Just a Loop

Photo by Tine Ivanič on Unsplash

Hey everyone! This post is dedicated to Lab 5. I’ll say upfront that this was the most challenging loop of my career. Yes! If you told assembly, you were right. Our professor was kind to provide necessary codes from each platform. He also gave us access to several machines with different hardware capabilities for both architectures. Combined with the previous lecture, we had everything that we need to succeed.

The goal was to create a program that shows “loop” ten times. Next, we had to change it to include the index number. Finally, extend it to show thirty times with the index suppressing the leading zero. Don’t forget, on BOTH architectures! If it were on C, C++, Java, JavaScript or Bash, I would have time to a coffee.

To be fair, the first task was to put the “hello world” with the script provided. That was easy, and the only one that I accomplished. To compile it, I added a new target to the make file provided. The result you can see below.

	BINARIES=hello-nasm hello-gas hello-loop hello-loop2
	all: ${BINARIES}
	AS_ARGS=-g
	hello-nasm: hello-nasm.s
	nasm -g -o hello-nasm.o -f elf64 hello-nasm.s
	ld -o hello-nasm hello-nasm.o
	hello-gas: hello-gas.s
	as ${AS_ARGS} -o hello-gas.o hello-gas.s
	ld -o hello-gas hello-gas.o
	hello-loop: hello-loop.s
	as ${AS_ARGS} -o hello-loop.o hello-loop.s
	ld -o hello-loop hello-loop.o
	hello-loop2: hello-loop2.s
	as ${AS_ARGS} -o hello-loop2.o hello-loop2.s
	ld -o hello-loop2 hello-loop2.o
	clean:
	rm ${BINARIES} *.o || true

view raw Makefile hosted with ❤ by GitHub

	.text
	.globl _start
	min = 0 /* starting value for the loop index; note that this is a symbol (constant), not a variable */
	max = 10 /* loop exits when the index hits this number (loop condition is i<max) */
	_start:
	mov $min,%r15 /* loop index */
	loop:
	/* ... body of the loop ... do something useful here ... */
	movq $len,%rdx /* message length */
	movq $msg,%rsi /* message location */
	movq $1,%rdi /* file descriptor stdout */
	movq $1,%rax /* syscall sys_write */
	syscall
	inc %r15 /* increment index */
	cmp $max,%r15 /* see if we're done */
	jne loop /* loop if we're not */
	mov $0,%rdi /* exit status */
	mov $60,%rax /* syscall sys_exit */
	syscall
	.section .rodata
	msg: .ascii "Loop\n"
	len = . - msg

view raw hello-loop-x86_64.s hosted with ❤ by GitHub

	.text
	.globl _start
	min = 0 /* starting value for the loop index; note that this is a symbol (constant), not a variable */
	max = 30 /* loop exits when the index hits this number (loop condition is i<max) */
	_start:
	mov x19, min
	loop:
	/* ... body of the loop ... do something useful here ... */
	mov x0, 1 /* file descriptor: 1 is stdout */
	adr x1, msg /* message location (memory address) */
	mov x2, len /* message length (bytes) */
	mov x8, 64 /* write is syscall #64 */
	svc 0 /* invoke syscall */
	add x19, x19, 1
	cmp x19, max
	b.ne loop
	mov x0, 0 /* status -> 0 */
	mov x8, 93 /* exit is syscall #93 */
	svc 0 /* invoke syscall */
	.data
	msg: .ascii "Loop\n"
	len= . - msg

view raw hello-loop-aarch64.s hosted with ❤ by GitHub

The next is the indexed loop. Remembering the previous lecture, if we use a place holder in the message and change it before displaying, it would produce the desired output. Easy to say, hard to do it! I got stuck on how to change that little “X.” Even with my professor’s tip – use the binary move – I couldn’t figure it out. Hey Siri, could replace the X placeholder with the loop index, please? Siri just put some music on – Dammit, Blink-182! Here is the result of my failure.

	.text
	.globl _start
	min = 0 /* starting value for the loop index; note that this is a symbol (constant), not a variable */
	max = 10 /* loop exits when the index hits this number (loop condition is i<max) */
	_start:
	mov $min,%r15 /* loop index */
	loop:
	/* ... body of the loop ... do something useful here ... */
	movb $1,%al
	mov (%al),$msg
	movq $len,%rdx /* message length */
	movq $msg,%rsi /* message location */
	movq $1,%rdi /* file descriptor stdout */
	movq $1,%rax /* syscall sys_write */
	syscall
	inc %r15 /* increment index */
	cmp $max,%r15 /* see if we're done */
	jne loop /* loop if we're not */
	mov $0,%rdi /* exit status */
	mov $60,%rax /* syscall sys_exit */
	syscall
	.section .rodata
	msg: .ascii "Loop: X\n"
	len = . - msg
	counter = . - msg + 2

view raw hello-loop-fail.s hosted with ❤ by GitHub

The last challenge, I know some ways. We could use an “if” statement to see if the value is less than 10 to hide the leading zero. We could separate the index into two “variables” (one for decimals and the other for the units). However, I lack the knowledge of the language and the hardware to make it happen. Do I get some marks doing it in C or C++? Dammit is still playing in the background.

This is it! Epic failure and a great time playing with x86_64 and ARMv8. See you.