Question

This is currently a homework project that me and my teammate are stuck on. We haven’t been given much of an introduction into Assembly, and this is supposed to be our first homework exercise. The task is to create a program that converts 0xAABBCCDD into 0xDDCCBBAA.

I’m not looking for an answer, as that would defeat the purpose, but we are getting severely frustrated over the difficulty of this stupid thing. We think we have a good start in creating a viable solution, but we just cannot come up with the rest of the program.

First, we mask every single tupel (aa), (bb), (cc), (dd) into a different register:

LDR R0, LittleEndian    // 0xAABBCCDD
AND R1, R0, #0xFF000000 // 0xAA
AND R2, R0, #0x00FF0000 // 0xBB
AND R3, R0, #0x0000FF00 // 0xCC
AND R4, R0, #0x000000FF // 0xDD

Then we try to re-align them into the R0 register, but hell if we could come up with a good solution…

Our best effort came from:

ORR R0, R1, LSL #24 
ORR R0, R2, LSL #8
ORR R0, R3, LSR #8
ORR R0, R4, LSR #24

which produced 0xBBBBCCDD for some odd reason; we really don’t know.

Any hints would be greatly appreciated. Again, we are asking for help, but not for a solution.

Cheers!

Answer 1

I think you need to do (ARM a bit rusty):

MOV R0, R1, LSR #24 
ORR R0, R2, LSR #8
ORR R0, R3, LSL #8
ORR R0, R4, LSL #24

That is, MOV in the first instruction as you don’t want anything from the old R0 to affect the new value.

I can’t see any way to make this more ‘hint’ like as you’ve effectively done all the work anyway.