; Hello World Program (Subroutines)
; Compile with: nasm -f elf helloworld-len.asm
; Link with (64 bit systems require elf_i386 option): ld -m elf_i386 helloworld-len.o -o helloworld-len
; Run with: ./helloworld-len
 
SECTION .data
msg     db      'Hello, solab2 world!', 0Ah
 
SECTION .text
global  _start
 
_start:
 
    mov     eax, msg       ; move the address of our message string into EAX
    call    strlen         ; call our function to calculate the length of the string
 
    mov     edx, eax       ; our function leaves the result in EAX
    mov     ecx, msg       ; this is all the same as before
    mov     ebx, 1
    mov     eax, 4
    int     80h
 
    mov     ebx, 0
    mov     eax, 1
    int     80h
 
strlen:                    ; this is our first function declaration
    push    ebx            ; push the value in EBX onto the stack to preserve it while we use EBX in this function
    mov     ebx, eax       ; move the address in EAX into EBX (Both point to the same segment in memory)
 
nextchar:                  ; this is the same as lesson3
    cmp     byte [eax], 0
    jz      finished
    inc     eax
    jmp     nextchar
 
finished:
    sub     eax, ebx
    pop     ebx            ; pop the value on the stack back into EBX
    ret                    ; return to where the function was called
