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39 lines
1.2 KiB
39 lines
1.2 KiB
; calc_str_len.asm
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; Prints calculates the length of the string "Hello, world!" and prints to stdout
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; Compile with "nasm -f elf64 calc_str_len.asm && ld calc_str_len.o -o calc_str_len"
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.text:
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global _start
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_start:
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mov rax, msg ; move msg to rax register
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call strlen ; call strlen subroutine
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mov rax, 1 ; system call for write
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mov rdi, 1 ; file handle for stdout
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syscall ; call kernel
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mov rax, 60 ; system call for exit
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mov rdi, 0 ; exit code
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syscall ; call kernel
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strlen:
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push rax ; push rax to stack
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mov rbx, rax ; move rax value (msg variable) to rbx register
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calc_str_len:
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cmp byte [rax], 0 ; check if the pointer of rax equals 0 (string delimeter)
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jz exit_loop ; jump to "exit_loop" if zero flag has been set
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inc rax ; increment rax (position along string)
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jmp calc_str_len ; jump to start of loop
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exit_loop:
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sub rax, rbx ; subtract rbx from rax to equal to length of bytes between them
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mov rdx, rax ; rax will now equal the length of the string
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pop rsi ; pop top value on stack to rsi for printing
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ret
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.data:
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; initialize doubleword msg variable
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msg db 'Hello, world!',0xa,0x0
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