§1.2 Memory and Storage

All data inside a computer is ultimately stored as binary — sequences of 0s and 1s. A bit is the smallest unit; 8 bits make a byte; units scale by 1,000 from there (kilobyte → megabyte → gigabyte → terabyte → petabyte). The same binary digits can represent different things — a number, a character, part of an image — depending on context and encoding. ASCII and Unicode define how characters are encoded; image and sound files are binary sequences whose file size can be calculated from colour depth × resolution and sample rate × duration × bit depth respectively.

Memory sits in a hierarchy. Cache (tiny, very fast) feeds the CPU first; RAM (fast, volatile) holds running programs and data; secondary storage (slow, non-volatile — magnetic, optical, solid-state) holds everything else when the power is off. When RAM is full, the OS moves data to a virtual memory area on secondary storage — slower, but it prevents crashes. The choice of secondary storage type depends on the application: SSDs are fast but expensive; HDDs are slow but cheap and large; optical media (CD/DVD/Blu-ray) is portable but slow.

What students must understand

Primary storage: RAM (volatile, fast, holds running programs) vs ROM (non-volatile, holds boot instructions)

Virtual memory: overflow from RAM to secondary storage when RAM is full; slower but prevents crashes

Cache: very fast memory between CPU and RAM; hit/miss; larger cache → fewer slow RAM accesses

Secondary storage: optical (CD/DVD/Blu-ray), magnetic (HDD), solid-state (SSD) — compare on capacity, speed, portability, durability, reliability, cost

Units: bit, nibble (4 bits), byte (8 bits), KB, MB, GB, TB, PB (each 1,000 × previous)

Binary to denary and back (up to 8 bits); binary addition including overflow

Hexadecimal: base-16; convert denary ↔ hex (2 digits, 00–FF); convert binary ↔ hex

Binary shifts: left shift multiplies by 2; right shift divides by 2

Characters: character sets; ASCII (8-bit, 256 characters); Unicode (up to 32-bit, covers all languages)

Images: pixels; colour depth and resolution affect quality and file size; metadata

Sound: sample rate (Hz), bit depth, duration; higher values → better quality, larger files

Compression: lossy (some data removed permanently, e.g. MP3, JPEG) vs lossless (data fully recoverable, e.g. PNG, ZIP)

Diagrams

A RAM module. Primary storage is fast and volatile, holding running programs and their data.

Diagram of the surface of an optical disc showing pits and lands and the binary data they encode

Optical storage records data as pits and lands. Each transition between them is read as a 1.

A grid of ones and zeros shown as black and white pixels, illustrating how a bitmap image is stored in binary

A bitmap is just binary. Each pixel's colour is stored as a binary value, here 1 bit per pixel.

An analogue sound wave with sample points marked at regular time intervals

Sampling. The analogue wave is measured at regular intervals, and each sample is stored as a binary number.

The full ASCII character table with decimal and 7-bit binary codes for each character

The ASCII character set. Each character maps to a 7-bit binary code.

Diagrams: PG Online (Paul Long), OCR J277 textbook. Used under the school site licence.

Key formulas and facts

What students must understand

Diagrams

Linking questions

Practice

Video Support