OUT sends the accumulator to an I/O port: AL as a byte, or AX as a word, in which case AL goes to port p and AH to port p+1. The port comes first, as in every destination-first 8086 instruction. It is an immediate from 0 to FFh or, for any port up to FFFFh, the value in DX. No other register can be the source, and no flags change.

A beep

Timer channel 2 of the 8253/8254 drives the PC speaker. Port 43h selects the channel and mode, port 42h takes the divisor of the 1193182 Hz input clock (low byte, then high byte), and bits 0 and 1 of port 61h connect the timer to the speaker. Port 61h holds other control bits too, so the program reads it, changes two bits, and writes it back.

A short beep; its length depends on the emulated CPU speed.
MOV     AL, 0B6h
OUT     43h, AL   ; timer 2, low then high byte, square wave
MOV     AX, 1193  ; 1193182 / 1193: about 1000 Hz
OUT     42h, AL   ; low byte of the divisor
MOV     AL, AH
OUT     42h, AL   ; high byte
IN      AL, 61h
OR      AL, 3     ; gate the timer, enable the speaker
OUT     61h, AL   ; sound on
MOV     BX, 20
DELAY1:
MOV     CX, 0FFFFh
DELAY2:
LOOP    DELAY2
DEC     BX
JNZ     DELAY1
IN      AL, 61h
AND     AL, 0FCh  ; clear bits 0 and 1
OUT     61h, AL   ; sound off

Ports above FFh

Video card registers sit at 3B0h-3DFh, so they need DX. The VGA palette takes a colour number at 3C8h, then three values (red, green, blue, 0 to 63 each) at 3C9h. INC DX moves from one to the next.

MOV     DX, 3C8h  ; VGA DAC write index
MOV     AL, 1     ; colour 1
OUT     DX, AL
INC     DX        ; 3C9h: the data port
MOV     AL, 63
OUT     DX, AL    ; red
MOV     AL, 32
OUT     DX, AL    ; green
MOV     AL, 0
OUT     DX, AL    ; blue: colour 1 is now orange

Pitfalls

Writing to a port tells hardware to do something, and a wrong value can hang a device or, on real machines, damage data: write only to ports you know. Keep the other bits of shared control ports such as 61h. Some old devices need a short pause between two writes to the same chip. Multi-step protocols (index, then data) break if an interrupt handler touches the same device in between; such sequences are often wrapped in CLI / STI.

Later CPUs

The 80186 adds OUTSB and OUTSW to write a block from DS:SI. The 80386 writes 32 bits with OUT DX, EAX, and protected mode checks the I/O privilege level before allowing the write.