questions about MOVB/MOVBS/MOVBU of arm

[Ben Shi]

1. MOVB R3, R4 is assembled the same code to MOVW R3, R4, why?

2. What is difference between MOVBU and MOVB


Ben Shi

[Russ Cox]

On Tue, Jun 13, 2017 at 5:44 AM, Ben Shi <power...@163.com> wrote:

1. MOVB R3, R4 is assembled the same code to MOVW R3, R4, why?

Because there are no byte registers on ARM, so if both args are registers it's fine to move the whole word.

 

2. What is difference between MOVBU and MOVB

The sign-extension behavior when reading from memory into a register: MOVBU zero-extends (treats the byte as unsigned) into the full-width register, while MOVB sign-extends (treats the byte as signed).

Russ

[Ben Shi]

There are thrree MOVB：

MOVB MOVBS MOVBU，

if MOVB imply sign extention

what is the difference between MOVB and MOVBS?

Ben Shi

[cherry]

It looks like MOVB does a sign extension for load (you have to extend one way or the other), but not for register move; MOVBS does a sign extension for both load and register move.

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[Ben Shi]

Shall we delete the movb reg, reg 

and movh reg, reg

They are quite confusing.

Ben Shi

[cherry]

No, don't delete instructions. It may break user code.

They are less confusing when you think it as "move a byte, I don't care what the high bits are"...