Why "or ah,ah"?

Question

I'm slowly trying to get into osdev just to play around.

This tutorial has a piece of assembly that waits for a drive to become ready:

reset_drive:
   mov ah, 0
   int 13h
   or ah, ah
   jnz reset_drive

I get that after the interrupt, ah will be zero if the device is ready. But what does or ah,ah do? Seems a bit redundant... it appears to do nothing. (at least by my logic) What does it do?

It's [an inefficient alternative to `test ah,ah`](https://stackoverflow.com/a/33724806/224132), which is especially bad for a high-8 register because writing them creates partial-register merging issues on Intel CPUs, even Skylake. — Peter Cordes, Aug 04 '17 at 00:28

score 12 · Accepted Answer · answered Dec 22 '12 at 15:36

12

It sets/unsets the ZERO flag depending on whether ah is zero.

Depending on the status of the flag, jnz reset_drive will jump: Only if ah is not zero.

In other words, it waits for the device to become ready since it stops repeating after ah becomes zero.

answered Dec 22 '12 at 15:36

phant0m

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oh wow. I didn't know about `ZF`. Thank you for the quick answer! – Matthew Sainsbury Dec 22 '12 at 15:41
2

@Matt Welcome, basically all arithmetic operations set/unset the `ZF` and the other flags, this is not exclusive to `or`. – phant0m Dec 22 '12 at 15:42
I have experience with the dcpu-16 assembler, but not so hot on x86. dcpu is much easier to understand – Matthew Sainsbury Dec 22 '12 at 15:45

Why "or ah,ah"?

1 Answers1