Brennan's Guide to Inline Assembly
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 names and other literals in bold type.
The Syntax
So, DJGPP uses the AT&T assembly syntax. What does that mean to you?
* Register naming:
Register names are prefixed with "%". To reference eax:
AT&T: %eax
Intel: eax
* 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, %eax
Intel: mov eax, _booga
Now let's load ebx with 0xd00d:
AT&T: movl $0xd00d, %ebx
Intel: mov ebx, d00dh
* Operator size specification:
You must suffix the instruction with one of b, w, or l to specify the
width of the destination register as a byte, word or longword. If you omit
this, GAS (GNU assembler) will attempt to guess. You don't want GAS to
guess, and guess wrong! Don't forget it.
AT&T: movw %ax, %bx
Intel: mov bx, ax
The equivalent forms for Intel 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:
AT&T: immed32(basepointer,indexpointer,indexscale)
Intel: [basepointer + indexpointer*indexscale + immed32]
You could think of the formula to calculate the address as:
immed32 + basepointer + indexpointer * indexscale
You don't have to use all those fields, but you do have to have at least
1 of immed32, basepointer and you MUST add the size suffix to the
operator!
Let's see some simple forms of memory addressing:
o Addressing a particular C variable:
AT&T: _booga
Intel: [_booga]
Note: the underscore ("_") is how you get at static (global) C
variables from assembler. This only works with global variables.
Otherwise, you can use extended asm to have variables preloaded
into registers for you. I address that farther down.
o Addressing what a register points to:
AT&T: (%eax)
Intel: [eax]
o Addressing a variable offset by a value in a register:
AT&T: _variable(%eax)
Intel: [eax + _variable]
o Addressing a value in an array of integers (scaling up by 4):
AT&T: _array(,%eax,4)
Intel: [eax*4 + array]
o You can also do offsets with the immediate value:
C code: *(p+1) where p is a char *
AT&T: 1(%eax) where eax has the value of p
Intel: [eax + 1]
o You can do some simple math on the immediate value:
AT&T: _struct_pointer+8
I assume you can do that with Intel format as well.
o Addressing a particular char in an array of 8-character records:
eax holds the number of the record desired. ebx has the wanted
char's offset within the record.
AT&T: _array(%ebx,%eax,8)
Intel: [ebx + eax*8 + _array]
Whew. Hopefully that covers all the addressing you'll need to do. As a
note, you can put esp into the address, but only as the base register.
Basic inline assembly
The format for basic inline assembly is very simple, and much like Borland's
method.
asm ("statements");
Pretty simple, no? So
asm ("nop");
will do nothing of course, and
asm ("cli");
will stop interrupts, with
asm ("sti");
of course enabling them. You can use __asm__ instead of asm if the keyword asm
conflicts with something in your program.
When it comes to simple stuff like this, basic inline assembly is fine. You can
even push your registers onto the stack, use them, and put them back.
asm ("pushl %eax\n\t"
"movl $0, %eax\n\t"
"popl %eax");
(The \n's and \t's are there so the .s file that GCC generates and hands to GAS
comes out right when you've got multiple statements per asm)
It's really meant for issuing instructions for which there is no equivalent in
C and don't touch the registers.
But if you do touch the registers, and don't fix things at the end of your asm
statement, like so:
asm ("movl %eax, %ebx");
asm ("xorl %ebx, %edx");
asm ("movl $0, _booga");
then your program will probably blow things to hell. This is because GCC hasn't
been told that your asm statement clobbered ebx and edx and booga, which it
might have been keeping in a register, and might plan on using later. For that,
you need:
Extended inline assembly
The basic format of the inline assembly stays much the same, but now gets
Watcom-like extensions to allow input arguments and output arguments.
Here is the basic format:
asm ( "statements" : output_registers : input_registers : clobbered_registers);
Let's just jump straight to a nifty example, which I'll then explain:
asm ("cld\n\t"
"rep\n\t"
"stosl"
: /* no output registers */
: "c" (count), "a" (fill_value), "D" (dest)
: "%ecx", "%edi" );
The above stores the value in fill_value count times to the pointer dest.
Let's look at this bit by bit.
asm ("cld\n\t"
We are clearing the direction bit of the flags register. You never know what
this is going to be left at, and it costs you all of 1 or 2 cycles.
"rep\n\t"
"stosl"
Notice that GAS requires the rep prefix to occupy a line of it's own. Notice
also that stos has the l suffix to make it move longwords.
: /* no output registers */
Well, there aren't any in this function.
: "c" (count), "a" (fill_value), "D" (dest)
Here we load ecx with count, eax with fill_value, and edi with dest. Why make
GCC do it instead of doing it ourselves? Because GCC, in its register allocating
might be able to arrange for, say, fill_value to already be in eax. If this is
in a loop, it might be able to preserve eax thru the loop, and save a movl once
per loop.
: "%ecx", "%edi" );
And here's where we specify to GCC, "you can no longer count on the values you
loaded into ecx or edi to be valid." This doesn't mean they will be reloaded for
certain. This is the clobberlist.
Seem funky? Well, it really helps when optimizing, when GCC can know exactly
what you're doing with the registers before and after. It folds your assembly
code into the code it's generates (whose rules for generation look remarkably
like the above) and then optimizes. It's even smart enough to know that if you
tell it to put (x+1) in a register, then if you don't clobber it, and later C
code refers to (x+1), and it was able to keep that register free, it will reuse
the computation. Whew.
Here's the list of register loading codes that you'll be likely to use:
a eax
b ebx
c ecx
d edx
S esi
D edi
I constant value (0 to 31)
q,r dynamically allocated register (see below)
g eax, ebx, ecx, edx or variable in memory
A eax and edx combined into a 64-bit integer (use long longs)
Note that you can't directly refer to the byte registers (ah, al, etc.) or the
word registers (ax, bx, etc.) when you're loading this way. Once you've got it
in there, though, you can specify ax or whatever all you like.
The codes have to be in quotes, and the expressions to load in have to be in
parentheses.
When you do the clobber list, you specify the registers as above with the %.
If you write to a variable, you must include "memory" as one of The Clobbered.
This is in case you wrote to a variable that GCC thought it had in a register.
This is the same as clobbering all registers. While I've never run into a
problem with it, you might also want to add "cc" as a clobber if you change the
condition codes (the bits in the flags register the jnz, je, etc. operators look
at.)
Now, that's all fine and good for loading specific registers. But what if you
specify, say, ebx, and ecx, and GCC can't arrange for the values to be in those
registers without having to stash the previous values. It's possible to let GCC
pick the register(s). You do this:
asm ("leal (%1,%1,4), %0"
: "=r" (x)
: "0" (x) );
The above example multiplies x by 5 really quickly (1 cycle on the Pentium).
Now, we could have specified, say eax. But unless we really need a specific
register (like when using rep movsl or rep stosl, which are hardcoded to use
ecx, edi, and esi), why not let GCC pick an available one? So when GCC generates
the output code for GAS, %0 will be replaced by the register it picked.
And where did "q" and "r" come from? Well, "q" causes GCC to allocate from eax,
ebx, ecx, and edx. "r" lets GCC also consider esi and edi. So make sure, if you
use "r" that it would be possible to use esi or edi in that instruction. If not,
use "q".
Now, you might wonder, how to determine how the %n tokens get allocated to the
arguments. It's a straightforward first-come-first-served, left-to-right thing,
mapping to the "q"'s and "r"'s. But if you want to reuse a register allocated
with a "q" or "r", you use "0", "1", "2"... etc.
You don't need to put a GCC-allocated register on the clobberlist as GCC knows
that you're messing with it.
Now for output registers.
asm ("leal (%1,%1,4), %0"
: "=r" (x_times_5)
: "r" (x) );
Note the use of = to specify an output register. You just have to do it that
way. If you want 1 variable to stay in 1 register for both in and out, you have
to respecify the register allocated to it on the way in with the "0" type codes
as mentioned above.
asm ("leal (%0,%0,4), %0"
: "=r" (x)
: "0" (x) );
This also works, by the way:
asm ("leal (%%ebx,%%ebx,4), %%ebx"
: "=b" (x)
: "b" (x) );
2 things here:
* Note that we don't have to put ebx on the clobberlist, GCC knows it goes
into x. Therefore, since it can know the value of ebx, it isn't considered
clobbered.
* Notice that in extended asm, you must prefix registers with %% instead of
just %. Why, you ask? Because as GCC parses along for %0's and %1's and so
on, it would interpret %edx as a %e parameter, see that that's non-existent,
and ignore it. Then it would bitch about finding a symbol named dx, which
isn't valid because it's not prefixed with % and it's not the one you meant
anyway.
Important note: If your assembly statement must execute where you put it, (i.e.
must not be moved out of a loop as an optimization), put the keyword volatile
after asm and before the ()'s. To be ultra-careful, use
__asm__ __volatile__ (...whatever...);
However, I would like to point out that if your assembly's only purpose is to
calculate the output registers, with no other side effects, you should leave off
the volatile keyword so your statement will be processed into GCC's common
subexpression elimination optimization.
Some useful examples
#define disable() __asm__ __volatile__ ("cli");
#define enable() __asm__ __volatile__ ("sti");
Of course, libc has these defined too.
#define times3(arg1, arg2) \
__asm__ ( \
"leal (%0,%0,2),%0" \
: "=r" (arg2) \
: "0" (arg1) );
#define times5(arg1, arg2) \
__asm__ ( \
"leal (%0,%0,4),%0" \
: "=r" (arg2) \
: "0" (arg1) );
#define times9(arg1, arg2) \
__asm__ ( \
"leal (%0,%0,8),%0" \
: "=r" (arg2) \
: "0" (arg1) );
These multiply arg1 by 3, 5, or 9 and put them in arg2. You should be ok to do:
times5(x,x);
as well.
#define rep_movsl(src, dest, numwords) \
__asm__ __volatile__ ( \
"cld\n\t" \
"rep\n\t" \
"movsl" \
: : "S" (src), "D" (dest), "c" (numwords) \
: "%ecx", "%esi", "%edi" )
Helpful Hint: If you say memcpy() with a constant length parameter, GCC will
inline it to a rep movsl like above. But if you need a variable length version
that inlines and you're always moving dwords, there ya go.
#define rep_stosl(value, dest, numwords) \
__asm__ __volatile__ ( \
"cld\n\t" \
"rep\n\t" \
"stosl" \
: : "a" (value), "D" (dest), "c" (numwords) \
: "%ecx", "%edi" )
Same as above but for memset(), which doesn't get inlined no matter what (for
now)
#define RDTSC(llptr) ({ \
__asm__ __volatile__ ( \
".byte 0x0f; .byte 0x31" \
: "=A" (llptr) \
: : "eax", "edx"); })
Reads the TimeStampCounter on the Pentium and puts the 64 bit result into llptr.
The End
"The End"?! Yah, I guess so.
If you're wondering, I personally am a big fan of AT&T/UNIX syntax now. (It
might have helped that I cut my teeth on SPARC assembly. Of course, that machine
actually had a decent number of general registers.) It might seem weird to you
at first, but it's really more logical than Intel format, and has no
ambiguities.
If I still haven't answered a question of yours, look in the Info pages for more
information, particularly on the input/output registers. You can do some funky
stuff like use "A" to allocate two registers at once for 64-bit math or "m" for
static memory locations, and a bunch more that aren't really used as much as "q"
and "r".
Alternately, mail me, and I'll see what I can do. (If you find any errors in the
above, please, e-mail me and tell me about it! It's frustrating enough to learn
without buggy docs!) Or heck, mail me to say "boogabooga".
It's the least you can do.
Related Usenet posts:
* local labels
* fixed point multiplies
Thanks to Eric J. Korpela <korpela@ssl.Berkeley.EDU> for some corrections.
Have
you seen the DJGPP2+Games Page? Probably.
Page written and provided by Brennan Underwood.
Copyright © 1996 Brennan Underwood. Share and enjoy!
Page created with vi, God's own editor.
Source page:
http://www.delorie.com/djgpp/doc/brennan/brennan_att_inline_djgpp.html
Intel Platform
