what does "outb" in AT&T asm mean?

Question

I am reading some code of Linux. The keyboard.S has something like

outb %al,$0x61

and

inb $0x61,%al

I think the pending 'b' means 'byte', but i still cannot find what these instructions mean.

Intel cores have an I/O bus, distinct from the memory bus. Port 0x61 is a keyboard control register. http://www.tek-tips.com/viewthread.cfm?qid=258828 — Hans Passant, Dec 03 '11 at 05:52
Possible duplicate of [What are IN & OUT instructions in x86 used for?](http://stackoverflow.com/questions/3215878/what-are-in-out-instructions-in-x86-used-for) — Ciro Santilli OurBigBook.com, Oct 30 '15 at 20:21

score 19 · Accepted Answer · edited Feb 20 '13 at 14:02

19

OUT and IN are instructions for transferring data between I/O ports. Those are a couple of links I found which provide a bit more information.

As you mentioned, the b is the byte form of those instructions, which is appropriate since you're using the lower byte of ax (al).

edited Feb 20 '13 at 14:02

nemo

answered Dec 03 '11 at 05:36

AusCBloke

score 5 · Answer 2 · answered Dec 03 '11 at 05:32

5

Yes, the out port and in port instructions can be encoded for 8 bits of data or 16 bits of data. The b indicates byte which is the 8 bit version.

answered Dec 03 '11 at 05:32

old_timer

2 Answers2