What is a Bus?

• A communication pathway connecting two or more devices


• Often grouped

---- A number of channels in one bus

---- e.g. 32 bit data bus is 32 separate single bit channels

Data Bus :

• Carries data

---- Remember that there is no difference between “data” and “instruction” at this level.

• Width is a key determinant of performance

---- 8, 16, 32, 64 bit

Address bus :

• Identify the source or destination of data

e.g. CPU needs to read an instruction (data) from a given location in memory

• Bus width determines maximum memory capacity of system

e.g. 8080 has 16 bit address bus giving 64k address space

Control Bus :

• Control and timing information

---- Memory read/write signal

---- Interrupt request

---- Clock signals

Bus Interconnection Scheme

Single Bus Problems

• Lots of devices on one bus leads to:

—   Propagation delays

—   Long data paths mean that co-ordination of bus use can adversely affect performance

—   If aggregate data transfer approaches bus capacity

• Most systems use multiple buses to overcome these problems

Traditional Bus Architecture

High Performance Bus

Bus Types

• Dedicated

--- Separate data & address lines

• Multiplexed

--- Shared lines

--- Address valid or data valid control line

Advantage

--- Fewer lines

Disadvantages

--- More complex control

--- Ultimate performance

Bus Arbitration

• More than one module controlling the bus


• e.g. CPU and DMA controller


• Only one module may control bus at one time


• Arbitration may be centralised or distributed

Centralised Arbitration

• Single hardware device controlling bus access

--- Bus Controller

---  Arbiter

• May be part of CPU or separate