Review

The main components of the computer of most significance to programmers are disk, RAM, and the CPU; the first two of these store programs and data that are used by the CPU.

Computers represent pieces of information (or data) as binary digits, universally referred to as bits. Each bit can have the value 0 or 1. The binary system is used instead of the more familiar decimal system because it is much easier to make devices that can store and retrieve 1 of 2 values, than 1 of 10. Bits are grouped into sets of eight, called bytes.

The disk uses magnetic recording heads to store and retrieve groups of a few hundred to a few thousand bytes on rapidly spinning platters in a few milliseconds. The contents of the disk are not lost when the power is turned off, so it is suitable for more or less permanent storage of programs and data.

RAM, which is an acronym for Random Access Memory, is used to hold programs and data while they're in use. It is made of millions of microscopic transistors on a piece of silicon called a chip. Each bit is stored using a few of these transistors. RAM does not retain its contents when power is removed, so it is not good for permanent storage. However, any byte in a RAM chip can be accessed in about 10 nanoseconds, which is about a million times as fast as accessing a disk. Each byte in a RAM chip can be independently stored and retrieved without affecting other bytes, by providing the unique memory address belonging to the byte you want.

The CPU (also called the processor) is the active component in the computer. It is also made of millions of microscopic transistors on a chip. The CPU executes programs consisting of instructions stored in RAM, using data also stored in RAM. However, the CPU is so fast that even the typical RAM access time of 10 nanoseconds is a bottleneck; therefore, computer manufacturers have added both external cache and internal cache, which are faster types of memory used to reduce the amount of time that the CPU has to wait. The internal cache resides on the same chip as the CPU and can be accessed without delay. The external cache sits between the CPU and the regular RAM; it's faster than the latter, but not as fast as the internal cache. Finally, a very small part of the on-chip memory is organized as registers, which can be accessed within the normal cycle time of the CPU, thus allowing the fastest possible processing.