Word size

Also called: word width, word length, bits per word

The number of bits in each word of a memory: the n in 2ᵏ × n. It sets the number of data lines, independently of how many words there are.

The word size of a memory is how many bits each address holds. In the 2ᵏ × n notation it's the second number, n.

Word size fixes the number of data lines, one per bit, and the width of the data bus. It has nothing to do with the number of address bits, which depends only on how many words there are. You can change one without the other:

4K × 8 → 4K × 16: twice as many data lines, same 12 address lines.
4K × 8 → 8K × 8: one more address line, same 8 data lines.

Common word sizes are 1, 4, 8 (a byte), 16 and 32 bits.

When chips have a smaller word size than you need, put them side by side: that's width expansion. The number of chips is the wanted word size divided by the chip's word size.

The word size also matters when converting a capacity to words: words = total bits ÷ word size.

Worked examples

Example

Changing the word size

A 16K × 4 memory is widened to 16K × 8. What changes?

1.
Data lines: 4 → 8.
2.
Address lines: still 14, since there are still 16K words (2¹⁴).
3.
Capacity doubles: 64K bits → 128K bits.

Example

Chips needed for a wider word

How many 2K × 4 chips make a 2K × 24 memory?

1.
Word sizes: 24 wanted, 4 per chip.
2.
24 ÷ 4 = 6 chips, side by side.
3.
The number of words is already right, so no decoder is needed.

Common mistakes

Thinking a wider word needs more address lines. Address lines depend only on the number of words.
Confusing word size with the number of words. In 4K × 16, the word size is 16.
Assuming a word is always 16 bits. In a memory, the word is whatever one address holds.

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