IBM PC Compatible Computer Project – Technical Analysis

In order to recreate the IBM PC we first need to understand what it is made up of and how it all plays together. Here is a block diagram of how everything is related. (This was taken from the IBM PC Technical Reference Manual. I colored the buses to make the paths easier to follow.)

Below is a listing of the major components and their purpose. (I’ll start at the top-left of the page and move top to bottom and then left to right.)

Clock Generator
This is based around the Intel 8284A “Clock Generator And Driver” chip. It is mostly responsible for generating the clock to drive the processor and some of control signals.

The timing is generated using a 14.31818MHz crystal which the chip divides down by three to create the processor clock output (CLK) of 4.77MHz witch runs at a non-symmetrical duty cycle of 33%. (This is recommended for proper operation of Intel’s 8088/8086 processors.) This output is connected to the main processor, auxiliary processor socket, expansion slots and a delayed version is sent to the DMA controller.

The processor clock is then divided in half to create the peripheral clock output (PCLK) running at 2.38MHz with a symmetrical duty cycle of 50%. This is connected to the Programmable Interval Timer and the Programmable Peripheral Interface chips.

This chip also generates synchronous reset (RESET) and ready (READY) signals which are used to control the processor. Reset in the original IBM PC was tied to the Power Good line from the power supply and held the machine from booting until the power supply was fully operational. The ready signal is used to generate wait states for the processor when accessing slower peripherals.

Main Processor
IBM decided to use the Intel 8088 Microprocessor as the “brain” of the computer. The processor is clocked at a rate of 4.77Mhz.