Word (memory)

Also called: memory word, memory location, storage location, memory contents

The fixed-size group of bits stored at one memory address and always read or written as a unit. Its width is the memory's word size.

A memory is a long row of numbered boxes. Each box is a location, and the bits inside it are a word.

Every word in a given memory has the same width, the word size. A ×8 memory holds 8-bit words, a ×16 memory 16-bit words. The word is the unit of transfer: one read returns a whole word and one write replaces a whole word.

Keep three ideas apart:

The address says which location. It is fixed by where the location sits.
The word (the contents) is what is stored there. It changes whenever you write.
The word size is how many bits each location holds. It equals the number of data lines.

Two locations can hold the same word; contents don't have to be unique, addresses do.

The number of words is a separate number again. In a 1K × 8 memory, 1K is the number of words (which fixes the address bits), and 8 is the word size.

Careful: "word" also means a CPU's natural data size (often 32 or 64 bits). In a memory question, a word is simply whatever one address holds, even a single bit or a byte.

128

Worked examples

Example

A tiny 4 × 8 memory

Four locations, each holding an 8-bit word.

1.
Address 0: 10110010. Address 1: 00000000. Address 2: 10110010. Address 3: 11110000.
2.
Reading address 3 returns the whole word 11110000.
3.
Addresses 0 and 2 hold the same word. That's allowed.
4.
Size: 4 words × 8 bits = 32 bits. The 4 counts words; the 32 counts bits.

Example

Writing a word

Write 00001111 to address 1 of the memory above.

1.
The old word at address 1, 00000000, is replaced completely.
2.
Address 1 now holds 00001111. No other location changes.
3.
The address is still 1; only the contents moved.

Common mistakes

Mixing up the address with the contents. Address 3 is the location's number; the word is what's stored there.
Using the bit count where the word count belongs. A 16 × 8 memory has 16 words, not 128.
Assuming a word is always 16 or 32 bits. In memory notation it is whatever one address holds.

Practice Word (memory)

Interactive questions with instant feedback and a worked solution for every wrong answer.

Memory lesson full course

Basic CPU / Computer Architecture lesson full course

A single-cycle CPU can fetch an instruction and read that instruction's data operand from the same single-ported memory within one clock cycle.

Practice exams

Learn it step by step

Word (memory) is taught in Memory and Registers.

Memory →Registers →

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