I'm trying to learn the basics of ARM assembly and wrote a fairly simple program to sort an array. I initially assembled it using the armv8-a option and ran the program under qemu while debugging with gdb. This worked fine and the program initialized the array and sorted it as expected.
Ultimately I would like to be able to write some assembly for my Raspberry Pi Pico, which has an ARM Cortex M0+, which I believe uses the armv6-m option. However, when I change the directive in my code, it compiles fine but behaves strangely in that the program counter increments by 4 after every instruction instead of the 2 that I expect for thumb. This is causing my program to not work correctly. I suspect that qemu is trying to run my code as if it were compiled for the full ARM instruction set instead of thumb, but I'm not sure why this is.
I am running on Ubuntu Linux 20.04 LTS, using qemu-arm version 4.2.1 (installed from the package manager). Does the qemu-arm executable only run full ARM binaries? If so, is there another qemu package I can install to run a thumb binary?
Here is my code if it is helpful:
.arch armv6-m
.cpu cortex-m0plus
.syntax unified
.thumb
.data
arr: .skip 4 * 10
len: .word 10
.section .text
.global _start
.align 2
_start:
ldr r0, arr_adr @ load the address of the start of the array into register 0
movs r1, #0 @ clear the counter register
movs r2, #100
init_loop:
str r2, [r0,r1] @ store r2's value to the base address of the array plus the offset stored in r1
subs r2, r2, #10 @ subtract 10 from r2
adds r1, #4 @ add 4 to the offset (1 word in bytes)
cmp r1, #40 @ check if we've reached the end of the array
bne init_loop
movs r1, #0 @ clear the offset
out_loop:
mov r3, r1 @ set the index of the minimum value to the current array index
mov r4, r1 @ set the inner loop index to the outer loop index
in_loop:
ldr r5, [r0,r3] @ load the minimum index's value to r5
ldr r6, [r0,r4] @ load the inner loop's next value to r6
cmp r6, r5 @ compare the two values
bge in_loop_inc @ if r6 is greater than or equal to r5, increment and restart loop
mov r3, r4 @ set the minimum index to the current index
in_loop_inc:
adds r4, #4
cmp r4, #40 @ check if at end of array
blt in_loop
ldr r5, [r0,r3] @ load the minimum index value into r5
ldr r6, [r0,r1] @ load the current outer loop index value into r6
str r6, [r0,r3] @ swap the two values
str r5, [r0,r1]
adds r1, #4 @ increment outer loop index
cmp r1, #40 @ check if at end of array
blt out_loop
loop:
nop
b loop
arr_adr: .word arr
Thank you for your help!
