69 lines
2.1 KiB
NASM
69 lines
2.1 KiB
NASM
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;; This is an includes file. It doesn't have its own compilation
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;; capabilities. There is no namespacing in assembly language;
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;; try not to use these names in your own code.
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;; sys/unistd_32.h
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%define SYS_write 4
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%define SYS_exit 1
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;; unistd.h
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%define STDOUT 1
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;; strlen() function. Takes eax as an argument - the pointer to
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;; the initial string. Returns eax as the result - the length of
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;; the string.
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strlen:
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push ebx ; We'll be borrowing this register, so we put its
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; current value on the stack.
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mov ebx, eax
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;; Note that even though these have underscores to indicate that
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;; outside users should not use them, they're still globally
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;; accesible in this program's namespace. A user could
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;; theoretically call _strlen_done from anywhere. Assembly gives
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;; you ALL the opportunities to shoot yourself in the foot!
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_strlen_next:
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cmp byte [eax], 0
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jz _strlen_done
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inc eax
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jmp _strlen_next
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_strlen_done:
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sub eax, ebx
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pop ebx ; Restore the register
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ret
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;; Puts() function - puts a string to the console. Takes EAX as
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;; its only argument - the pointer to the beginning of the string.
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puts:
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push edx
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push ecx
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push ebx
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push eax
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call strlen ; Uses EAX as the pointer to the beginning of the string.
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; Returns EAX as the length of the string
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mov edx, eax ; Move the length of the string into EDX, where WRITE expects
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pop eax ; Restore EAX from the stack
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push eax ; Put the value BACK on the stack.
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mov ecx, eax ; Put the pointer to the message into ECX, where WRITE expects
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mov ebx, STDOUT
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mov eax, SYS_write
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int 80h
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pop eax ; Restore registers in reverse order.
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pop ebx
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pop ecx
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pop edx
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ret
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;; exit(). Straight from the original.
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exit_program:
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mov ebx, 0
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mov eax, SYS_exit
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int 80h
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