All of the various kinds of systems that we have discussed so far in this chapter can readily be implemented on practical computers. But none of them at an underlying level actually work very much like typical computers. Register machines are however specifically designed to be very simple idealizations of present-day computers.
Under most everyday circumstances, the hardware construction of the computers we use is hidden from us by many layers of software. But at the lowest level, the CPUs of all standard computers have registers that store numbers, and any program we write is ultimately converted into a sequence of simple instructions that specify operations to be performed on these registers.
Most practical computers have quite a few registers, and support perhaps tens of different kinds of instructions. But as a simple idealization one can consider register machines with just two registers—each storing a number of any size—and just two kinds of instructions: "increments" and "decrement-jumps". The rules for such register machines are then idealizations of practical programs, and are taken to consist of fixed sequences of instructions, to be executed in turn.
Increment instructions are set up just to increase by one the number stored in a particular register. Decrement-jump instructions, on the other hand, do two things. First, they decrease by one the number in a particular register. But then, instead of just going on to execute the next instruction in the program, they jump to some specified other point in the program, and begin executing again from there.
Since we assume that the numbers in our registers cannot be negative, however, a register that is already zero cannot be decremented. And decrement-jump instructions are then set up so that if they are applied to a register containing zero, they just do essentially nothing: they leave the register unchanged, and then they go on to execute the next instruction in the program, without jumping anywhere.
This feature of decrement-jump instructions may seem like a detail, but in fact it is crucial—for it is what makes it possible for our register machines to take different paths depending on values in registers through the programs they are given.