65 lines
1.8 KiB
NASM
65 lines
1.8 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 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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;; strlen() function. Takes rdx as an argument - the pointer to
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;; the initial string. Returns rdx as the result - the length of
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;; the string.
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strlen:
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push rsi
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mov rsi, rdx
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strlen_next:
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cmp byte [rdx], 0
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jz strlen_done
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inc rdx
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jmp strlen_next
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strlen_done:
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sub rdx, rsi
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pop rsi
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ret
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;; puts() function - puts a string to the console. Takes RSI 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 rdx ; RDX (data) is used as the length of the message
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push rax ; RAX is the instruction to WRITE
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push rdi ; RDI (destination) is used as an instruction to WRITE
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mov rdx, rsi
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call strlen
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;; Because I chose to use RDX as the target for strlen, I don't need to
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;; do the pop/push dance as I did in the 32-bit version.
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mov rdi, STDOUT ; using STDOUT (see definition above)
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mov rax, SYS_write ; Using WRITE in 32-bit mode?
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syscall
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;; Likewise, because I chose RSI as the Source (pointer) to my
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;; message, it remains unchanged during the lifetime of this
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;; routine (unless there are any bugs in strlen) and I don't
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;; have to restore it either.
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pop rdi
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pop rax
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pop rdx
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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 rdi, 0
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mov rax, SYS_exit
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syscall
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