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CS 456 - Systems Programming
Spring 2024
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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
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