Bus

Also called: shared bus, bus lines, system bus

A set of shared wires that carries a group of related signals, such as an address or a data word, between the parts of a digital system.

A bus is a group of wires treated as one path, carrying one multi-bit value: an address, a data word, a set of control signals. Its width is the number of wires. An 8-bit data bus has 8 wires, D7–D0.

In a simple computer there are three:

the address bus, from the processor to every memory chip (address lines)
the data bus, carrying words in both directions
the control bus: chip selects, read/write and the like

What makes a bus a bus is sharing. Many chips connect to the same wires, so connecting a new chip costs no new wiring. The price is a rule: at any moment, only one device may drive the bus. Everyone else either listens or keeps its outputs at high impedance with tri-state buffers. Breaking the rule is bus contention.

Widths are independent. A memory system might have a 16-bit address bus (64K addresses) and an 8-bit data bus (bytes), because the address width counts words and the data width is the word size.

Inside a CPU, buses connect registers, the ALU and memory interface in the datapath.

128

Worked example

Example

Who drives the data bus?

A RAM, a ROM and an I/O chip share an 8-bit data bus. The processor reads address 2000₁₆, which belongs to the RAM.

1.
The address decoding activates the RAM's chip select only.
2.
The RAM enables its tri-state outputs and drives the stored word, say 10110100, onto D7–D0.
3.
The ROM and I/O chip keep their outputs at Z.
4.
The processor latches 10110100 from the bus. When the read ends, the RAM lets go too.

Common mistakes

Thinking a bus is a single wire. A bus is a group of wires, one per bit.
Letting two devices drive the same bus at once. Only one driver may be enabled at a time.
Assuming the address and data buses must be the same width. One counts words, the other is the word size.

Practice Bus

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

Basic CPU / Computer Architecture lesson full course

On a single shared bus, two registers may drive the bus in the same clock cycle as long as both values are needed.

Learn it step by step

Bus is taught in Memory and Basic CPU / Computer Architecture.

Memory →Basic CPU / Computer Architecture →

All terms

Bus