CPE 431531 Chapter 5 The Processor: Datapath and Control

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CPE 431/531Chapter 5 - The Processor Datapath
and Control
Swathi T. Gurumani Modified From Slides of Dr.
Rhonda Kay Gaede UAH
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5.1 Introduction Implementation Basics

• Performance Factors
• Instruction Count
• Cycle Time
• CPI
• A Basic MIPS Implementation
• Simple subset lw, sw, add, sub, and, or, slt,
  beq, j

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5.1 Introduction Implementation Overview

• All instructions begin the same way
• Send PC to memory and fetch the instruction
• Read registers using fields of instruction
• Then, it depends on the instruction
• Use ALU after reading registers
• lw sw - Address calculation
• add et.al. - operation execution
• beq - comparison
• Simplicity and regularity of instruction set
  simplifies the implementation

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5.1 Introduction Implementation Datapath,
Datapath Control

• Datapath

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5.1 Introduction Implementation Datapath,
Datapath Control
Datapath Control

• Why two memories?

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5.2 Logic Design Conventions Classes and Values

• Two classes of logic
• Elements that operate on data values -
  Combinational
• Elements that contain state State elements
  (some internal storage) - sequential
• Two logic values
• Asserted - high
• Deasserted - low

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5.2 Logic Design Conventions Clocking
Methodology

• A clocking methodology defines when signals can
  be read and when they can be written.
• We assume an edge-triggered clocking methodology.

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5.3 Building a Datapath -Instruction Fetch and
Sequencing
Datapath element Functional unit used to
operate on or hold data within a
processor Program counter (PC) Register
containing the address of current instruction
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5.3 Building a Datapath -R-type Instruction
Requirements
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5.2 Building a Datapath lw/sw Instruction
Requirements
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5.3 Building a Datapath beq Instruction
Requirements
Branch Target Address Address specified in A
branch, which becomes the new PC if branch is
taken. Branch taken, branch not taken
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5.3 Building a Datapath Creating a Single
Datapath R-type lw/sw
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5.3 Building a Datapath Creating a Single
Datapath
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5.4 A Simple Implementation Scheme Defining
Necessary Control
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5.4 A Simple Implementation Scheme - Adding a
Control Unit