49 lines
1.6 KiB
ArmAsm
49 lines
1.6 KiB
ArmAsm
;; Hello World Program #3
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;; Compile with: nasm -f elf64 strlen.s
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;; Link with: ld -o strlen strlen.o
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;; Run with: ./strlen
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;; sys/unistd_64.h
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%define SYS_write 1
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%define SYS_exit 60
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;; unistd.h
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%define STDOUT 1
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global _start
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section .data
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msg db "Hello You Beautiful Human, You're Looking Mighty Fine!", 0Ah, 00h
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section .text
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_start:
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mov rsi, msg ; Move the address of the message into rsi
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mov rax, rsi ; Move the address of the message into rax
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; (Register-to-register copying is faster that a constant!)
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nextchar:
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cmp byte [rax], 0 ; Compare the byte pointed to by rax with zero
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;; Small detail: cmp and sub use the same internal architecture,
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;; except cmp doesn't copy the results into the first operand and
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;; sub does. cmp sets flags; does sub? This is why 'jz' works,
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;; because if they're equal the result of subtraction is zero.
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jz counted ; Jump if the zero flag set
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inc rax ; Increment rax by 1
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jmp nextchar ; Jump to the beginning of the loop
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counted:
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sub rax, rsi ; Substract source from endpointer, leaving counter
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mov rdx, rax ; Length of the message
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mov rsi, msg ; Address of the message
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mov rdi, STDOUT ; using STDOUT (see definition above)
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mov rax, SYS_write ; Using WRITE in 64-bin mode.
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syscall ; Call the kernel
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mov rdi, 0
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mov rax, SYS_exit
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syscall
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