Computer Design Basics

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Chapter 9

• Computer Design Basics

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9-2 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

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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.

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

G select S 2..0Cin
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Symbol for an n-bit ALU (Fig. 9-2)
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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.

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9-3 The ALU

• This section introduction the design of ALU
• The basic component of an arithmetic circuit is
  a parallel adder

Fig. 9-3 Block diagram of an arithmetic circuit
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Function of arithmetic circuit
G XYCin
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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.

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B input logic

• logic design for B input logic (one stage)

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Diagram of a 4-bit arithmetic circuit (Fig. 9-5)
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Logic Circuit Design
Fig 9-6 One stage of logic circuit
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One stage of ALU (Fig. 9-7)
Gi
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Function Table of ALU (Table 9-2)
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9-4 The Shifter

• one bit is shifted in a clock
• To shift an operant by mgt1 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. 9-8
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Barrel shifter

• A combinational circuit
• Shifts or rotates the input data by the number of
  bit position specified by a binary value

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

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

• MF, G and H are coded in terms of FS
• MFF3.F2, G3F3, G2F2, G1F1, G0F0, H1F1, H0F0