PC operations, including the detailed steps to obey JMP, JNE & JGE

As mentioned above, we have to fetch each instruction, which means:

• send the contents of PC to memory to fetch the instruction to be obeyed
• increment the contents of PC to point to the next instruction

For JMP instructions, and JNE and JGE instructions when the test condition is true, we have to:

• copy the operand-location to PC

To do these actions, we could have another set of connections to memory and another adder. However, to simplify the hardware we can instead link the PC register to the existing ALU and through it to the memory. See figure 4.

  

Figure 4: MU0 + PC operations

The STP instruction simply has to stop the next instruction being fetched.