Basic Computers — Page 304, Lesson 367

We’re starting a brand-new topic now: the hardware of a basic digital computer. Last time we talked about the language computers work in — binary, the ones and zeros that carry out calculations and get stored in memory. Now I want to walk you through the physical construction, the components that actually make that happen. Every computer, whether it’s a PC or an aircraft system, is built from the same basic set of components. Let me take you through them one by one, starting with the heart of it all. The Central Processing Unit — the CPU. This is the component that performs, organizes, and controls all the operations the computer can carry out. It really is the brain of the computer. And here’s a key point: what the CPU is actually able to do is controlled by something called its instruction set. That instruction set defines the range of operations the CPU can perform. Now, the CPU itself is made up of three parts. The first is the Arithmetic Logic Unit — the ALU. This performs arithmetic calculations and logical operations, and it does all of this in the binary number system. So addition, subtraction, comparisons, that kind of thing — all done in binary. The second part is the Shift Registers. These are temporary stores. One of them in particular is called the accumulator, and it contains the data that is actually being processed at that moment. So think of the accumulator as the working register — the data you’re actively working on sits there. The third part is the Control Unit. This contains the computer’s clock. And that clock is a crystal-controlled oscillator which generates timing pulses at a fixed frequency — typically between 120 and 45 MHz. That’s a wide range, but the point is it’s a fixed frequency for a given computer. This clock synchronizes all the computer’s operations. Everything happens in step with those timing pulses. Now, the CPU doesn’t work in isolation. It needs to get data in and send results out. That’s where the Input and Output Devices come in. The CPU accepts digital signals from input devices. In the case of a PC, those would be things like a keyboard, a mouse, or a modem, and they come in via the input port. But in an aircraft, the inputs are different — they’re sensors. Things like Rad Alt — that’s radar altimeter — Baro Alt, barometric altitude, TAS, true airspeed, fuel flow, and so on. All of those sensors feed digital signals into the CPU. After the CPU has processed that data, it feeds the results out via its output port. On a PC, that output goes to a Visual Display Unit — the monitor — or to a printer. In an aircraft, the output might be fed to an EFIS Symbol Generator — that’s the Electronic Flight Instrument System — or to the FMS Control and Display Unit, the CDU. So the whole picture is: sensors and devices feed digital signals in through the input port, the CPU — with its ALU, shift registers, and control unit — processes that data in binary, and the results go out through the output port to displays or other systems. Take a look at Figure 23.1 — it shows the basic digital computer and how all these components connect together.