Registers: the CPU's own tiny worktop

By the end of this lesson

By the end you can say what a register is, tell general purpose from special purpose registers, and explain the jobs of the accumulator (ACC), the index register (IX) and the status register.

Why the CPU needs its own tiny storage

Main memory is where the program lives, but it is slow to reach from inside the CPU. The instructions currently being worked on need somewhere faster.

A register is a tiny storage location inside the CPU, holding a fixed number of bits (usually one word — 32 or 64 bits on a modern machine).

Registers are the fastest storage in the computer, and there are only a handful of them. That is why they are used for the one thing happening right now.

Definition

A small, very fast storage location inside the processor, typically holding one word of data or one address, used during the fetch–execute cycle.

Registers are few and fast; main memory is large and slower.

General purpose vs special purpose

The syllabus splits registers into two groups:

Kind	Who decides what it holds	Examples
General purpose registers (GPR)	The programmer or compiler — whatever is convenient: a value, a total, an address	Often named R1, R2 …
Special purpose registers	The CPU itself, for one fixed job	PC, MAR, MDR, CIR, ACC, IX, status register

The difference to write down in an exam: a general-purpose register has no fixed job, while a special-purpose register always does one specific job.

The three special registers of this lesson

Accumulator (ACC). The ALU's own register. Arithmetic and logic results land here — hence the name: it accumulates the results. Almost every calculation passes through it.
Index register (IX). Holds a value used as an offset — a position relative to a starting point. Its classic job is stepping through the elements of an array: the base address stays put, IX counts 0, 1, 2 … along it.
Status register (also called the flags register). Holds single-bit flags that record the outcome of the last operation: the zero flag (result was 0), carry flag, negative flag and overflow flag. Conditional instructions ("jump only if the result was negative") read these flags.

Worked example: Using ACC and IX

A program adds up the 4 values in an array starting at address 500.

Have a go first?

Quick check

Multiple choiceQuestion 1 · Recall

Multiple choiceQuestion 2 · Apply

One-line answerQuestion 3 · Recall

Answer the 3 questions above to continue.