Hi this is Tecca, and in this post I will be demonstrating my findings with assembly language in the AArch64, an ARM architecture, and x86_64, an x86 architecture.
The goal of this lab is to generate output like the following in both AArch64 and x86_64:
Loop: 0
Loop: 1
Loop: 2
Loop: 3
Loop: 4
...
Loop: 30
etc.
Source code for this lab in x86_64
.text
.globl _start
_start:
print: mov $0,%r15 /* loop index */
loop: mov %r15,%r14
add $'0',%r14
movb %r14b,msg+6
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 the index */
cmp $10,%r15 /* check if we're done */
jne loop /* keep looping if we're not */
movq $0,%rdi /* exit status */
movq $60,%rax /* syscall sys_exit */
syscall
.section .data
msg: .ascii "Loop: #\n"
len = . - msg
Result:
Moving onto AArch64 source code:
.text
.globl _start
_start:
mov x19, 0
loop:
add x20, x19, '0' // Create digit character
adr x17, msg+6 // Get a pointer to desired location of digit
strb w20, [x17] // Put digit to desired location
mov x0, 1 /* file descriptor: 1 is stdout */
adr x1, msg /* message location (memory address) */
mov x2, len /* message length (bytes) */
mov x8, 64
svc 0 /* invoke syscall */
add x19, x19, 1
cmp x19, 10
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
This is done from storing value defined from the given range into a register, an ASCII value is then converted from the value within the register.
The value ‘0’ is stored into a register, which in turn is converted into an ASCII value of 48 in decimal. After the loop incremented it to 10, the program would not print an integer character as expected, there is no associated ASCII reference for '10'.
I've tried adding 10 to the ASCII value of ‘0’, the resulting ASCII value prints out a colon “:” instead of the expected number 10 because 48+10=58 and 58 is a ":".
I'll focus on making it loop past 10 and onward in the next post.
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