Registers

Registers are tiny, very fast storage locations built into the CPU. They are not locations in RAM: they sit inside the processor itself, holding the particular instruction or data value that the CPU is handling at that instant.

A register holds just one item at a time: it may be an instruction, a data value or an address in memory. Because a register is built into the CPU and holds a single item, registers are the fastest storage in a computer. There are only a few of them, though, so the total amount they can store is tiny, smaller than the cache.

What the named registers hold

Register	What it stores
Program Counter (PC)	The address of the next instruction
Memory Address Register (MAR)	The address in memory that the CPU will access next for a read or a write operation
Memory Data Register (MDR)	Data or an instruction that has just been read, or data waiting to be written
Accumulator	Data, including results from calculations

An address tells the CPU where something is; data is the value itself. For example, if the next instruction is at address 5120, the Program Counter holds 5120. Once that instruction is fetched, the instruction itself is held in the Memory Data Register.

The Program Counter holds the address 5120; it points to that address in memory, where the instruction is stored. Once fetched, the instruction itself is held in the Memory Data Register, showing an address is not the same as data.

Describe a register by what it stores rather than by an action it performs: the Program Counter stores the address of the next instruction, so it does not fetch the instruction, and it does not count how many programs have run.

What the named registers hold

Practise this