Show understanding of the purpose and role of registers, including the difference between general purpose and special purpose registers

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Computer Science 9618 – 4.1 CPU Architecture: Registers

4.1 Central Processing Unit (CPU) Architecture

Objective

Show understanding of the purpose and role of registers, including the difference between general‑purpose and special‑purpose registers.

Why a CPU Needs Registers

Registers are small, fast storage locations inside the CPU. They hold data that the processor is currently working with, reducing the need to access slower main memory. This improves execution speed and allows the control unit to coordinate operations efficiently.

Classification of Registers

  • General‑purpose registers (GPRs) – used by programmers and the CPU for a wide range of data manipulation tasks.

  • Special‑purpose registers (SPRs) – dedicated to specific control or status functions within the CPU.

General‑Purpose Registers

GPRs are typically named R0, R1, … or given functional names in specific architectures. They can store operands, intermediate results, or addresses.

RegisterTypical UseExample Architecture
Accumulator (ACC)Holds results of arithmetic/logic operations8085, ARM
Base Register (BR)Provides a base address for indexed addressingIBM System/360
Counter Register (CTR)Controls loop iteration countsMotorola 68000
Data Register (DR)Temporarily stores data being transferred to/from memoryIntel x86
Index Register (IX)Used for array indexing and pointer arithmeticZ80, MIPS (as \$t0‑\$t9)

Special‑Purpose Registers

SPRs support the internal operation of the CPU and are not generally available for arbitrary data storage.

RegisterFunctionTypical Symbol
Program Counter (PC)Holds the address of the next instruction to fetchPC
Instruction Register (IR)Stores the currently decoded instructionIR
Memory Address Register (MAR)Contains the address of memory location being accessedMAR
Memory Data Register (MDR)Holds data read from or to be written to memoryMDR
Status Register / Flags (SR/FLAGS)Indicates the outcome of the last operation (zero, carry, overflow, sign, etc.)SR, FLAGS

Key Differences Between GPRs and SPRs

  1. Purpose

    • GPRs: General data manipulation, accessible to assembly‑level programmers.

    • SPRs: Control and status functions, usually not directly manipulated by user code.

  2. Number and Naming

    • GPRs: Vary widely (e.g., 8, 16, 32, 64) and often have simple numeric names.

    • SPRs: Fixed set with well‑defined names (PC, IR, MAR, etc.).

  3. Visibility to the Programmer

    • GPRs: Can be used explicitly in instructions (e.g., ADD R1, R2).

    • SPRs: Updated implicitly by the CPU during the fetch‑decode‑execute cycle.

  4. Impact on Instruction Set Design

    • GPRs influence the number of operand fields an instruction can contain.

    • SPRs dictate how the CPU controls flow and interacts with memory.

Typical Data Path Involving Registers

A simplified data path for an arithmetic instruction can be described as:

\$\$\text{IR} \leftarrow \text{Memory}[ \text{PC} ] \quad

\text{PC} \leftarrow \text{PC} + 1 \quad

\text{ACC} \leftarrow \text{ACC} \; \text{op} \; \text{R}_n\$\$

where op is an arithmetic or logical operation, R_n is a general‑purpose register, and the result is stored back in the accumulator.

Suggested diagram: Block diagram of a CPU showing the flow between PC, MAR, MDR, IR, GPRs, and the ALU.

Summary

Registers are essential for the rapid execution of instructions. General‑purpose registers provide flexible storage for data and addresses, while special‑purpose registers manage the internal control flow, memory interfacing, and status reporting of the CPU. Understanding their distinct roles is fundamental to analysing instruction sets and CPU performance.