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Shifter

Description​

The shifter shifts its input to the right by one if enabled (otherwise it implements the identity function). If the shifter is enabled, the most significant bit of the output can be selected using the CARRY_SELECT control line from one of the following:

  • Carry flag: Current value of CF in the flags register
  • Zero: Constant 0
  • Sign bit: Current value of SF in the flags register. The sign bit of the input operand.
  • Serial input: Value from I/O unit to enable certain I/O operations

Implementation in Hardware​

The shifter itself is implemented with two 4-bit mux chips (74-157 shifterLow and shifterHigh). The selection of the correct value to shift in is done using a decoder (74-153 carryMux). If the shifter is not enabled, the carry output from the ALU chips should just be passed through. If it is enabled, the last bit of the shifter input should be used as carry output. This selection is implemented using (our favorite part) a mux (74-157 shiftCarry).

Schematic​

ALU shifter schematic

As well as a separate carryMux schematic which was easier to use for the final nested schematics.

ALU carry mux