The Shifting Operations

1
The Shifting Operations

• Left Shift Logical / Open Left Shift.
• Right Shift Logical / Open Right Shift.
• Right Shift Arithmetic.
• Rotate Left / Closed Shift Left thro Carry.
• Rotate Right / Closed Shift Right thro. Carry.

2
The Open Left Shift Operation
LSHIFT gt Open ( Logically) left shift
Operand a by 1 bit.
cnPresent ? a ((i-1) Prev. for i
1..(n-1) n7/15/31 CARRY/BORROW Flag
Present?a(n-1)Prev. i..e. Prev.
CARRY/BORROW is

OVERWRITTEN.
c0Present? 0 Affects ZERO SIGN Flags too,
Does not affect OVERFLOW Flag
a(n-1) a(n-2) a(i) a(i-1)
a(0)
c(n-1) c(n-2) c(i1) c(i) c(i-1)
c(1) c(0)
CY Flag
0
3
The Open Right Shift Operation
RSHIFT L gtOpen / Logically Right Shift
Operand a by 1 bit
cnPresent ? a(i1) Prev. for i 0..(n-2)
n7/15/31 CARRY/BORROW Flag Present?
a(0)Prev. i..e. Prev. CARRY/BORROW

is OVERWRITTEN
c(n-1)Present? 0 Affects ZERO SIGN
Flags too, Does not affect OVERFLOW Flag
a(n-1) a(i1) b(i)
a(2) a(1) a(0)
c(n-1) c(n-2) c(i) c(i-1)
c(1) c(0)
CY Flag
0
4
The Arithmetic Right Shift Operation
ARSHIFT gt Right Shifted Arithmetic Operand
a. Where ciPresent ? a(i1)Prev. for i
0..(n-2) n7/15/31 CARRY/BORROW Flag
Present? a0Prev. i.e. Prev. CARRY/BORROW

is OVERWRITTEN.
c(n-1)Present? a(n-1)Prev. . Sign is
Preserved in its designated Position.
Affects ZERO Flag too, does not affect OVERFLOW
SIGN Flags.
a(n-1) a(i1) a(i)
a(2) a(1) a(0)
c(n-1) c(n-2) c(i) c(i-1)
c(1) c(0)
CY Flag
5
The Rotate Left ( Closed Left Shift) Operation
RAL gt CLOSED Left Shift Operand a through
Carry once where CARRY/BORROW Flag Present ?
a(n-1) Prev. c(i) Present ? a(i-1)
Prev. for i 1..(n-1) n7/ 15/31 c0
Present ? CARRY/BORROW Flag Prev.Cy_In i.e.
CARRY/BORROW is Preserved in the LSBit.
Affects CARRY/BORROW, ZERO SIGN Flags only,
does not affect OVERFLOW Flag.
a(n-1) a(n-2) a(i) a(i-1)
a(1) a(0)
c(n-1) c(i1) c(i)
c(2) c(1) c(0)
CY Flag
6
The Rotate Right ( Closed Right Shift) Operation
RAR CLOSE Right Shift Operand a thro. Carry
Where CARRY/BORROW Flag Present? a0 Prev.
c(i)Present ? a(i1) Prev. for i
0..(n-2) n7/15/31 c(n-1) Present ?
CARRY/BORROW Flag Prev.Cy_In. i.e .
CARRY/BORROW is Preserved in the MSBit.
Affects CARRY/BORROW, ZERO SIGN Flags only,
does not affect OVERFLOW Flag.
a(n-1) a(i1) a(i)
a(2) a(1) a(0)
c(n-1) c(n-2) c(i) c(i-1)
c(1) c(0)
CY Flag
7
The Shifting Operations Table

• Sl. No. Operation
  ALU Function
  Code
• 1. Open/ Logical Left Shift
  
  1 X 0 0 0
• 2. Closed Left Shift / Rotate Left
  
  1 X 0 0 1
• 3. Arithmetic Right Shift
  
  1 X 1 0 0
• 4. Open Right Shift
  
  1 X 0 1 0
• 
  
• 5. Closed Right Shift / Rotate Right
  1
  X 0 1 1

8
The Shifter bit Selection Table

• Sl. No. Operation Key ALU
  Function Code Cy c(n-1) c(i) c(0)
• 
  ( S2 S1 S0 )
• 1. Open/ Logical Left Shift
  0 0 0 a(n-1) a(n-2)
  a(i-1) 0
• --------------------------------------------------
  --------------------------------------------------
  ---------------
• 2. Closed Left Shift / Rotate Left
  0 0 1 a(n-1) a(n-2)
  a(i-1) Prev.Cy
• ---------------------------------------------
  --------------------------------------------------
  ----------------
• 3. Arithmetic Right Shift
  1 0 0 a(0)
  a(n-1) a(i1) a(1)
• --------------------------------------------------
  --------------------------------------------------
  ---------------
• 4. Open Right Shift
  0 1 0 a(0)
  0 a(i1) a(1)
• --------------------------------------------------
  --------------------------------------------------
  ---------------
• 5. Closed Right Shift / Rotate Right
  0 1 1 a(0) Prev. Cy
  a(i1) a(1)
• --------------------------------------------------
  --------------------------------------------------
  ----------------

9
The Carry Generator Structure

a(0)
a(n-1)
a(0)
a(0)
Cy
I4 I3 I2 I1 I0
S2
5 1 MUX
S1
S0
Shifted O/P To Cy
10
The MSB Generator Structure

0
a(n-1)
a(n-2)
Cy
a(0)
I4 I3 I2 I1 I0
S2
5 1 MUX
S1
S0
Shifted O/P To C(n-1)
11
The Intermediate Bits Generator Structure

a(i1)
a(i-1)
a(i-1)
a(i1)
a(i1)
I4 I3 I2 I1 I0
S2
5 1 MUX
S1
S0
Shifted O/P To C(n-2) .. C (1)
12
The LSB Generator Structure

a(1)
a(1)
0
Cy
a(1)
I4 I3 I2 I1 I0
S2
5 1 MUX
S1
S0
Shifted O/P To C(0)