Computer memory

A computer stores everything as bits: binary digits, 0 or 1. Bits are grouped into bigger units:

• 4 bits make a nibble; 8 bits make a byte.
• 1 KB (kilobyte) = 1024 bytes; 1 MB = 1024 KB; 1 GB = 1024 MB; 1 TB = 1024 GB; and so on through PB, EB, ZB and YB (peta, exa, zetta, yotta).

Why 1024 and not 1000? Because 1024 = 2¹⁰, a power of two, which suits binary.

Primary memory is of two kinds:

• RAM (random access memory) holds the programs and data the computer is working on right now. It is much faster than secondary memory, but volatile: its contents vanish when the power goes off. That is why unsaved work is lost in a power cut.
• ROM (read-only memory) is non-volatile: it keeps its contents without power. It holds things that rarely change, such as the boot loader, the small program that starts the computer when it is switched on.

Cache memory is a very high speed memory placed between the CPU and RAM. It keeps copies of the data and instructions the CPU uses most often. The CPU looks in the cache first, and goes to RAM only if what it needs is not there. This speeds up the computer.

Secondary memory (hard disk drive HDD, solid state drive SSD, CD/DVD, memory card, pen drive) is non-volatile, much larger, slower and cheaper per byte. The CPU cannot use it directly: its contents must first be brought into main memory (RAM).

In speed: cache > RAM > secondary memory. In size and cheapness, the order is reversed.