Basic Computers — Page 315, Lesson 369

Let's start with the BIOS, because it's the bridge between the computer and everything outside it. 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 back into a form that the operator, or another aircraft system, can understand. So think of it as the translator at the boundary of the computer. Now, the heart of the matter is memory. A computer needs a working memory to run the programme of instructions — that's the software — that it has to execute. Now here's the key distinction. If the programme is fixed, like in a computer-controlled piece of equipment, then the memory only has to be read. For that, we use a Read Only Memory, which we call ROM. This ROM is programmed by the manufacturer, so you can't change it. But what if the user wants to construct or modify the programme himself and keep it permanently in memory? Then we'd use a Programmable Read Only Memory — PROM. Or an Erasable Programmable Read Only Memory — EPROM. Or an Electronically Erasable Programmable Read Only Memory — EEPROM. Each one gives the user progressively more ability to write and erase the programme. Now, there's a fundamental classification of memory based on power. 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. That's the whole distinction — non-volatile survives, volatile doesn't. Now, 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. And here's the catch — RAM is normally volatile memory. So when power goes, the work in RAM is lost. That brings us to permanent memory. Since RAM is volatile and work is lost when power is removed, permanent storage for computer programs and the work they generate can be stored in three different ways. First, an electronic chip. This is solid state memory, often known as "Flash Memory". It has very fast access and is re-writable, and it may be re-written indefinitely. Second, an optical disk. These are compact discs, CDs, read with a laser. They may be either read only or re-writable, but here's the limitation — there will only be a very limited number of times they can be re-written. Third, a 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. Removable "floppy disks" are now virtually obsolete. Now let's move to software, and 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. And importantly, the code is usually defined by the hardware manufacturer. So it's the most direct way to talk to the processor, one step above raw machine code. Let me show you how all this fits together in a basic digital computer. That figure shows the Central Processing Unit — the CPU — which performs, organizes, and controls all the operations the computer carries out. So you have the CPU at the centre, the BIOS translating at the edges, and the memory hierarchy — ROM for fixed programmes, RAM for changing data, and the permanent storage options for keeping work when power is off.