Activity Learning:
During the design of a 16-bit processor with a single address format and indirect addressing mode, I gained significant insights into processor architecture, including the detailed working of various modules like the ALU, Program Counter, Instruction Register, and Control Unit. I learned how indirect addressing adds flexibility by allowing dynamic memory access through pointers. Implementing the instruction lifecycle (fetch-decode-execute-write back) helped reinforce my understanding of how a CPU processes instructions step-by-step. Additionally, working with simulation tools like Logisim provided practical experience in designing and testing digital circuits.
Challenges Faced and Solutions:
Designing the Control Unit:
Challenge: Generating accurate control signals for each microinstruction in indirect addressing mode was difficult because it required additional steps for fetching the effective address.
Solution: I broke down each instruction into smaller microinstructions, ensuring that control signals were correctly sequenced. Using a state-based approach simplified the process of handling indirect addressing.
Operand Fetch in Indirect Addressing:
Challenge: Unlike direct addressing, where the operand is immediately available, indirect addressing required an extra memory access to retrieve the actual operand.
Solution: I added an extra operand fetch cycle in the instruction lifecycle. By carefully timing the read and write signals in the control unit, I ensured that the data was fetched correctly.
Testing and Debugging the Processor:
Challenge: Ensuring that all components worked together seamlessly was challenging. Initially, I faced issues where the fetched instruction was not being executed correctly.
Solution: I used a stepwise simulation approach, testing each module independently before integrating them. Debugging was done by monitoring intermediate registers and control signals during each clock cycle.
Handling Edge Cases:
Challenge: Some edge cases, like fetching data from uninitialized memory locations or handling jumps correctly, caused incorrect results during testing.
Solution: I added initialization routines and boundary checks to ensure that the program counter and memory accesses remained within valid ranges.
Overall, this activity taught me how to approach complex digital design problems methodically. By dividing the problem into smaller, manageable tasks and iteratively testing each part, I was able to complete the processor design successfully. It also helped me develop problem-solving skills and gain confidence in working with computer architecture concepts.