assembly_park/4_unlink/src/unlink.s

.text					# Section for machine istructions to be executed
	.global _start			# Needed by ld to find his default entry point
_start:					# _start symbol
					# cmdline check
	pop	%rax			# Put number of arguments in %rax register
	cmp	$2, %rax		# Must be 2 arguments (1+executable name)
	jne	exit_cmd_line_error	# jump to error if not equal

	mov	$263, %rax		# Put "263" in %rax register. 263 = sys_unlinkat call.
	mov	$-100, %rdi		# dirfd of directory for relative paths. -100 = AT_FDCWD
					# special for current working directory
	mov	8(%rsp), %rsi		# Put address of string that is the name of the file
					# to unlink in %rsi register. This is expected to be
					# the second cmdline argument of this program and is
					# found at %rsp + 64bit (=8byte) after the pop above.
	xor	%rdx, %rdx		# Zero %rdx register = no special flags for call
	syscall				# Ask Linux to perform syscall index by rax

	test	%rax, %rax		# Test if return value of syscall in %rax register is
					# zero
	jz	exit_zero		# If yes, exit zero

	neg	%rax			# If not negate error value (linux return negative numbers
					# in %rax for system error (range -4096:-1)
	mov	$10, %bl		# Set to "10" the 8 low bits od %rbx register

	div	%bl			# "div" istruction works like this in this case:
					# %ax : %bl = %al (quotient) + %ah (reminder)
					# NOTE: the negated error value of sys_unlink is expected to
					# be *in any case* a number of two digits in decimal base.
					# If I divide it by "10" the quotient is the first digit
					# and the reminder is the second one (from left to right)
	mov	$48, %bl		# Put ASCII "0" in 8 low bits of %rbx register
	add	%bl, %al		# Add ASSCII "0" one digit number in 0-byte of %rax register
	add	%bl, %ah		# Add ASSCII "0" one digit number in 1-byte of %rax register
	push	%ax			# Push 16 low bits of %rax register (contains concatened ASCII
					# bits for the two digits of our error code)

	movb	$0x0A, 2(%rsp)		# Put an "endline" at %rsp + 16bit. (Put "\n" ASCII code
					# in stack just after the digits of our error code).
					# (All this is to avoid 2 sys_write calls above)

					# Write generic part of system error message to stderr
	mov	$1, %rax
	mov	$1, %rdi
	mov	$err_msg, %rsi
	mov	$13, %rdx
	syscall

	mov	$1, %rax		# Write error code digits + endline to stderr
	mov	$1, %rdi
	lea	(%rsp), %rsi
	mov	$3, %rdx
	syscall

	mov	$1, %rdi		# Put "1" in $rax %rdi register. (1= exit code)
	jmp	exit			# Exit

print_help:				# Print cmdline help to stdout
	mov	$1, %rax
	mov	$1, %rdi
	mov	$help_msg, %rsi
	mov	$19, %rdx
	syscall

	ret				# Return to main program execution

exit:					# Ask kernel to terminate execution with some exit
					# code
	mov	$60, %rax		# Put "60" in %rax register. 60 = sys_exit call
					# exit status argument of sys_exit.
	syscall				# Ask Linux to perform syscall indexed by %rax

exit_zero:				# Exit with 0 exit code
	xor	%rdi, %rdi		# Set %rdi register to "0".
	jmp	exit			# Jump to "exit" routine

exit_cmd_line_error:			# Exit with 1 exit code printing cmdline help
	call	print_help		# Call print_help function
	mov	$1, %rdi		# Put "1" exit code in %rdi register
	jmp	exit			# Jump to "exit" routine

.section .rodata
	help_msg:			# 19 bytes; cmdline help message
		.ascii "Usage: unlink FILE\n"
	err_msg:			# 13 bytes; generic string for system error
		.ascii "System error "