Processing Unit Design - Fundamentals of Computer Organization and Architecture - page 90

5.3.2. Two-Bus Organization

Using two buses is a faster solution than the one-bus organization. In this case, gen-
eral-purpose registers are connected to both buses. Data can be transferred from two
different registers to the input point of the ALU at the same time. Therefore, a two-
operand operation can fetch both operands in the same clock cycle. An additional
buffer register may be needed to hold the output of the ALU when the two buses
are busy carrying the two operands. Figure 5.4a shows a two-bus organization.
In some cases, one of the buses may be dedicated for moving data into registers
(in-bus), while the other is dedicated for transferring data out of the registers
(out-bus). In this case, the additional buffer register may be used, as one of the
ALU inputs, to hold one of the operands. The ALU output can be connected directly
to the in-bus, which will transfer the result into one of the registers. Figure 5.4b
shows a two-bus organization with in-bus and out-bus.

5.3.3. Three-Bus Organization

In a three-bus organization, two buses may be used as source buses while the third
is used as destination. The source buses move data out of registers (out-bus), and

Bus 1

PC

IR

ALU

General
Purpose
Registers

MAR

A

MDR

Memory

Bus 2

( a )

A

In-bus

PC

IR

General
Purpose
Registers

ALU

MAR

MDR

Memory Bus

Out-bus

( b )

Figure 5.4 Two-bus organizations. (a) An Example of Two-Bus Datapath. (b) Another
Example of Two-Bus Datapath with in-bus and out-bus