Barrel Shifter
Instructions
The barrel shifter is capable of shifting a 40-bit value, up to 16bits right or left, in a single cycle. The source can be either of the two DSP accumulators or the X bus, to support multi-bit shifts of register or memory data. The shifter requires a signed binary value to determine both the magnitude (number of bits) and direction of the shift operation. A positive value shifts the operand right. A negative value shifts the operand left. A value of ‘0’ does not modify the operand.
The barrel shifter can be used by both MCU and DSP instructions. The barrel shifter is 40 bits wide, thereby obtaining a 40-bit result for DSP shift operations and a 16-bit result for MCU shift operations. Data from the X bus is presented to the barrel shifter between bit positions 16 to 31 for right shifts, and between bit positions 0 to 16 for left shifts.
- Single cycle ±16-bit shift
- Shift left
- Logical shift right - ignores sign bit
- Arithmetic shift left - retains sign bit
- ASR, LSR & SL are identical in dsPIC® DSC and PIC24
- Rotate instructions (RRC, etc.) do not use barrel shifter
Barrel shifter instructions
| Instruction | Description |
|---|---|
| ASR | Arithmetic multi-bit right shift of data memory |
| LSR | Logical multi-bit right shift of data memory |
| SL | Multi-bit shift left of data memory |
| SAC | Store DSP accumulator with optional shift |
| SAC.R | Store Rounded DSP accumulator with optional shift |
| SFTAC | Shift DSP Accumulator |
ASR, LSR, and SL operate on the ALU, while SAC, SAC.R, and SFTAC operate on the accumulators.
Examples
The first example is using the arithmetic multi-bit right shift of data memory, asr instruction, while the second example utilizes the logical multi-bit right shift of data memory, lsr instruction. The third example performs a multi-bit left shift, and the last example shows the DSP Accumulator shift.
