microinstructions

Question

Give the binary microinstruction corresponding to each of the following sets of micro-operations. should be 32 bits for each.

1. Increment SP by 1. Store the results back in the SP. Load the MAR with the SP goto 0.
2. Load the contents of register IR into MAR; Read from main memory. 

3.Perform the Boolean AND of the contents of IR and AMASK;

Store the result in register AC.

4.Test the contents of register TIR for its sign bit N;

If N is set, got to the microinstruction at address 50;

Shift the contents of TIR one bit to the left;

Store the result back into the TIR register.

Answer 1

First, for incrementing the stack pointer (SP) by 1, and saving it back, then loading the MAR with the updated SP, and jumping to address 0 — you basically want to let the SP send its value to the ALU, tell the ALU to add 1, then write that result back into the SP. After that, output the SP again so the MAR can load it, enable writing into MAR, and finally set the microinstruction to jump back to address 0. Next, to load what’s inside the instruction register (IR) into the MAR, and then read from main memory, you just let IR output its value to MAR, enable MAR to write that value, and trigger the memory read signal so it fetches data. For doing a Boolean AND between IR and AMASK, then storing the result in the accumulator (AC), you enable both IR and AMASK outputs into the ALU, set the ALU to perform an AND operation, and then write the result into AC. Finally, for checking the sign bit (N) of the temporary instruction register (TIR), if that bit is set (meaning it’s 1), you jump to microinstruction 50. Otherwise, you shift TIR left by one bit, and store the result back in TIR. So it’s basically: check the sign bit, branch if needed, or do the shift and store. Once you’ve got the control signal bits mapped out — like which bits control register output, ALU ops, memory access, branching, and next addresses. You can translate these steps into the exact 32-bit binary microinstructions, otherwise respond to my message. You should be good to go!