Basic Computers — Page 304, Lesson 367

I want to walk you through the hardware of a basic digital computer. We've already talked about binary being the language computers work in — the way calculations are carried out and information is stored in memory. Now let's look at the physical construction, the components that actually do that work. Every computer, whether it's a PC or an aircraft system, is built from the same basic components. Let me introduce you to the heart of it: the Central Processing Unit, or CPU. The CPU performs, organizes, and controls all the operations the computer can carry out. It's really 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's the fixed list of operations the CPU knows how to perform. Now, the CPU itself is made up of three main parts. The first is the Arithmetic Logic Unit, which we call the ALU. The ALU performs arithmetic calculations and logical operations, and it does all of this in the binary number system — ones and zeros. 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 actually being processed at any given moment. So while the ALU does the math, the accumulator holds the number we're working on right now. The third part is the Control Unit. This is where the computer's clock lives. The clock is a crystal-controlled oscillator that generates timing pulses at a fixed frequency — typically between 120 and 45 MHz. That timing is what synchronizes all the computer's operations. Everything happens in step with those pulses. Now let's look at how data gets in and out. The CPU accepts digital signals from input devices. On a PC, those inputs are things like a keyboard, a mouse, or a modem, and they come in through the input port. But in an aircraft, the inputs are different — they're sensors. Things like Rad Alt, which is radar altimeter; Baro Alt, the barometric altimeter; TAS, true airspeed; and fuel flow. All of those feed digital signals into the CPU. After the CPU processes that data, it feeds the results out through its output port. On a PC, the output goes to a Visual Display Unit — that's the monitor — or to a printer. But in an aircraft, the output might go to an EFIS Symbol Generator, which is what creates the electronic flight instrument displays, or to the FMS Control and Display Unit, the CDU — that's the interface you use to talk to the Flight Management System. So the whole picture is this: sensors feed data in through the input port, the CPU processes it using the ALU, shift registers, and control unit, and then the results go out through the output port to the displays. That's the basic architecture of every digital computer, and it's exactly the same principle whether you're looking at a desktop PC or the avionics in the cockpit.