RAM and ROM

Primary storage is memory the CPU can access directly. The two main types:

• RAM (Random Access Memory) — volatile (loses its contents when power is off). Holds the data, programs and OS currently in use. It is read/write.
• ROM (Read Only Memory) — non-volatile (keeps its contents without power). Holds the boot program (BIOS) that starts the computer. It is read-only.

Nail it: RAM is volatile and can be written to; ROM is non-volatile and (normally) only read. When you turn a computer on, ROM boots it, then the OS is loaded into RAM.

Which is volatile?

Virtual memory

If the RAM becomes full, the computer can use part of the secondary storage (e.g. the hard disk) as extra, temporary memory. This is called virtual memory.

• Data not currently needed is moved from RAM to virtual memory to free up RAM.
• Because secondary storage is much slower than RAM, relying on virtual memory makes the computer run more slowly.

Fix: adding more physical RAM reduces the need for virtual memory and speeds the computer up.

Secondary storage: magnetic, optical & solid state

Secondary storage holds data long term (non-volatile). Three technologies:

• Magnetic (HDD, magnetic tape) — data stored as magnetised patterns on spinning platters. High capacity, cheap per GB, but slower and has moving parts that can fail.
• Optical (CD, DVD, Blu-ray) — data read/written with a laser as pits and lands. Cheap and portable, but low capacity and easily scratched.
• Solid state (SSD, USB flash, memory card) — data stored in flash memory with no moving parts. Fast, silent, robust, low power, but more expensive per GB.

Choosing storage: weigh up capacity, speed, portability, durability, reliability and cost per GB for the job.

Which storage technology?

Tap a device, then tap the technology it uses.

Units of data & capacity

The smallest unit is the bit (a single 1 or 0). Bits are grouped into larger units:

OCR uses the decimal (×1000) convention. To work out how many bits/bytes a file needs, multiply the number of items by the size of each item.

Remember: nibble = 4 bits, byte = 8 bits. Going up a unit you divide by 1000; going down you multiply by 1000.

Your turn — bytes to KB

Your turn — image capacity

Binary, denary & hexadecimal

Computers use binary (base 2). Humans use denary (base 10). Hexadecimal (base 16) is a shorthand for binary — one hex digit = 4 bits (a nibble).

Hex digits: 0–9 then A=10, B=11, C=12, D=13, E=14, F=15. One hex digit ↔ one nibble (4 bits).

Your turn — binary to denary

Denary to hex

Binary addition & shifts

You must be able to add two 8-bit binary numbers and perform binary shifts.

• Addition rules: 0+0=0, 0+1=1, 1+1=10 (0 carry 1), 1+1+1=11 (1 carry 1).
• An overflow error happens if the result needs more than 8 bits (a carry out of the leftmost column).
• A left shift by 1 place multiplies by 2; a right shift by 1 place divides by 2. Bits shifted off the end are lost; new positions are filled with 0.

Nail it: shift left = ×2 per place; shift right = ÷2 per place.

Your turn — binary shift

Characters, images & sound

Everything is stored as binary. How different data is coded:

• Characters — each has a binary code in a character set. ASCII uses 7 bits (128 characters); Unicode uses more bits to cover many languages & emoji.
• Images — a bitmap is a grid of pixels; each pixel's colour is a binary number. Colour depth = bits per pixel; resolution = number of pixels. More of either → bigger, better-quality file.
• Sound — an analogue wave is sampled at intervals. Sample rate (samples per second, Hz) and bit depth (bits per sample) both raise quality and file size.

Character codes: in ASCII the letters are in order, so if 'A' = 65 then 'C' = 67 and 'a' = 97. Higher colour depth or sample rate = more bits = larger file.

ASCII letters

Compression: lossy vs lossless

Compression makes a file smaller so it needs less storage and transfers faster. Two types:

• Lossy — permanently removes some data (e.g. detail the ear/eye barely notices). Much smaller files, but quality is reduced and the original cannot be recovered (e.g. MP3, JPEG).
• Lossless — reduces size using patterns (e.g. an index of repeats) with no loss of data; the exact original can be perfectly rebuilt (e.g. PNG, ZIP, FLAC).

Choose: use lossless when every bit matters (text, program code, archives); use lossy when small size matters more than perfect quality (streaming music/photos).

Which compression?

Match term to meaning

Tap a term on the left, then its matching description on the right.

The big ideas to know

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