Thus far, there has been no need to understand the inner workings of the computer in order to do useful experiments. It has been a black box which responds in a reliable way when given instructions. Just as in using a car, many times this is sufficient; however, to utilize its capabilities as a tool in the laboratory fully, the internal operation of the computer should be understood. In this chapter, we will look under the hood to explore the internal organization of the IBM-PC and to learn to program the 8088 microprocessor directly.

Inside the IBM-PC

A first glance under the cover of the IBM-PC shows a circuit board with a row of connectors which contain other circuit boards standing vertically. The horizontal board (the mother board) contains the 8088 microprocessor chip and various other chips which control the keyboard and screen and contain the memory cells. The microprocessor is the CPU which controls the system and executes the program instructions. The boards in the connectors perform a variety of other functions. Figure 6.1 shows the general organization.

The different chips and circuit boards communicate with each other via the buss: a group of 62 wires which carry digital signals. There are 20 address lines, 8 data lines, and 34 auxiliary lines. The data lines contain the 8 bits of data which are to be transferred by the CPU. The bits on the address lines specify the binary number of the location from or to which the data will be transferred. The operation of the computer is, at the lowest level, a controlled transfer and manipulation of bits of data among various chips and devices.