How to print a character in Linux x86 NASM?

Question

I\'m trying to print a single character or a number using NASM, targeting an x86 GNU/Linux architecture.

Here\'s the code I\'m using:

Answer 1

The system call you are executing expects ecx to contain an address in memory. This can be an arbitrary literal address (i.e. in your code, "A" translates to the address 041h), an address on the stack, or an address defined by a label in the program.

Here's an example of defining a byte in memory and writing it to the standard output stream of your terminal:

    section .rodata  ; This section contains read-only data
buffer:    db 'A'    ; Define our single character in memory

    section .text
    global start
_start:
    ; Prints the letter 'A', then exits

    mov eax, 4      ; sys_write()
    mov ebx, 1      ; ... to STDOUT
    mov ecx, buffer ; ... using the following memory address
    mov edx, 1      ; ... and only print one character
    int 80h         ; SYSCALL

    mov eax, 1      ; Return to system
    mov ebx, 0      ; Exit zero, success
    int 80h         ; SYSCALL