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Circuit Description

The control unit ('Steuerwerk') of the PRIMA processor. It takes the following inputs:

• clock (rising edge)
• nreset (active low)
• external switch_in input

• current instruction (contents of the BR register)
• current accumulator contents
• current ALU overflow signal output

• enable signals for the four registers (AR,BR,PC,AKKU)
• control signals for the multiplexers
• ALU function selection
• RAM write enable signal for the store instruction.

The upper part of the circuit calculates the conditional branch flag used to control the PC multiplexer. The value is based on the current instruction, the current accumulator value (e.g. zero), and the current sequencer state.

The bottom part of the circuit contains the flipflop and logic to manage the overflow flag.

Finally, the flipflop on the right is used to generate a (hazard-free) impulse required for the write-enable signal for the main memory.