G.H. PATEL COLLEGE OF ENGINEERING AND TECHNOLOGY

COMPUTER ORGANIZATION

SHIFT MICROOPERATIONS

PRESENTED BY:DARSHIT METALIYA

( 130110107020 )CHIRAG SONAGRA

( 130110107055 )

Shift Microoperations

• Shift microoperations are used for serial transfer of data. They are also used in conjunction with arithmetic, logic, and other data-processing operations.

• The contents of a register can be shifted to the left or the right. At the same time that the bits are shifted, the first flip-flop receives its binary information from the serial input.

• There are three type of shifts :

1. Logical Shift

2. Circular Shift

3. Arithmetic Shift

1. Logical Shift

• A logical shift is one that transfers 0 through the serial input.

• We will adopt the symbol shl and shr for logical shift-left and shift-right microoperations.

• For example :• R1 shl R1

• R2 shr R2

• These are two microoperations that specify a 1-bit shift to the left of the

content of register R1 and 1-bit shift to the right of the content of register R2.

• The register symbol must be same on both sides of arrow.

• The bit transferred to the end position though the serial input is assumed to be 0 during a logical shift

• Use a Logical Left Shift Operation on the bit pattern10011000.

• Solution• The solution is shown below. The leftmost bit is lost and a 0 is

inserted as the rightmost bit.

• Discarded

• Added

• As an Example

2. Circular shift

• The circular shift circulates the bits of the register around the two ends without loss of information.

• This is accomplished by connecting serial output of the shift register to its serial input.

• We will use the symbols cil and cir for the circular shift left and right, respectively.

• For example :

• R1 cil R1

• R2 cir R2

• Use a Circular Left Shift Operation on the bit pattern10011000.

• Solution• The solution is shown below. The leftmost bit is circulated and

becomes the rightmost bit.

• As an Example :

3. Arithmetic Shift

• An arithmetic shift is a microoperation that shifts a signed binary number to the left or right.

• Arithmetic right shift divides the number by two.

• Arithmetic left shift multiplies a signed binary number by two.

• Arithmetic shifts must leave the sign bit unchanged because the sign of the number remains the same when it is multiplied or divided by two.

• Use an Arithmetic right shift operation on the bit pattern10011001. The pattern is an integer in two’s complement format.

• Solution• The solution is shown below. The leftmost bit is retained and also

copied to its right neighbor bit.

• The original number was −103 and the new number is −52, which is theresult of dividing −103 by 2 truncated to the smaller integer.

• As an example :

• Use an Arithmetic left shift operation on the bit pattern 11011001.The pattern is an integer in two’s complement format.

• Solution• The solution is shown below. The leftmost bit is lost and a 0 is inserted

as the rightmost bit.

• The original number was −39 and the new number is −78. The originalnumber is multiplied by two. The operation is valid because nounderflow occurred.

• As an example

• Use an Arithmetic left shift operation on the bit pattern 01111111.

The pattern is an integer in two’s complement format

• Solution• The solution is shown below. The leftmost bit is lost and a 0 is

inserted as the rightmost bit.

• The original number was 127 and the new number is 0. Here the resultis not valid because an overflow has occurred. The expected answer 127× 2 = 254 cannot be represented by an 8-bit sign pattern.

• As an Example

• Hardware Implementation

S

01

H0MUX

S

01

H1MUX

S

01

H2MUX

S

01

H3MUX

Select0 for shift right (down)

1 for shift left (up)Serialinput (IR)

A0

A1

A2

A3

Serial

input (IL)

4-bit combinational circuit shifter

• A combinational circuit shifter can be constructed with multiplexers as shown in Fig.

• The 4-bit shifter has four data inputs, A0 through A3, and four data outputs, H0 through H3.

• There are two serial inputs , one for shift left(IL) and the other for shift right(IR).

• When the selection input S=0, the input data are shifted right (down in the diagram). When S=1, the input data are shifted left (up in the diagram).

• The function table in Fig. shows which inputs goes to each output after shift.

• A shifter with n data inputs and outputs requires n multiplexers. The two serial inputs can be controlled by another multiplexer to provide the three possible types of shifts.

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