, execute STI, and exit.

if you're using C you may be able to use signal instead of messing with
interrupts...

 RS> It's supposed to increment a global variable called _count, but
 RS> since the function is asynchronous, I can't rely on the value of
 RS> DS; I have to load it myself.  If I don't, it'll merrily increment
 RS> random memory locations until my machine crashes.  This is what I
 RS> can't figure out how to do.

if you need the names of the sections check out the assembler code
egnerated by gpp (it'll be AT&T syntax but should be readable)
OTOH

nm some_unstripped.exe

should list all the names (labels) in that exe...

 RS> The only way I can think of to get around this is to create an
 RS> alias data descriptor for my code segment, and use that to write
 RS> the value directly into my code segment at runtime.  Not only does
 RS> this seem like a kludge, but it also seems to be bad programming
 RS> practice. Isn't there some mechanism similar to

 RS> mov ax, DGROUP

I guess you've been through the NASM manual and the djgpp into tree a few
times already...

doesn't DJGPP support an "interrupt" function-type, if it does compile one
to assembler and see how it's built.

OTOH I'm told that djgcc uses tiny model with DS,CS, and SS all pointing to
the same memory (if not the same segment descriptor) and they never change,

I'm not sure how the DPMI stub handles interrupts...

If nothing works check the ALLEGRO source it'll have an example of
interrupts for shure.

 -=> Bye <=-
---
 * Origin: Every time I think I know where it's at, the move it. (3:640/531.42)

.