The i8254 timer controller

The i8254 PIT has several counters. Lab 2 focuses on Timer 0, the periodic interrupt source.

Field	Purpose
timer select	choose which counter is programmed
access mode	decide low byte, high byte, or both
operating mode	choose counting behavior
BCD/binary	choose encoding
divisor	convert base frequency into output frequency

The base frequency is divided by the programmed divisor. A lower divisor means more frequent interrupts. You do not write the desired frequency; you compute the divisor that produces approximately that frequency.

Control words

A control word is a byte made from fields. Lab 1 matters here: construct the value by shifting and OR-ing fields, not by guessing a decimal constant.

Online examples often use fixed hexadecimal control words. That is fine for a narrow demo, but here build the byte from named fields so the timer, access mode, operating mode, and BCD setting are visible.

IMPORTANT

Timer programming has order. If the access mode says low byte then high byte, write the low byte first.