Chapter 9

Computer Design Basics

9-2 Datapaths

RemindingA digital system (or a simple

computer) contains datapath unit and control unit.

Datapath: • A set of registers• Microoperations on these registers• Control interface

9-2 Datapaths

The arithmetic/logic unit (ALU) and shifter are designed to perform the microoperations (arithmetic and logic).

Shifter is often separated from ALU for shift operation, but sometimes is implemented in ALU.

The combination of a set of registers with a shared ALU and shifter and interconnecting path is the datapath of the system.

Block diagram of a generic datapath (Fig. 9-1)

G select: S 2..0||Cin

Symbol for an n-bit ALU (Fig. 9-2)

Requirements for ALU and Shifter

The sets of values must be generated and must become available on the corresponding control lines early in the clock cycle.

Binary data from the two source registers must be propagated through the multiplexers and the ALU and on into the inputs of the destination register, all during the remainder of the same clock.

To achieve fast operations, ALU and shifter are constructed with combinational logic having limited number of levels.

9-3 The ALU

Fig. 9-3 Block diagram of an arithmetic circuit

This section introduction the design of ALU

The basic component of an arithmetic circuit is a parallel adder

Function of arithmetic circuit

G = X+Y+Cin

B input logic

The B input logic in Fig. 9-3 can be implemented with 4-to-1 multiplexer (How?).

To reduce the gates number, it can be implemented through logic design of one bit (one stage) instead of using multiplexer.

B input logic logic design for B input logic (one stage)

10 SBSBY iii

Diagram of a 4-bit arithmetic circuit (Fig. 9-5)

Logic Circuit Design

Fig 9-6 One stage of logic circuit

One stage of ALU (Fig. 9-7)

Gi

Function Table of ALU (Table 9-2)

9-4 The Shifter

Fig. 9-8

one bit is shifted in a clock To shift an operant by m>1 bit positions, the shifter should

perform a series of m 1-bit position shift, taking m clock cycle

What is the operation when S is set as 00 (0), 01(1), 10 (2)?

Barrel shifter A combinational circuit Shifts or rotates the input data by the number of bit

position specified by a binary value

4-bit Barrel shifter (Fig. 9-9)Note: i positions of left rotation is the same as 2n-i bits of right rotation

9-5 Datapath representation

Reduce the complexity of the datapath in Fig. 9-1

A computer with 32 or more registers are common

Register file: a set of registers having common microoperations performed on them

Datapath using register file and function unit (Fig. 9-10)

Function select

MF, G and H are coded in terms of FS

MF=F3.F2, G3=F3, G2=F2, G1=F1, G0=F0, H1=F1, H0=F0