From: brennan AT mack DOT rt66 DOT com (Brennan "The Rev. Bas" Underwood)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: ASM - Accessing structures
Date: 11 Nov 1996 15:11:01 -0700
Organization: Rasterfari
Lines: 50
Message-ID: <56889l$ib8@mack.rt66.com>
References: <199611101834 DOT MAA26637 AT ghgcorp DOT ghgcorp DOT com>
NNTP-Posting-Host: mack.rt66.com
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

In article <199611101834 DOT MAA26637 AT ghgcorp DOT ghgcorp DOT com>,
Bachtel <rbachtel AT ghgcorp DOT com> wrote:
>Could Brennan, if he's watching this, please tell me also how to 
>access structures in extended asm? Unless I'm a complete idiot (by no 
>means a remote possibility:), it is not explained in the info pages. 
>I'm trying to translate sliver.asm for Mr. Ogles, and this is 
>stopping me.

Heeeeeeeeeeeeeeeeeeeeeeeere I come to save the DAY!!! Hopefully.

Since a struct is just a pile of memory, you need to know the location
of the beginning of the struct and the offset of the element you want.
The difficulty of this goes up (slightly) since gcc likes to pad structures,
so you can't just count bytes (unless you __attribute__((packed)) everything,
which is going to slow down your accesses, which asm won't compensate
for, so don't do that.)

In C you can get the address of a struct element via &structname.element.
You can get the offset in bytes via
((int)&structname.element - (int)&structname)

You should probably have the program calculate these offsets at startup,
in case the structs ever change.
Then, armed with the offset:

movl $_mystruct, %%eax
movl $_element1offset, %%ebx
movb (%%eax,%%ebx), %%cl /* or something to that effect */

That's not the most efficient way to do the access, but hopefully this gets
the idea across.

If the structs never change, you could calculate the offsets once and build
them into the access: (pretend 16 is the offset)

movl $_mystruct, %%eax
movb 16(%%eax), %%cl

That's how gcc (usually) does it, since it has intimate knowledge of the
struct internals.

And, if it's a static struct (not likely)
movb _mystruct+16, %%cl


--Brennan
-- 
 brennan AT rt66 DOT com |  "Developing for Windows is not fun." -- John Carmack
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