嵌入汇编编程

Brennan's Guide to Inline Assembly

www.delorie.com /djgpp/doc/brennan/brennan_att_inline_djgpp.html

by Brennan "Bas" Underwood

Document version 1.1.2.2

Ok. This is meant to be an introduction to inline assembly under DJGPP. DJGPP is based on GCC, so it uses the

AT&T/UNIX syntax and has a somewhat unique method of inline assembly. I spent many hours figuring some of

this stuff out and told Info that I hate it, many times.

Hopefully if you already know Intel syntax, the examples will be helpful to you. I've put variable names, register

Source/Destination Ordering:

In AT&T syntax (which is the UNIX standard, BTW) the source is always on the left, and the destination is

always on the right.

So let's load ebx with the value in eax:

AT&T: movl %eax,

%ebx

Intel: mov ebx, eax

Constant value/immediate value format:

You must prefix all constant/immediate values with "$".

Let's load eax with the address of the "C" variable booga, which is static.

AT&T: movl $_booga,

AT&T: movl $0xd00d,

AT&T: movw %ax,

%bx

is byte ptr, word ptr, and dword ptr, but that is for when you are...

Referencing memory:

DJGPP uses 386-protected mode, so you can forget all that real-mode addressing junk, including the

restrictions on which register has what default segment, which registers can be base or index pointers.

Now, we just get 6 general purpose registers. (7 if you use ebp, but be sure to restore it yourself or

compile with -fomit-frame-pointer.)

Here is the canonical format for 32-bit addressing:

You don't have to use all those fields, but you do have to have at least 1 of immed32, basepointer and you

Let's see some simple forms of memory addressing: