[Tutorial] How to inline ASM in C (gcc)

[harleyg]

How to inline ASM in C (gcc)

In your code you will use the asm(); function, which you can use to inline asm in C. If you don't understand what inline means, it's basically using ASM in C.

First, set headers, module info and start main.

Code:

#include <pspkernel.h>
#include <pspdebug.h>

PSP_MODULE_INFO("Hello World in ASM", 0, 1, 1);

int main(void) {

When inlining ASM in C, you can set a variable in C, in out case we will call it msg, and use it in the ASM code.

Code:

char *msg = "Hello world\n";

Setup the psp screen.

Code:

pspDebugScreenInit();

Now, using the asm() function you can make the psp print this code...

Code:

asm("lui $4, %%hi(%0)\n"
"jal pspDebugScreenPrintf\n"
"addiu $4,$4,%%lo(%0)\n"
"nop\n"
: : "g" (msg));

lui, load upper immediate.
$4, $4 will be loaded later.
%%hi, this will grab the upper 16bits of (val).
$0, this is this first argument passed into the function.

jal, jump and load.
pspDebugScreenPrintf, the jal loads pspDebugScreenPrintf and that prints $4.

addiu, add immmediate unsigned.
$4,$4,%%lo(%0), $4 + %%lo(%0) (lower 16 bits of the first arguement passed into the function) = $4.

nop, no operation. This means that instead of setting $4 after jal, you can set before. This is used for aligning code.

: : "g" (msg));, loads the variable msg ($0) and ends the asm function. This requires -02 on gcc, if that isnt by default.


Return 0 and close main.

Code:

return 0;
}

Your code shoud look like this.

Code:

#include <pspkernel.h>
#include <pspdebug.h>

PSP_MODULE_INFO("Hello World", 0, 1, 1);

int main(void) {
char *msg = "Hello world!\n";
pspDebugScreenInit();
asm("lui $4, %%hi(%0)\n"
"jal pspDebugScreenPrintf\n"
"addiu $4,$4,%%lo(%0)\n"
"nop\n"
: : "g" (msg));
return 0;
}

Your Makefile.

Code:

TARGET = hello
OBJS = main.o

CFLAGS = -O2 -G0 -Wall
CXXFLAGS = $(CFLAGS) -fno-exceptions -fno-rtti
ASFLAGS = $(CFLAGS)

EXTRA_TARGETS = EBOOT.PBP
PSP_EBOOT_TITLE = Hello World in ASM

PSPSDK=$(shell psp-config --pspsdk-path)
include $(PSPSDK)/lib/build.mak

Ta-da, it should say hello world!



Yes, this way probably does suck, but im still learning MIPS ASM myself
Big thanks to pspdev and siberian for helping me

[nathan42100]

nice!

[boom]

cool, it does look REALLY CONFUSING
do you use the same method of making a C/C++ eboot just with the asm code cuz the thing that lets you make C codes wont work on my computer
         

WHHHYYYYYYYYYYYYYYYYYYYYYYYYYY!!!!!!!!!!!!!!!!

[Zettablade]

Of course, it's inlined. Just type 'make', nothing has changed.

Anyways, awsome tut. And good luck with your mips asm.  

[SG57]

Here's another good reference taken from an answer to my question at ps2dev...

A good comparison from a C routine of an addition test between variables, to an assembly routine of an addition test between variables.

I use this as a sample to learn from...  Like how to use a loop for example.   I make little guesses of how they work, then check it with a quick search of the internet.  Keeps me going and interested to see if Im right

Code:

void AddTest()
{


int num1=5;
int num2=10;
int out=0;
int to = 5;
float r17,r16;
r16=0;
r17=0;
for(int i=0;i<to;i++)
{
   float res = num1*num2;
   r16 = res;
   r17 = r16+r17;
}
printf("C Res:%f",r17);

num1 =5;
num2 = 10;
out = 0;
to = 5;

asm __volatile__ ("\n\
ori $5,$0,0\n\
ori $17,$0,0\n\
loop:\n\
mul %1,%2\n\
mflo $16\n\
addu $19,$16,$17\n\
move $17,$19\n\
addiu $5,$5,1\n\
bne $5,%3,loop\n\
nop\n\
move %0,$17\n\
":"=r"(out):"r"(num1),"r"(num2),"r"(to)
);

printf("Asm Result:%d \n",out);
}

harleyg uses the " to start and stop a line, while you can easily use the backwards slash (\) to show 'next line' without a need for the multiple ".  I find it a tad easier to read with the \'s since my editor doesnt highlight hte entire text behind it, where as the " and ", everything in between is highlighted, making it look like a string for printing  But that's just me, as I too am learning ASM step by step, so thought Id share with you if thats ok

[harleyg]

Sure, i was only showed that form what SiberianSTAR said.

[PaulC]

How to inline ASM in C (gcc)

When I run this, it prints out a garbage value. Anyone know why?

[macthefork]

The problem is the newer compilers (from when that was probably written) are doing some aggressive inline assembly optimisation (or alternatively dumb ). When you compile that you get:

Code:

; ======================================================
; Subroutine main - Address 0x00000280
main:           ; Refs: 0x0000009C
        addiu      $sp, $sp, -0x8                           ; 0x00000280: 0x27BDFFF8 '...''
        sw         $ra, 0x0($sp)                            ; 0x00000284: 0xAFBF0000 '....'
        jal        pspDebugScreenInit                       ; 0x00000288: 0x0C00029D '....'
        nop                                                 ; 0x0000028C: 0x00000000 '....'
        jal        pspDebugScreenPrintf                     ; 0x00000290: 0x0C0002A5 '....'
        lui        $a0, 0x1                                 ; 0x00000294: 0x3C040001 '...<'
        addiu      $a0, $a0, -0xA3C                         ; 0x00000298: 0x2484F5C4 '...$'
        nop                                                 ; 0x0000029C: 0x00000000 '....'
        lw         $ra, 0x0($sp)                            ; 0x000002A0: 0x8FBF0000 '....'
        move       $v0, $zr                                 ; 0x000002A4: 0x00001021 '!...'
        jr         $ra                                      ; 0x000002A8: 0x03E00008 '....'
        addiu      $sp, $sp, 0x8                            ; 0x000002AC: 0x27BD0008 '...''

If you look at the call to pspDebugScreenPrintf the final loading of the pointer is now after the call has finished so the function will try and print what ever is at address 0x10000 (well it will be relocated of course).

Changing the asm to:

Code:

    asm __volatile__ ("la $4, %0\n"
        "jal pspDebugScreenPrintf\n"
        : : "g" (msg));

Will fix it for the time being, you should always double check that the assembly code you wanted it outputted correctly. Even better just don't use inline asm for anything but the cases where you have no choice (like VFPU code) as in reality the compiler can probably do a better job at generating decent asm code

[PaulC]

Thanks!

I'm sure the compiler can do a better job and I know better than to start going low-level if I can help it, but I need to start learning this for part of my coursework and it's nice that there's some relevant assembly on here. The "Hello World" example should be a good entry point once I know enough to start adding my own code.

[macthefork]

If you want to write assembly I would suggest doing it in it's own file, that way the compiler doesn't play around with the ordering of instructions.

[Raiden]

I know this thread is old, but I have to bump it.
I´ve a real noob question and I hope you aren´t bothered by me
Is there a chance to compile this without cygwin? I´m currently using pspsdk, which is as far as I know only for C.
Hope you can answer my question!

[mowglisanu]

As the title says "[Tutorial] How to inline ASM in C (gcc)"
so it is C at least in gcc.