ARM Assembly loop using PC?

1
votes

I am currently learning arm assembly and I have some questions. When reading docs, I've found that the register nº 15 is the program counter that stores the next instruction adress, and when an instruction is done, it is incremented by 4 (bytes, or 2 in thumb mode).

So, my question is, if I run an instruction that changes PC by itself less 4 bytes, would it return to the instruction before, won't it? Then back and over and over again so it will be an infinite loop?

Thanks, and sorry if it is an obvious question. Regards, Pedro.

3
loopsassemblyarm
Yes, you can code an infinite loop. You can try this kind of stuff in a debugger. - Jester

3 Answers

1
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You have to look on an instruction by instruction basis, as some have modification of the PC being unpredictable, but for those where it is legal modification of the program counter essentially causes a jump to the address you save in the program counter. You dont have to worry about the two instructions ahead thing (it is 8 and 4 bytes not 4 and 2, two instructions ahead).

0
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Yes - a jump/branch instruction is exactly what you're describing - it's an instruction which modifies the PC. If you arrange the result of the jump to put the program counter back where it was then, yes, you'll loop on the spot.

0
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Note that this is not really the address of the next instruction but the address of the current instruction +4 (In Thumb mode) or +8 (In ARM mode). So in ARM this is 2 instructions later, but in Thumb it may not be (As instructions can be 16-bit or 32-bit)