Computers as Components, 2nd Edition

160 CHAPTER 4 Bus-Based Computer Systems

Get

data

CPU

Ack

Wait Wait

See

ack

Start

here

Device

Send

data

Ack

Adrs

Done

Release

ack

Start here

Adrs

FIGURE 4.7

State diagrams for the bus read transaction.

The state machine view of the bus transaction is also helpful and a useful com-

plement to the timing diagram. Figure 4.7 shows the CPU and device state machines

for the read operation. As with a timing diagram, we do not show all the possible

values of address and data lines but instead concentrate on the transitions of control

signals.When the CPU decides to perform a read transaction,it moves to a new state,

sending bus signals that cause the device to behave appropriately ...