Introduction to Computer

Organization and Architecture

Micro Program

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Model of Control Unit

How the Control Unit Generate the control

signal?

Model of Control UnitGet instruction from

Instruction RegisterWorking steps with the timing generator and conditions from flag, the signal from IR passed through the Decoder circuit into the CU to generate the control signal out.

Block Diagram of the Control

Unit

Control Unit with Decoded Inputs

Microprograms ?• 1950 Maurice V. Wikes

( Cambridge Univer.)• 360IBM System/ tt ttttttt ttttttttttt-ttt ttt tttt tttttt t ttt t tttttt.• Microprogramming allows a ttttttt ttttttt tttt ’: PCU to be designed sequ ences known as MicroproMicroprogramsgrams are placed in a spe cial control memory in theCPU.

Wilkes's Microprogrammed

Control Unit

- so that instruction from t he CPU’s main instruction set is executed by invokin g and executing the corre sponding microprogram. * * * * * * *

- CPU with no floating point arithmetic circuits can exe -cute by means of fixed po int arithmetic circuits.

Microprograms ?

Digital Systems : CPU Data Path Unit

network of functional and s torage units capable of pe rforming certain operation on data words. Control Unit issue control signal to the data path for selecting the

function to be performed at specific times and route

through the appropriate p arts of the datapath unit.

Data Paths and Control Signals

Digital Systems : CPU Hardwired

fixedl ogi c ci rcui ts to genera te the control signals.

Microprogrammed stores the control signals -in sequence of micro in structi ons ( microprogramicroprogra

msms ) ina control memory. * provide a systematic & fl

exi bl e method*

Microprogram med Control

Use microprograms to select, interpret, and e xecute instruction set.

CU contain logic to ge -nerate micro instructi

on addresses and to f etch and decode from

control memory.

Fig.15.4

Control Unit Microarchitecture

Basic Concepts Instruction is implement

ed by a sequence of on e or more sets of concu

-rrent micro operations.

-Each micro operatio n is associated with a group of control l

ines that must be a ctivated in a prescr

ibed sequence to tr -igger the micro operations.

Basic Concepts Program Execution

Instructi on Cycle

Instructi on Cycle

. ..

Fetch

Execute

Interrup

t m

OP

mOP

mOP

. .. . ..

Basic Concepts Microprogramming is a metho

d of - control unit design in which t

he control signal selection and

sequencing information is sto red in a

ROMROM or RAMRAM called . . . . .

Control Me Control Memorymory : CM

Functioning of Microprogrammed Control Unit

Basic Concepts

The control signals control signals could beactivatedactivated at any time that a

re specified by a -micro instr-micro instruction,uction,

which is fetched from CM in much the

same way an instruction is f etched from

main memory.

Basic Concepts Instruction Cycle Instruction Cycle

•Fetch•Indirect•Execute

•Interrupt

Flowchart for Instruction Cycle

Basic Concepts Fetch Cycle Fetch Cycle 3 steps and

t tttt-tttttttttt4

t 1 --- : MAR < (PC)

t 2 --- : MBR < Memory

--- PC < (PC) + IIt 3 --- : IR < (MBR) II : Instruction Len

gth

Sequence of Events, Fetch Cycle

Basic Concepts Indirect Cycle Indirect Cycle -3 micro operatio

ns

t 1 --- :MAR< (I R (address) )

t 2 --- : MBR < Memory

t 3 : --- IR(Address) < (MBR(ad

dress))

Basic Concepts Execute Cycle Execute Cycle -vary on op co

de such as 1ADD R , X will -have 3 micro operations

t 1 --- : MAR < (IR(address))t 2 --- : MBR < Memory

t 3 --- : R1 < (R1) + (MBR)

Basic Concepts Execute Cycle Execute Cycle as ISZ X (Inc

rement and Skip instruction)

t 1 --- : MAR < (IR(address))t 2 --- : MBR < Memoryt 3 --- : MBR < (MBR) + 1t 4 --- : Memory < (MBR) If ((MBR) = 0) then (

--- PC < (PC)+1)

Basic Concepts Execute Cycle Execute Cycle as BSA X (su

- -broutine call : Branch and sav- e address instruction)t 1 --- : MAR < (IR(address))

--- MBR < (PC) t 2 --- : PC < (IR(address)) --- Memory < (MBR)t 3 --- : PC < (PC) + 1

Basic Concepts Interrupt Cycle Interrupt Cycle -3micro operati

ons

t 1 --- : MBR < (PC)

t 2 --- : MAR < Save_Address

--- PC < Routine_Addresst 3 --- : Memory < (MBR)

Basic Concepts - Each micro instruction also e

xplicitly or implicitly specifi -es the next micro instructio

n to be used, thereby provi ding the necessary informat

- ion for micro operation sequencing.

set of micro-instruction

forms

microprmicroprogramogram

Organization of

Control Memory

Microinstruction Encoding

Microinstruction Encoding

Basic ConceptsAdvantageAdvantage Microprogra

m can be changed relati vely easily by changing

the contents of CM. (fleflexiblexible)

DisadvantageDisadvantage The tim e required to access the

microinstructions from C M. Chip area and circuit

delay must both be minimized.

Used in CISC’s as the Penti um and MC6 8 0 X0

-Micro instructions Horizontal Formats

long formats, little encoding of the control fields, and t

he ability to control many- micro operation in parallel.

Vertical Formats short formats, considerable

- control field encoding, and limited parallelism.

-interpreted by nano inst ruction that directly con

trol the hardware.

Micro-instruction Types

• Each micro-instruction specifies single (or few) micro-operations to be performed— (vertical micro-programming)

• Each micro-instruction specifies many different micro-operations to be performed in parallel—(horizontal micro-programming)

Vertical Micro-programming

• Width is narrow

• n control signals encoded into log2 n bits

• Limited ability to express parallelism• Considerable encoding of control

information requires external memory word decoder to identify the exact control line being manipulated

Horizontal Micro-programming

• Wide memory word• High degree of parallel operations possible• Little encoding of control information

Alternative Microinstruct

ion Formats for a Simple

Machine

Alternative Microinstruction Formats for a Simple Machine

Next Address Decision

• Depending on ALU flags and control buffer register—Get next instruction

– Add 1 to control address register

—Jump to new routine based on jump microinstruction

– Load address field of control buffer register into control address register

—Jump to machine instruction routine– Load control address register based on opcode in IR

Functioning of Microprogrammed Control Unit

Design Considerations

• Size of microinstructions• Address generation time

—Determined by instruction register– Once per cycle, after instruction is fetched

—Next sequential address– Common in most designed

—Branches– Both conditional and unconditional

Sequencing Techniques

• Based on current microinstruction, condition flags, contents of IR, control memory address must be generated

• Based on format of address information—Two address fields—Single address field—Variable format

Branch Control Logic: Two Address Fields

Branch ControlLogic: Single Address Field

Branch Control Logic: Variable

Format

Address Generation

Explicit Implicit

Two-field Mapping

Unconditional Branch Addition

Conditional branch Residual control

Execution

• The cycle is the basic event• Each cycle is made up of two events

—Fetch– Determined by generation of microinstruction

address

—Execute

Execute

• Effect is to generate control signals• Some control points internal to processor• Rest go to external control bus or other

interface

Control Unit Organization

A Taxonomy of Microinstructions

• Vertical/horizontal• Packed/unpacked• Hard/soft microprogramming• Direct/indirect encoding

How to Encode• K different internal and external control signals • Wilkes’s:

— K bits dedicated — 2K control signals during any instruction cycle

• Not all used— Two sources cannot be gated to same destination— Register cannot be source and destination— Only one pattern presented to ALU at a time— Only one pattern presented to external control bus at a time

• Require Q < 2K which can be encoded with log2Q < K bits• Not done

— As difficult to program as pure decoded (Wilkes) scheme— Requires complex slow control logic module

• Compromises— More bits than necessary used— Some combinations that are physically allowable are not

possible to encode

Specific Encoding Techniques

• Microinstruction organized as set of fields• Each field contains code• Activates one or more control signals• Organize format into independent fields

—Field depicts set of actions (pattern of control signals)

—Actions from different fields can occur simultaneously

• Alternative actions that can be specified by a field are mutually exclusive—Only one action specified for field could occur

at a time

Microinstruction Encoding

Microinstruction Encoding

-Micro instructions Machine instruction is execu

ted by a microprogram whi - ch acts as a real time inter

preter for the instruction. Microinstruction Format

IBM System/370 Model 145

Control

Operand1

Operand2 CM Addressing

0 8 1

6

2

4

3

1

IBM 3033 Microinstruction Format

Other Samples

•LSI computer

Simplified Block Diagram

of the LSI-11 Processor

Organization of the LSI-11 Control Unit

LSI-11 Microinstruction

Format

Introduction to Computer

Organization and Architecture

Machine Language

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