Compile/run assembler in Linux?

Question

I'm fairly new to Linux (Ubuntu 10.04) and a total novice to assembler. I was following some tutorials and I couldn't find anything specific to Linux. So, my question is, what is a good package to compile/run assembler and what are the command line commands to compile/run for that package?

Answer 1

The GNU assembler is probably already installed on your system. Try man as to see full usage information. You can use as to compile individual files and ld to link if you really, really want to.

However, GCC makes a great front-end. It can assemble .s files for you. For example:

$ cat >hello.s <<"EOF"
.section .rodata             # read-only static data
.globl hello
hello:
  .string "Hello, world!"    # zero-terminated C string

.text
.global main
main:
    push    %rbp
    mov     %rsp,  %rbp                 # create a stack frame

    mov     $hello, %edi                # put the address of hello into RDI
    call    puts                        #  as the first arg for puts

    mov     $0,    %eax                 # return value = 0.  Normally xor %eax,%eax
    leave                               # tear down the stack frame
    ret                            # pop the return address off the stack into RIP
EOF
$ gcc hello.s -no-pie -o hello
$ ./hello
Hello, world!

The code above is x86-64. If you want to make a position-independent executable (PIE), you'd need lea hello(%rip), %rdi, and call puts@plt.

A non-PIE executable (position-dependent) can use 32-bit absolute addressing for static data, but a PIE should use RIP-relative LEA. (See also Difference between movq and movabsq in x86-64 neither movq nor movabsq are a good choice.)

If you wanted to write 32-bit code, the calling convention is different, and RIP-relative addressing isn't available. (So you'd push $hello before the call, and pop the stack args after.)

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You can also compile C/C++ code directly to assembly if you're curious how something works:

$ cat >hello.c <<EOF
#include <stdio.h>
int main(void) {
    printf("Hello, world!\n");
    return 0;
}
EOF
$ gcc -S hello.c -o hello.s

See also How to remove "noise" from GCC/clang assembly output? for more about looking at compiler output, and writing useful small functions that will compile to interesting output.