Basic Computers — Page 315, Lesson 369

Let's start with the BIOS, because it's the gatekeeper of the whole system. BIOS stands for Basic Input Output System. Its job is to convert the input signals into a form the computer can actually work with, and then convert the outputs into a form that the operator—or another aircraft system—can understand. So think of it as the translator between the raw electrical signals and the meaningful data the computer processes. Now, the heart of the matter is memory. A computer needs a working memory to run the programme of instructions—that's the software—it has to execute. If that programme is fixed, like in a computer-controlled piece of equipment, the memory only has to be read. For that, we use a Read Only Memory, or ROM. This ROM is programmed by the manufacturer, so you can't change it. But what if you, the user, want to construct or modify the programme yourself and keep it permanently? Then you'd use a Programmable Read Only Memory, a PROM. Or an Erasable Programmable Read Only Memory, an EPROM. Or an Electronically Erasable Programmable Read Only Memory, an EEPROM. The key difference is in how they're programmed and erased—PROM is one-time programmable, EPROM can be erased with ultraviolet light, and EEPROM can be erased electronically. But all of them let you write your own permanent programme. Now, here's a critical distinction for aviation. Memory that retains data when the power is switched off is called NON-VOLATILE MEMORY. Memory that loses data in the event of a power failure or switch off is called VOLATILE MEMORY. So ROM is non-volatile—it keeps its programme even without power. RAM, which we'll get to, is typically volatile. If the programme has to be changed during operation, then the memory must be able to be written to as well as read. That's where Random Access Memory, or RAM, comes in. RAM allows instructions to be written in, read out, and altered at will. It's also required to store the data for processing, because that data changes continually. But remember, RAM is normally volatile memory—so if you lose power, you lose the work. Now, for permanent storage—because RAM is volatile and work is lost when power is removed—we have three different ways to store computer programs and the work they generate. Let me walk you through each. First, the electronic chip. This is solid state memory, often known as "Flash Memory." It has very fast access and is re-writable. And importantly, it may be re-written indefinitely. Second, the optical disk. These are compact discs, or CDs, read with a laser. They may be either read only or re-writable. But here's the catch: if they're re-writable, there will only be a very limited number of times they can be re-written. So unlike flash memory, they don't last forever. Third, the magnetic disk. These may be internal, external, or removable. They may be re-written indefinitely. The internal "hard drive" on a computer is traditionally a magnetic disk. And removable "floppy disks" are now virtually obsolete. Finally, let's talk about software, specifically Assembly Language. This is a low-level programming language for computers and other programmable devices. It's a symbolic representation of the machine codes, and other constants, needed to program a given CPU. The code is usually defined by the hardware manufacturer. So instead of writing in binary, you use symbols that represent the machine codes—but it's still very close to the hardware level. Let me tie this together for you. In an aircraft system, you have the BIOS translating signals, ROM holding the fixed programme, RAM for the changing data during operation, and permanent storage—flash, optical, or magnetic—for the long-term programs and generated work. Each type of memory has its role, and knowing which is volatile and which is non-volatile is crucial for understanding what happens when power is lost.