Data lines

Also called: data pins, data I/O lines, data inputs and outputs, D lines

The pins a memory uses to send and receive words, one per bit. A 2ᵏ × n memory has n data lines, equal to its word size.

A memory chip's data lines carry the word being read or written, one wire per bit. In a 2ᵏ × n memory there are n data lines, usually named D(n − 1) down to D0.

So the second number in the 2ᵏ × n notation tells you the data lines directly, while the first number tells you the address lines through log₂:

2K × 8: 11 address lines, 8 data lines
64K × 16: 16 address lines, 16 data lines
1M × 1: 20 address lines, 1 data line

On most RAM chips the data lines are bidirectional: inputs during a write, outputs during a read, and high impedance when the chip isn't selected. On a ROM they are outputs only.

Data lines connect the chip to the system's data bus. When several chips sit side by side for width expansion, each chip's data lines go to its own slice of the bus.

Worked examples

Example

Counting pins

A 4K × 8 chip has one pin per address bit and per data bit. How many address and data pins is that?

1.
4K = 2¹², so 12 address pins.
2.
Word size 8, so 8 data pins.
3.
12 + 8 = 20 pins, before power, ground and control pins.

Example

From pins to notation

A chip has 13 address lines and 4 data lines.

1.
Words: 2¹³ = 8K.
2.
Word size: 4.
3.
It is an 8K × 4 memory, holding 32K bits.

Common mistakes

Using log₂ for data lines. Data lines equal the word size directly; only address lines use log₂.
Swapping the two numbers. In 2K × 8, the 8 is the data lines; 2K sets the address lines.
Thinking data lines carry the address. Addresses travel on separate address lines.

Practice Data lines

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Practice exams

Memory: Exam 2 · 1 question

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Data lines is taught in Memory.

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