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.

Answer 1

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).

Answer 2

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.