6-1 Chapter 6 - Datapath and Control

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Principles of Computer ArchitectureMiles Murdocca and Vincent Heuring

Chapter 6: Datapath and Control

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Chapter Contents6.1 Basics of the Microarchitecture6.2 A Microarchitecture for the ARC6.3 Hardwired Control6.4 Case Study: The VHDL Hardware Description Language

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The Fetch-Execute Cycle• The steps that the control unit carries out in executing

a program are:(1) Fetch the next instruction to be executed from memory.(2) Decode the opcode.(3) Read operand(s) from main memory, if any.(4) Execute the instruction and store results.(5) Go to step 1.

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High Level View of Microarchitecture• The microarchitecture consists of the control unit and

the programmer-visible registers, functional units such as the ALU, and any additional registers that may be required by the control unit.

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ARC Instruction Subset

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ARC Instruction Formats

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ARC Datapath

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ARC ALU Operations

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Block Diagram of ALU

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Gate-Level Layout of Barrel Shifter

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Truth Table for (Most of the) ALU LUTs

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Design of Register %r1

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Outputs to Control Unit fromRegister %ir

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Microarch-itecture of the ARC

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Microword Format

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Settings for the COND Field of the Microword

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DECODE Format for Microinstruction Address

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Timing Relationships for the Registers

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Partial ARC

Micro-program

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Partial ARC Microprogram

(cont’)

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Branch Decoding• Decoding tree for

branch instructions shows corresponding microprogramlines:

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Assembled ARC

Microprogram

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Assembled ARC

Microprogram(cont’)

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Example: Add the subcc Instruction• Consider adding instruction subcc (subtract) to the ARC instruction

set. subcc uses the Arithmetic format and op3 = 001100.

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Branch Table• A branch table for trap handlers and interrupt service routines:

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Microprogramming vs. Nanoprogramming

• (a) Micropro-gramming vs. (b) nano-programming.

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Hardware Description Language

• HDL sequence for a resettablemodulo 4 counter.

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Circuit Derived from HDL• Logic design for a modulo 4 counter described in HDL.

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HDL for ARC

• HDL description of the ARC control unit.

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HDL for ARC (cont’)

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HDL ARC Circuit

• The hardwired control section of the ARC: generation of the control signals.

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HDL ARC Circuit (cont’)• Hardwired

control section of the ARC: signals from the data section of the control unit to the datapath.

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Case Study: The VHDL Hardware Description Language

• The majority function. a) truth table, b) AND-OR implementation, c) black box representation.

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VHDL SpecificationInterface specification for the majority component

--Interface

entity MAJORITY is

port

(A_IN, B_IN, C_IN: in BIT

F_OUT: out BIT);

end MAJORITY;

Behavioral model for the majority component

-- Body

architecture LOGIC_SPEC of MAJORITY is

begin

--compute the output using a Boolean expression

F_OUT <= (not A_IN and B_IN and C_IN) or

(A_IN and not B_IN and C_IN) or

(A_IN and B_IN and not C_IN) or

(A_IN and B_IN and C_IN) after 4 ns;

end LOGIC_SPEC;

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VHDL Specification (cont’)-- Package declaration, in library WORK

package LOGIC_GATES is

component AND3

port (A, B, C : in BIT; X : out BIT);

end component;

component OR4

port (A, B, C, D : in BIT; X : out BIT);

end component;

component NOT1

port (A : in BIT; X : out BIT);

end component;

--Interface

entity MAJORITY is

port

(A_IN, B_IN, C_IN: in BIT

F_OUT: out BIT);

end MAJORITY;

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VHDL Specification (cont’)-- Body-- Uses components declared in package LOGIC_GATES --in the WORK library--import all the components in WORK.LOGIC_GATESuse WORK.LOGIC_GATES.all architecture LOGIC_SPEC of MAJORITY is--declare signals used internally in MAJORITYsignal A_BAR, B_BAR, C_BAR, I1, I2, I3, I4: BIT;begin--connect the logic gatesNOT_1 : NOT1 port map (A_IN, A_BAR);NOT_2 : NOT1 port map (B_IN, B_BAR);NOT_3 : NOT1 port map (C_IN, C_BAR);AND_1 : AND3 port map (A_BAR, B_IN, C_IN, I1);AND_2 : AND3 port map (A_IN, B_BAR, C_IN, I2);AND_3 : AND3 port map (A_IN, B_IN, C_BAR, I3);AND_4 : AND3 port map (A_IN, B_IN, C_IN, I4);OR_1 : OR3 port map (I1, I2, I3, I4, F_OUT);end LOGIC_SPEC;