Systems architecture: register

The idea

A register is a small, very fast storage location built into the central processing unit. Registers hold the values the CPU is working on at that moment: the address of the next instruction, the address currently being accessed, data just fetched from memory, or intermediate results from the arithmetic logic unit. Because they sit inside the CPU and are extremely fast, registers allow the fetch–decode–execute cycle to run quickly. They differ from main memory, which is larger and slower and holds the whole program and its data. Named examples include the program counter, memory address register, memory data register and accumulator. A multiple-choice question on this term is likely to test the definition, the speed and location of registers, or the job of a named register, so learn each register's specific role rather than treating them as interchangeable.

What earns the marks

• A register is a small, fast storage location inside the CPU.
• Registers hold the current instruction, addresses and intermediate results needed during processing.
• Named registers have specific roles, for example the program counter holds the address of the next instruction.
• Registers are faster than main memory because they are built into the CPU and hold only the values in current use.

Common mistakes and how to put them right

• Treating a register as the same as main memory.

  Put it right: A register is small and inside the CPU, while main memory is larger and separate.

• Assuming all registers do the same job.

  Put it right: Each named register, such as the program counter or accumulator, has a specific role.

• Thinking registers store the whole program.

  Put it right: They hold only the current values being processed, such as an address or intermediate result.

Exam tips

• Read all options before choosing, and eliminate any that describe main memory rather than a register.
• Link each named register to its specific job so you can recognise the correct description.
• Remember the key features: small, fast and located inside the CPU.