[Total recall] asm #1 – fancy hello word.

During the “Total Recall” episodes I’m going to recall the knowledge I used to bury deep in corners of my mind.

Here is a simple output:

[root@asm workspace]# nasm -f elf64 -o hello.o hello.asm && ld hello.o -o hello && ./hello
Hey,I'm alive!
[root@asm workspace]#

I’m going to share some asm code with comments here as a note:

section .data ; section where you define data before compilation
        textWait db  "wait...#      ",10 ; db = define bytes "wait...#X \n"
        textAlive db "Hey,I'm alive!", 10
section .text ; section where you put the code
        global _start ; label that is known by the linker. 
        global _alive
        global _addWait
        global _continue
        ; loop part 
        inc rcx ; increase rcx - part of the loop for (i=0;i<=5;i++) 

        ; Jump if Lower or equal (like "if then" condition)     
        cmp rcx ,5
        jle _addWait

        xor rsi, rsi
        mov rsi, textAlive ; fill rsi with pointer to "Hi, I'm alive"
        jmp _continue

        xor rsi,rsi
        mov rax, rcx
        add rax, '0'; make text symbol
        mov rsi, textWait ; fill rsi with " wait..."
        mov [rsi+8], al ; fill with COUNTER from ($rcx) as character

        ; execute the syscall and pass the data from 64bit registers
        ; syscall to write the data to stdout.
        push rcx   ; save rax to the stack
        xor rax,rax ; set rax to 0
        mov rax, 1 ; sys_write
        mov rdi, 1 ; stdout
        ;rsi register is filled in "_alive" and "_addWait"

        mov rdx, 15 ; buf len
        syscall; syscall takes rax, rdi, rsi, rdx

        ; loop part
        pop rcx ; get looddp's "i" value from the stack
        cmp  rcx, 5 ; compare rax and 5
        jle _start ; if smaller/equal to 5 

        ; syscall to exit the logic with argument
        mov rax, 60 ; sys_exit
        mov rdi, 0 ; return 0