The CPU contains a number of registers. These are small scratch-pad areas, similar to memory slots, except that the CPU can get hold of them quickly and easily without bothering the memory itself. It uses the registers to hold numbers temporarily before writing them out to memory. Different processors have different numbers of registers - any number from 3 to about 8 or so. They are usually referred to by letters of the alphabet. The diagram has four registers, called A, B, C and D. All CPUs have a program counter (called PC). This is a special register that keeps track of which instruction in the program the CPU is carrying out. If the program starts in memory at address 200, then the PC will start with the number 200. When it moves to the next instruction (which may well be in memory address 201), then the PC will hold this new value.