I am trying to figure out how arrays work in ARM assembly, but I am just overwhelmed. I want to initialize an array of size 20 to 0, 1, 2 and so on.
A[0] = 0
A[1] = 1
I can't even figure out how to print what I have to see if I did it correctly. This is what I have so far:
.data
.balign 4 @ Memory location divisible by 4
string: .asciz "a[%d] = %d\n"
a: .skip 80 @ allocates 20
.text
.global main
.extern printf
main:
push {ip, lr} @ return address + dummy register
ldr r1, =a @ set r1 to index point of array
mov r2, #0 @ index r2 = 0
loop:
cmp r2, #20 @ 20 elements?
beq end @ Leave loop if 20 elements
add r3, r1, r2, LSL #2 @ r3 = r1 + (r2*4)
str r2, [r3] @ r3 = r2
add r2, r2, #1 @ r2 = r2 + 1
b loop @ branch to next loop iteration
print:
push {lr} @ store return address
ldr r0, =string @ format
bl printf @ c printf
pop {pc} @ return address
ARM confuses me enough as it is, I don't know what i'm doing wrong. If anyone could help me better understand how this works that would be much appreciated.
Answer
This might help down the line for others who want to know about how to allocate memory for array in arm assembly language here is a simple example to add corresponding array elements and store in the third array.
.global _start
_start:
MOV R0, #5
LDR R1,=first_array @ loading the address of first_array[0]
LDR R2,=second_array @ loading the address of second_array[0]
LDR R7,=final_array @ loading the address of final_array[0]
MOV R3,#5 @ len of array
MOV R4,#0 @ to store sum
check:
cmp R3,#1 @ like condition in for loop for i>1
BNE loop @ if R3 is not equal to 1 jump to the loop label
B _exit @ else exit
loop:
LDR R5,[R1],#4 @ loading the values and storing in registers and base register gets updated automatically R1 = R1 + 4
LDR R6,[R2],#4 @ similarly
add R4,R5,R6
STR R4,[R7],#4 @ storing the values back to the final array
SUB R3,R3,#1 @ decrment value just like i-- in for loop
B check
_exit:
LDR R7,=final_array @ before exiting checking the values stored
LDR R1, [R7] @ R1 = 60
LDR R2, [R7,#4] @ R2 = 80
LDR R3, [R7,#8] @ R3 = 100
LDR R4, [R7,#12] @ R4 = 120
MOV R7, #1 @ terminate syscall, 1
SWI 0 @ execute syscall
.data
first_array: .word 10,20,30,40
second_array: .word 50,60,70,80
final_array: .word 0,0,0,0,0