Computer Design Basics Chapter 8 Computer Design Basics

8-2 Datapaths Reminding A digital system (or a simple computer) contains datapath unit and control unit. Datapath: A set of registers Microoperations on these registers Control interface

8-2 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. 8-1) G select: S 2..0||Cin

Symbol for an n-bit ALU (Fig. 8-2)

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.

8-3 The ALU This section introduction the design of ALU The basic component of an arithmetic circuit is a parallel adder Fig. 8-3 Block diagram of an arithmetic circuit

Function of arithmetic circuit G = X+Y+Cin

B input logic The B input logic in Fig. 8-3 can be implemented with n-bits 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)

Diagram of a 4-bit arithmetic circuit (Fig. 8-5)

Logic Circuit Design Fig 9-6 One stage of logic circuit

One stage of ALU (Fig. 8-7) Gi

Function Table of ALU (Table 8-2)

8-4 The Shifter 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)? Fig. 8-8

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. 8-9) Note: i positions of left rotation is the same as 2n-i bits of right rotation

8-5 Datapath representation Reduce the complexity of the datapath in Fig. 8-1 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. 8-10)

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