雲林SONG: Linux：組合語言(使用GNU assembler與 ld)

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底下幾個範例是給了解CPU register 與懂一點組語的人所看的。

範例一：輸出訊息。

	//----------------------------------------------------------------
	// first.s: Output "Welcome to Assembly World."
	//
	// compile using: $ as first.s -o first.o
	// link using: $ ld first.o -o first
	// execute using: $ ./first
	//
	//----------------------------------------------------------------

	.section .data
	msg: .string "Welcome to Assembly World.\n" # the output message
	len = . - msg # the length of the message

	.section .text
	_start: movl $4, %eax # system-call number for sys_write
	movl $1, %ebx # device-file id-number for stdout
	movl $msg, %ecx # address of the message string
	movl $len, %edx # length of the message string
	int $0x80 # trap-instruction for system-call

	movl $1, %eax # system-call number for sys_exit
	movl $0, %ebx # value for the exit-code
	int $0x80 # trap-instruction for system-call

	.globl _start # makes entry-point visible (can omit this)

view raw first.s hosted with ❤ by GitHub

範例二：輸出訊息(ASCII code方式)。

	//----------------------------------------------------------------
	// Holan.s: print the 'Holan' in ascii code
	//
	// compile using: $ as Holan.s -o Holan.o
	// link using: $ ld Holan.o -o Holan
	// execute using: $ ./Holan
	//
	//----------------------------------------------------------------
	.text # this section is program
	.global _start # entry point

	_start:
	# write our string to stdout
	movl $len,%edx # third argument: message length
	movl $msg,%ecx # second: pointer to message
	movl $1,%ebx # Holan: file handle (stdout)
	movl $4,%eax # system call number (sys_write)
	int $0x80 # call kernel
	# and exit
	movl $0,%ebx # Holan argument: exit code
	movl $1,%eax # system call number (sys_exit)
	int $0x80 # call kernel

	.data # this section is data
	msg:
	.byte 72,111,108,97,110,10
	len = . - msg # the length of the message

view raw Holan.s hosted with ❤ by GitHub

範例三：加法。

	//----------------------------------------------------------------
	// sum.s:
	//
	// compile using: $ as sum.s -o sum.o
	// link using: $ ld sum.o -o sum
	// execute using: $ ./sum
	//
	//----------------------------------------------------------------
	.text # this section is program
	.global _start # entry point

	_start:

	movl $1, %eax # put 1 into eax
	addl $8, %eax # add 8 into eax
	addl $48, %eax # convert digit to ASCII
	movl $msg, %ecx # put address of msg into ecx
	movb %al, (%ecx) # store byte in al in location ecx
	# points to

	# write our string to stdout
	movl $len,%edx # third argument: message length
	movl $msg,%ecx # second: pointer to message
	movl $1,%ebx # sum: file handle (stdout)
	movl $4,%eax # system call number (sys_write)
	int $0x80 # call kernel
	# and exit
	movl $0,%ebx # sum argument: exit code
	movl $1,%eax # system call number (sys_exit)
	int $0x80 # call kernel

	.data # this section is data
	msg:
	.byte 72,10
	len = . - msg

view raw sum.s hosted with ❤ by GitHub

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