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Control unit of Basic Computer



Hardwired Control : When the control signals are generated by hardware using conventional logic design techniques like gates,flip-flop,decoder and other digital circuit .• it has the advantage that it can be optimized to produce a fast mode of operation • Requires changes in the wiring among the various components

Micro programmed Control : In the micro programmed organization the control information is stored in a control memory.• The control memory is programmed to initiate the required sequence of micro operations • In the Micro programmed control any required changes or modifications be done by updating the microprogram in control memory.

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• Control Unit : Control unit in digital computer is to initiate sequence of microoperations

• The control unit initiates a series of sequential steps of microoperation during any given time certain microoperation are to be initiated ,while other remain idle.

• It consists of two decoders ,a sequence counter, a number of control logic gates.

• An introduction read from memory is placed in the instruction register(IR).• The operation code in bits 12 through 14 are decoded with a 3 × 4

decoder .• The eight output of the decoder are designated by the symbols D0

through D7.

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Microprogrammed control organization

A memory that is part of a control unit is called a control Memory.

A computer that uses a microprogrammed control unit usually has two separate memories -

1.Main memory

2.Control memory.

Main memoryThe main memory is available to the user for storing their programs. The contents of main memory may change when the data are manipulated and every time the program is changed. The user' program in main memory

consists of machine instructions and data.

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Microprogrammed control organization

control memory holds a fixed microprogram that cannot be altered by the occasional user. usually a ROM, which stores all control information permanently

Microprogram SequencerA microprogram sequencer is the next address generator, as it determines the address sequence that is read from control memory. The address of the next microinstruction can be specified in several ways depending on the sequencer inputs. The functions of a microprogram sequencer are incrementing the control address register by one

Control data register (DR)The control DR(data register) stores the present microinstruction while the next address is computed and read from memory. The data register is also called a pipeline register. It allows the execution of the microoperations specified by the control word simultaneously with the generation of the next microinstruction

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Microprogrammed control organizationControl Address Register holds the microinstruction read from memory. The Microinstruction contains a control word that specifies one or more microoperation for the data processor.

Instruction Format

Microinstruction format

The 20 bits of the microinstruction are divided into four functional parts. The three fields F1, F2, and F3 specify microoperation for the computer.CD: Condition for BranchingBR: Branch fieldAD: Address field

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Selection of Address for control Memory

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Addition and Subtraction

We consider addition,subtraction,multiplication,and division

for the following types of data:

• Fixed-Point binary data in signed-magnitude


• Fixed-Point binary data in signed-2’s compliment


• Floating point binary data

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Addition and SubtractionA processor has an arithmetic processor(as a sub part of

it) that executes arithmetic operations. The data

type, assumed to reside in processor, registers during the execution of an arithmetic instruction. Negative

numbers may be in a signed magnitude or signed complement representation.

There are three ways of representing negative fixed point - binary numbers

1.signed magnitude,

2. signed 1’s complement

2. Signed 2’s complement.

Most computers use the signed magnitude epresentation for the mantissa

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Addition and Subtraction With Signed-Magnitude Data

Addition and Subtraction With Signed-Magnitude Numbers

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hardware implementation for signed magnitude addition and subtraction

As and Bs is a sign flip-flopOutput carry is transferred to flip-flop EAVF holdes the overflow bit

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hardware implementation for signed magnitude addition and subtraction

As and Bs is a sign flip-flopOutput carry is transferred to flip-flop EAVF holdes the overflow bit

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Flowchart for add and subtract operation

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Addition and Subtraction With Signed-2’s Compliment Data

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Addition and Subtraction With Signed-2’s Compliment Data

• The leftmost bit of a binary number represents the sign : 0 to denote positive and 1 to denote negative.

• If the sign bit is 1, then we represent number in 2's complement form.

• Thus + 33 is represented as 00100000 and -33 as 11011110. Note that 11011110 is the 2's complement of 00100000

• The addition of two numbers in signed 2's complement form by adding the numbers with the sign bits treated the same as the other bits of the number. We discard the carry of the sign-bit position.

• The subtraction consists of first taking the 2's complement of the subtrahend and then adding it to the minuend.

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• We can detect an overflow by inspecting the last two carries of the addition. When the two carries are applied to an exclusive-OR gate, the verflow is detected when the output of the gate is equal to 1.

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Multiplication AlgorithmMultiplication of two fixed-point binary numbers in signed magnitude representation

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Hardware for Multiply operation

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Multiplication Algorithm

• The multiplier is stored in the register • and its sign in Qs. • The sequence counter SC is initially set bits in

the multiplier. After forming each partial product the counter is decremented. When the content of the counter reaches zero, the product is complete and we stop the process.

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Booth multiplication algorithm

Booth algorithm needs examination of the multiplier bits and shifting of the

partial product. Prior to the shifting, the multiplicand added to the partial

product, subtracted from the partial product, or left unchanged by the

following rules:

1. The multiplicand is subtracted from the partial product when we get the first

least significant 1 in a string of 1' in the multiplier.

2. The multiplicand is added to the partial product when we get the first Q

(provided that there was a previous 1) in a string of 0' in the multiplier.

3. The partial product does not change when the multiplier bit is the same as

the previous multiplier bit.

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