80386 Barrel Shifter
15 hours ago
- #barrel-shifter
- #microcode
- #80386
- The 80386 uses a hybrid barrel shifter with two stages: a coarse shifter (32x8 crossbar) and a fine shifter, allowing shifts of 0-31 bits in one cycle.
- All shift and rotate instructions (ROL, ROR, SHL, SHR, SAR) share the same microcode sequence, reducing operations to right shifts by configuring the 64-bit input differently.
- Left shifts are implemented by placing the operand in the high half of the shifter input and right-shifting by (width - count).
- RCL and RCR (rotate through carry) require special handling: the carry flag is embedded into the shifter input, and counts are reduced modulo (width + 1) in microcode.
- Bit test instructions (BT, BTS, BTR, BTC) reuse the barrel shifter via a rotate-modify-rotate strategy, avoiding dedicated bit extraction hardware.
- The 386's design emphasizes hardware reuse and parameterized microcode, with dedicated logic handling EA calculation and stack addressing.