Final answer:
To add support for the BLT instruction, extend the control unit, modify instruction decoding, and add circuitry to use ALU flags for branching, ensuring no disruption to other instructions.
Step-by-step explanation:
To add support for the BLT (Branch Less Than) instruction in a processor design, you need to augment the control unit to handle this additional functionality. First, understand that the control unit usually generates signals that determine the flow of data and control within the CPU based on the instruction being executed.
Since the 'branchtype' output signal specifies whether the instruction is BEQ (Branch if Equal) when it is 1 or BLT when it is 0, the control unit needs to differentiate between these two branch instructions.
To implement this, you will need to extend the instruction set architecture (ISA) to include the BLT instruction with its unique opcode. In the control logic, add circuitry that can decode this opcode and set the 'branchtype' signal accordingly.
As for the branch decision, typically the ALU (Arithmetic Logic Unit) flags would be used to determine if a branch condition has been met. The ALU already provides a zero flag for BEQ, so for BLT, you would use the negative flag or a comparison result where the ALU checks if one value is less than another, depending on your ALU design.
The main steps you'll need to follow include:
Extend the control unit to output the 'branchtype' signal based on the detected instruction.
Modify the instruction decoder within the control unit to recognize the BLT opcode.
Add the necessary logic to the ALU or condition flag circuit to provide the correct branch decision for BLT.
Ensure the new design does not disrupt the execution of other instructions.
This addition will require detailed knowledge of the existing processor design to ensure compatibility and correct functionality.