CS 456 - Systems Programming Spring 2024

Displaying ./code/asm/helloworldLen.asm

; Hello World Program - (Calculating String Length)
; Compile with: nasm -f elf helloworld.asm
; Link with (64 bit systems require elf_i386 option): ld -m elf_i386 helloworld.o -o helloworld
; Run with: ./helloworld
 
SECTION .data
msg     db      'Hello, brave new World!', 0Ah     ; assign msg variable with your message string
 
SECTION .text
global  _start
 
_start:

    mov     ebx, msg        ; move the address of our message string into EBX
    mov     eax, ebx        ; move the address in EBX into EAX as well (Both now point to the same segment in memory)
 
nextchar:
    cmp     byte [eax], 0   ; compare the byte pointed to by EAX at this address against zero (Zero is an end of string delimiter)
    jz      finished        ; jump (if the zero flagged has been set) to the point in the code labeled 'finished'
    inc     eax             ; increment the address in EAX by one byte (if the zero flagged has NOT been set)
    jmp     nextchar        ; jump to the point in the code labeled 'nextchar'
 
finished:
    sub     eax, ebx        ; subtract the address in EBX from the address in EAX
                            ; remember both registers started pointing to the same address (see line 15)
                            ; but EAX has been incremented one byte for each character in the message string
                            ; when you subtract one memory address from another of the same type
                            ; the result is number of segments between them - in this case the number of bytes
 
    mov     edx, eax     ; number of bytes to write - one for each letter plus 0Ah (line feed character)
    mov     ecx, msg    ; move the memory address of our message string into ecx
    mov     ebx, 1      ; write to the STDOUT file
    mov     eax, 4      ; invoke SYS_WRITE (kernel opcode 4)
    int     80h

    mov     ebx, 0      ; return 0 status on exit - 'No Errors'
    mov     eax, 1      ; invoke SYS_EXIT (kernel opcode 1)
    int     80h