Barrel Shifter in CPU Design
BY
BY
A HARISH
A HARISH
108W1D8002
108W1D8002
Barrel Shifter
Combinational logic circuit with n data inputs, n data outputs and a set of
control inputs
Control i/p’s specify how to shift the data between input and output
Part of µp CPU that specifies the direction of shift(left or right), type of shift
and amount of shift from 0 to n-1 bits
Shift operation is controlled by 6 bits: Four bits for the length, one bit for
direction, and one bit for type shift/rotate
Cont..
The 2 main blocks of barrel shifter are: shift-and-rotate array (
SARA)
and
the control logic
SARA performs the actual shift-and-rotate task on available data while its
controlling signals comes from control logic
SARA occupies most of the chip area, determines the critical path delay of
the barrel shifter and so implemented in dynamic or D
3
L logic
SARA
For a 16 bit barrel shifter, SARA is designed using 5 stages each with
sixteen cells
Basic cell used in this array is an AO22 gate that is called q-mux
Implements the function F= Ci1* In1 + Ci2* In2 ,where
Ci1,Ci2 come for control logic and
In1,In2 come from external inputs or previous stage o/p’s
D
3
L Logic
Uses local data instead of a global clock to maintain correct pre-charge and
evaluation phases
Eliminating the clock from dynamic gates using D
3
L logic yields less power
consumption and faster gate operation
A D
3
L gate operates in two phases, pre-charge ,evaluate and combination of
inputs plays the role of the clock signal
Low power consumption and faster gate operation are advantages of D
3
L
logic
SARA Block Diagram
Elimination of clk signal is done by substitution of suitable input combinations
with external inputs (In1,In2) and control inputs (Ci1,Ci2)
Control logic o/p’s are set low in pre-charge phase to charge the entire circuit
When the condition In1=In2=0 is satisfied , each qmux cell is pre-charged and
transition in In1 or In2 starts the evaluation phase
Advantage over domino logic is conditional evaluation and less power
consumption
vhdl coding
16 bit barrel shifter is implemented using behavioral model through modelsim
Inputs to the barrel shifter are a 16 bit input, 4 bit control input whose decimal
eq gives no of bits of shift or rotate(0000-1111) , 3 bit opsel indicates type of
operation and a carry bit
Types of operations performed here are 4 shift operations(shr,shl,sar,sal) and 4
rotate operations(ror,rol,rcl,rcr) that are represented by opsel
Finally, We get 16 bit output after shift or rotate and an o/p carry bit
Simulations
For 2 bits shift or rotate:
Let, 16 bit input (a) = 1011001011000101;
4 bit control i/p (b)= 0010 ;
opsel =000,010,100,110 ;
c_in= 0
Indicates the operations of logical shift left, arithmetic shift left, rotate left
and rotate carry left operations by 2 bit positions
Barrel shifter is a vital component in CPU architecture, enabling shifting and rotating operations on data inputs based on control signals. The shifter consists of two main blocks - Shift-and-Rotate Array (SARA) and Control Logic. SARA, designed with multiple stages of cells, executes shift and rotate tasks based on control signals. Implementing D3L logic in SARA enhances power efficiency and gate speed. This technology replaces the global clock with local data for pre-charging and evaluation phases, reducing power consumption. Check out the detailed implementation and advantages of the Barrel Shifter Chip in the CPU design process.
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Presentation Transcript
BY A HARISH 108W1D8002
Barrel Shifter Combinational logic circuit with n data inputs, n data outputs and a set of control inputs Control i/p s specify how to shift the data between input and output Part of p CPU that specifies the direction of shift(left or right), type of shift and amount of shift from 0 to n-1 bits Shift operation is controlled by 6 bits: Four bits for the length, one bit for direction, and one bit for type shift/rotate
Cont.. The 2 main blocks of barrel shifter are: shift-and-rotate array (SARA) and the control logic SARA performs the actual shift-and-rotate task on available data while its controlling signals comes from control logic SARA occupies most of the chip area, determines the critical path delay of the barrel shifter and so implemented in dynamic or D3L logic
SARA For a 16 bit barrel shifter, SARA is designed using 5 stages each with sixteen cells Basic cell used in this array is an AO22 gate that is called q-mux Implements the function F= Ci1* In1 + Ci2* In2 ,where Ci1,Ci2 come for control logic and In1,In2 come from external inputs or previous stage o/p s
D3L Logic Uses local data instead of a global clock to maintain correct pre-charge and evaluation phases Eliminating the clock from dynamic gates using D3L logic yields less power consumption and faster gate operation A D3L gate operates in two phases, pre-charge ,evaluate and combination of inputs plays the role of the clock signal Low power consumption and faster gate operation are advantages of D3L logic
SARA Implementation Elimination of clk signal is done by substitution of suitable input combinations with external inputs (In1,In2) and control inputs (Ci1,Ci2) Control logic o/p s are set low in pre-charge phase to charge the entire circuit When the condition In1=In2=0 is satisfied , each qmux cell is pre-charged and transition in In1 or In2 starts the evaluation phase Advantage over domino logic is conditional evaluation and less power consumption
vhdl coding 16 bit barrel shifter is implemented using behavioral model through modelsim Inputs to the barrel shifter are a 16 bit input, 4 bit control input whose decimal eq gives no of bits of shift or rotate(0000-1111) , 3 bit opsel indicates type of operation and a carry bit Types of operations performed here are 4 shift operations(shr,shl,sar,sal) and 4 rotate operations(ror,rol,rcl,rcr) that are represented by opsel Finally, We get 16 bit output after shift or rotate and an o/p carry bit
Simulations For 2 bits shift or rotate: Let, 16 bit input (a) = 1011001011000101; 4 bit control i/p (b)= 0010 ; opsel =000,010,100,110 ; c_in= 0 Indicates the operations of logical shift left, arithmetic shift left, rotate left and rotate carry left operations by 2 bit positions
