X86 Inline Assembly
Hello!
I'm new to inline assembly, whilst I do have experience with assembly, it is mostly geared towards programming micro controllers. I'm trying to get an assignment done, and I have been researching inline assembly. My goal is to successfully implement the functionality of what getchar and putchar would do using inline assembly.(Reading a character from stdin, and putting a character on stdout).
I don't know if this means anything but I'm running this on a Linux machine, compiling with gcc.
Apparently it uses AT&T style as opposed to Intel style, This must use x86 inline assembly.


I'm trying to get the steps down.
Write to std_out

It seems I have to load, register %%eax with 4 (the call for sys\_write).
Load %%ebx with the file descriptor, in this case 1.
Load %%edx with the size, in this case 1.
Load %%ecx with a const char *, a pointer to the character.

My first implementation is here:

int main()
{
char p = 'p';
int ret;
__asm__ __volatile__(
"syscall"
:"=a"(ret)
:"a"(4), "b"(1), "c"(&p), "d"(1)
: "memory"
);
return 0;
}

This code above doesn't output anything, but ret is equivalent to -14/

A second implementation, without using extended Assembly
int main()
{
char c = 'c';
char * pointer = &c;
__asm__ __volatile__(
"mov $1, %%ebx\t\n"
"mov $1, %%edx\t\n"
"mov %1, %%ecx\t\n"
"mov $4, %%eax\t\n"
"syscall"
:
:"r"(pointer)
:
);
// printf("foo+bar=%d\n", foo);
return 0;
}

This code doesn't compile, I get a invalid 'asm': operand number out of range.


I'm trying to work through this before attempting writing putchar, as I may have foundational errors.

I'd appreciate any possible aid/hints!


Thanks in advance!
🗣StandardSBUStudent

Are you compiling for 32-bit or 64-bit? They have different system call numbers and calling conventions.

For 32-bit: Pass parameters in eax/ebx/ecx/edx (i.e. syscall number in eax) and use int $0x80 to execute. sys_write is 4.

For 64-bit: Pass parameters in rax/rdi/rsi/rdx and use syscall. sys_write is 1.

For your second example note that the first operand is %0. See here
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