Systems architecture: Register.

The idea

A register is a small, very fast storage location inside the CPU that holds a single item of data or an instruction while it is being processed. Registers are the fastest form of memory in a computer because they are part of the processor itself. Different registers have specific roles in the fetch-decode-execute cycle: the program counter holds the address of the next instruction, the memory address register holds the address being accessed, the memory data register holds the data or instruction fetched, and the accumulator holds results of calculations. Because registers are so few and so small, they hold only the data needed at that instant, not whole programs. Learn each named register by what it stores and when it is used.

What earns the marks

• A register is a small, fast storage location inside the CPU.
• Registers hold the data or instructions currently being processed rather than whole programs.
• The program counter holds the address of the next instruction to be fetched.
• The memory address register holds the address of the memory location being read from or written to.
• The memory data register holds the data or instruction fetched from or to be written to memory.
• The accumulator holds the result of arithmetic or logic operations carried out by the ALU.

Common mistakes and how to put them right

• Treating a register as a large store for whole programs.

  Put it right: A register holds only a single item of data or one instruction at a time.

• Confusing the program counter with the memory address register.

  Put it right: The program counter holds the address of the next instruction, while the memory address register holds the address currently being accessed.

• Thinking registers are slower than RAM.

  Put it right: Registers are the fastest memory because they are inside the CPU.

• Mixing up the memory data register with the memory address register.

  Put it right: The data register holds the data or instruction itself, while the address register holds the location.

Exam tips

• Link each named register to one clear job in the fetch-decode-execute cycle to avoid mixing them up.
• When asked what a register holds, state whether it is an address, data or a result.
• Remember registers are inside the CPU, so they are faster than cache, RAM and secondary storage.