How to create an array of booleans in arm assembly? - gcc

Question

I need to specify each boolean manually like in a fixed table, so using
Array: .skip 400
I will be declaring an array of 400 bytes,so how can i set the boolean values?

Answer 1

ARM registers are 32 bits each. You only need a bit to represent a boolean. So you can use the following 'C' code to access an array,
uint32_t load_bool(uint32_t index)
{
return (bool_array[index>>2] & (1<<(index&3)));
}
void store_bool(uint32_t index, int value)
{
uint32_t target = bool_array[index>>2];
if(value)
target |= (1<<(index&3));
else
target &= ~(1<<(index&3));
bool_array[index>>2] = target;
}
Use a compiler to target your CPU; for instance tuning godbolt output on a Cortex-A5 gives,
load_bool(unsigned int):
ldr r3, =bool_array
mov r2, r0, lsr #2
ldr r3, [r3, r2, asl #2]
and r0, r0, #3
mov r2, #1
and r0, r3, r2, asl r0
bx lr
store_bool(unsigned int, int):
ldr r3, =bool_array
mov r2, r0, lsr #2
cmp r1, #0
ldr r1, [r3, r2, asl #2]
and r0, r0, #3
mov ip, #1
orrne r0, r1, ip, asl r0
biceq r0, r1, ip, asl r0
str r0, [r3, r2, asl #2]
bx lr
The instructions tst, bclr, etc might be useful if you choose a macro instead of a function call (bit index known at compile/assemble time). Also, ldrb or byte access might be better on older platforms/CPUs. Most ARM CPUs have a 32bit bus, so the cycles for ldrb and ldr are equal.

Answer 2

Boolean variables in C and C++ are basically treated as a native integer assigned 1 for true and 0 for false; in ARM's case it would be a 32-bit integer. So if you need to access the structure as an array of Booleans in C/C++ you would need to access them as 32-bit integers aligned on a 4-byte boundary. However if you only need to access it from other assembly code you can use each byte as it's own boolean variable and simply manipulate the array on a byte level.
In ARM assembly, this would be the difference between accessing the array with LDR vs with LDRB.