Building a CPU from the Ground up Part 8 - The Programme Counter

The programme counter (PC)is a complex beast. In its simplest mode 'increment', (when I is set to 1) it is just a 4 bit counter and allows the CPU to step through the 16 instructions stored in the ROM. However, if the CPU is to have any branch functions to allow loops in the code then the PC needs to be able to jump to any 4 bit number. To do this it has a load mode. When L is set to 1 the PC reads an address off the address bus and sets its value to this. The ROM will consequently then jump to this address on its next instruction.

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