Organisasi Komputer I I

Micro-programmed Control

(Kontrol Termikroprogram)

Organisasi Unit Kontrol

Micro-programmed Control

Use sequences of instructions (see earlier notes) to control complex operationsCalled micro-programming or firmware

CONTROL UNIT MICROPROGRAMMED

Terbagi 2 yaitu :1. Control Vertikal

Jenis implementasi dimana signal kontrol di kode ke dalam pada bit , kemudian digunakan setelah dikode.

2. Control HorizontalControl dimana setiap bit kontrol mengatur 1 operasi gate ataumesin.

KOMPONEN-KOMPONEN POKOK CONTROL UNIT

MICROPROGRAMMED

1. Instruction Register Menyimpan instruksi register mesin yang dijalankan.

2. Control Store berisi microprogrammedUntuk semua instruksi mesin.Untuk startup mesin.Untuk memprosesan interupt

3. Address Computing CircuitingMenentukan alamat Control Store dari mikroinstruksiberikutnya yang akan dijalankan.

4. Microprogrammed CounterMenyimpan alamat dari mikroinstruksi berikutnya.

5. Microinstruction BufferMenyimpan mikroinstruksi tersebut selama dieksekusi.

6. Microinstruction DecoderMenghasilkan dan mengeluarkan mikroorder yang didasarkan pada mikroinstruksi dan opcode instruksiyang akan dijalankan

Implementat ion (1)

All the control unit does is generate a set of control signalsEach control signal is on or offRepresent each control signal by a bitHave a control word for each micro-operationHave a sequence of control words for each machine code instructionAdd an address to specify the next micro-instruction, depending on conditions

Implementat ion (2)

Today s large microprocessorMany instructions and associated register-level hardwareMany control points to be manipulated

This results in control memory thatContains a large number of words

co-responding to the number of instructions to be executed

Has a wide word width Due to the large number of control points to be manipulated

Micro-program Word Length

Based on 3 factorsMaximum number of simultaneous micro-operations supportedThe way control information is represented or encodedThe way in which the next micro-instruction address is specified

Micro-instruct ion 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)

Vert ical Micro-programming

Width is narrown control signals encoded into log2 n bitsLimited ability to express parallelismConsiderable encoding of control information requires external memory word decoder to identify the exact control line being manipulated

Horizontal Micro-programming

Wide memory wordHigh degree of parallel operations possibleLittle encoding of control information

Typical Microinstruct ion Formats

Compromise

Divide control signals into disjoint groupsImplement each group as separate field in memory wordSupports reasonable levels of parallelism without too much complexity

Organizat ion ofControl Memory

Control Unit

Control Unit Funct ion

Sequence login unit issues read commandWord specified in control address register is read into control buffer registerControl buffer register contents generates control signals and next address informationSequence login loads new address into control buffer register based on next address information from control buffer register and ALU flags

Next Address Decision

Depending on ALU flags and control buffer register

Get next instructionAdd 1 to control address register

Jump to new routine based on jump microinstructionLoad address field of control buffer register into control address register

Jump to machine instruction routineLoad control address register based on opcode in IR

Functioning of MicroprogrammedControl Unit

Wilkes Control

1951Matrix partially filled with diodesDuring cycle, one row activated

Generates signals where diode presentFirst part of row generates controlSecond generates address for next cycle

Wilkes's Microprogrammed Control Unit

Advantages and Disadvantages of Microprogramming

Simplifies design of control unitCheaperLess error-prone

Slower

Tasks Done By MicroprogrammedControl Unit

Microinstruction sequencingMicroinstruction executionMust consider both together

Design Considerat ions

Size of microinstructionsAddress generation time

Determined by instruction registerOnce per cycle, after instruction is fetched

Next sequential addressCommon in most designed

BranchesBoth conditional and unconditional

Sequencing Techniques

Based on current microinstruction, condition flags, contents of IR, control memory address must be generatedBased on format of address information

Two address fieldsSingle address fieldVariable format

Branch Control Logic: Tw o Address Fields

Branch ControlLogic: Single Address Field

Branch Control Logic: Variable Format

Address Generat ion

Residual controlConditional branch

AdditionUnconditional Branch

MappingTwo-field

ImplicitExplicit

Execution

The cycle is the basic eventEach cycle is made up of two events

FetchDetermined by generation of microinstruction address

Execute

Execute

Effect is to generate control signalsSome control points internal to processorRest go to external control bus or other interface

Control Unit Organizat ion

A Taxonomy of Microinstruct ions

Vertical/horizontalPacked/unpackedHard/soft microprogrammingDirect/indirect encoding

Improvements over Wilkes

Wilkes had each bit directly produced a control signal or directly produced one bit of next addressMore complex address sequencing schemes,using fewer microinstruction bits, are possibleRequire more complex sequencing logic moduleControl word bits can be saved by encoding and subsequently decoding control information

How to EncodeK different internal and external control signals Wilkes s:

K bits dedicated 2K control signals during any instruction cycle

Not all usedTwo sources cannot be gated to same destinationRegister cannot be source and destinationOnly one pattern presented to ALU at a timeOnly one pattern presented to external control bus at a time

Require Q < 2K which can be encoded with log2Q < K bitsNot done

As difficult to program as pure decoded (Wilkes) schemeRequires complex slow control logic module

CompromisesMore bits than necessary usedSome combinations that are physically allowable are not possible to encode

Specific Encoding Techniques

Microinstruction organized as set of fieldsEach field contains codeActivates one or more control signalsOrganize 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

Microinstruct ion EncodingDirect Encoding

Microinstruct ion EncodingIndirect Encoding