Word line

Also called: word lines, row line, row select line

A wire driven by a memory's address decoder that runs along one row of cells. When it is 1, that row is connected to the data (bit) lines.

Inside a memory, cells are arranged in rows. Each row has a word line, one output of the address decoder, running along it.

When a word line is 1, every cell on that row is connected to its column's bit line, so the row can be read or written.
When it is 0, the row's cells are isolated and keep their values.

Because the decoder's outputs are one hot, exactly one word line is active for any address, and so exactly one row takes part in each access.

In a simple memory, one row is one word, so word line Wᵢ selects word i. In a large chip with two dimensional decoding, a word line selects a whole row of the grid, and a column decoder then picks the bits you want from it.

In a ROM, a stored 1 is a connection from a word line to a data bit's OR gate. When the word line goes high, exactly the bits with connections become 1.

Worked example

Example

Reading a ROM through its word lines

A 4 × 3 ROM has D2 = W0 + W2, D1 = W1 + W3, D0 = W3. Read address 11.

1.
Address 3: only W3 is 1.
2.
W3 feeds the D1 and D0 gates, not D2.
3.
Output D2 D1 D0 = 011.

Common mistakes

Thinking several word lines can be active at once. With the decoder enabled, exactly one is.
Mixing up word lines and data lines. Word lines pick the row; data (bit) lines carry the bits.

Practice Word line

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

Memory lesson full course

Learn it step by step

Word line is taught in Memory.

Memory →

All terms