What is Barrel Shifter in ARM with example and its working operation
The Barrel Shifter is a key component of the ARM processor's datapath responsible for shifting and rotating operations on data. It's an essential part of the ARM's instruction set architecture, providing efficient ways to manipulate data.
Hardware Implementation of barrel shifter :
The Barrel Shifter is implemented as a combinational circuit with multiple stages, allowing for efficient shifting
and rotating operations in a single clock cycle.
The shifting and rotating operations are performed in parallel across the 32 bits of the input data,
improving throughput and minimizing latency.
How Barrel Shifter works
A barrel shifter uses a combination of multiplexers to achieve the desired shift in one clock cycle. For an n-bit word, it consists of log2(n) stages of multiplexers. Each stage shifts the input by powers of 2 (1, 2, 4, 8, etc.) depending on the control signals.
Steps
Stage 1: Shifts the input by 1 bit if the least significant bit of the shift amount is set.
Stage 2: Shifts the result of stage 1 by 2 bits if the second bit of the shift amount is set.
Stage 3: Shifts the result of stage 2 by 4 bits if the third bit of the shift amount is set.
Continue similarly for more stages if necessary.
By combining these stages, the barrel shifter can achieve any arbitrary shift in a single cycle.
Let us understand how the Barrel Shifter works.
• Functionality: The Barrel Shifter can perform various bitwise operations on data, including left shifts, right shifts,
logical shifts, and rotations.
It operates on a 32-bit input, which can be either a register value or an immediate value.
The Barrel Shifter takes two inputs: the data to be shifted or rotated and a shift amount specified by the instruction.
Shift Amount: The shift amount can be specified in three ways:
• Immediate Shift: A constant value encoded in the instruction. For example,
LSL R0, R1, #5 would left-shift the value in register R1 by 5 bits.
• Register Shift: The value of a register. For example, LSL R0, R1, R2 would left-shift the value in
register R1 by the number of bits specified in register R2.
• Logical Shift Left (LSL): Shifts the bits of the input to the left by the specified number of bits.
Zeros are shifted into the least significant bits, and bits shifted out from the most significant end are discarded.
• Logical Shift Right (LSR): Shifts the bits of the input to the right by the specified number of bits. Zeros are shifted
into the most significant bits, and bits shifted out from the least significant end are discarded.
• Arithmetic Shift Right (ASR): Similar to LSR, but it preserves the sign bit (the most significant bit)
during the shift, effectively performing a sign-extending right shift.
• Rotate Left (ROL): Shifts bits to the left, and the bits that fall off the left end are shifted into the right end.
• Rotate Right (ROR): Rotates the bits of the input to the right by the specified number of bits.
Bits shifted out from one end are shifted back in from the opposite end.
• Rotate Right with Extend (RRX): Similar to ROR, but with an additional carry bit shifted in from the carry flag.
Example
Consider an 8-bit word '10110011' and a 3 bit shift amount '3'.
Question -1 : Shifting '10110011' left by 3 positions.
Answer -1 :
Original: 10110011
Shifted: 10011000
Question -1 : Shifting '10110011' right by 3 positions.
Answer -1 :
Original: 10110011
Shifted: 00010110
In summary, the Barrel Shifter in ARM processors provides efficient and versatile operations for shifting and rotating data, enabling various bitwise manipulations essential for many arithmetic, logical, and data processing tasks in software executed on ARM-based systems.
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