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.

This makes the timer a good example of hardware that is configured indirectly. You do not write the desired frequency into the device. You compute the divisor that will make the device produce approximately that frequency. The controller then keeps counting independently from your program until it is reconfigured.

Control Words

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

When you read examples online, you may see control words written as fixed hexadecimal constants. That is acceptable for a narrow demo, but it is not a good learning strategy. In this lab, build the byte from named fields. The code should show which timer is selected, which access mode is used, which operating mode is requested, and whether BCD is enabled.

IMPORTANT

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