Typically, the program counter is advanced to the next instruction, and then the current instruction is executed. Also known as a "sequence control register" and the "instruction pointer." See ...

Easy just use the Program Counter to cycle through all the code-space keeping R0 and R2 as zero. When you hit an instruction that makes R2 the same value as the stuff in 0x00000 (as R0 is 0x00000 ...

Except they don’t change instructions close to the instruction pointer but in a logically separate ‘program ... A program counter can be made from a shift register and a half adder.