← IGCSE Computer Science Paper 1

Registers

A very small, very fast storage location inside the CPU (not in RAM) that holds one item of data or one address, such as the PC, MAR, MDR, CIR or ACC.

Part of: IGCSE Computer Science 0478 — 3.1 Computer Architecture — Module 8, IGCSE Computer Science 0478 — 4.1 Types of Software and Interrupts — Module 43

Understanding Registers in IGCSE Computer Science Paper 1

What it does: The registers are listed inside the CPU boundary, separate from RAM, which is why they are faster to reach. Each special-purpose register has a fixed role, so the PC and CIR hold different items. A register is a tiny storage location built into the CPU itself. RAM is a separate component outside the CPU. A register is very small. It holds a single piece of data or a single address. Programs are stored in RAM. The PC holds the address of the next instruction. The ACC holds the results of calculations from the ALU. This mixes up registers with RAM. Registers are tiny, fast and located in the CPU.

Common mistakes: The instruction is copied from the MDR into the CIR, which holds the current instruction. The line should be CIR ← MDR. The MAR holds an address, not data. The data fetched from RAM is in the MDR, so the line should be ACC ← MDR.

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This module's key terms are also in a printable crossword (Modules 218–227) and word search (Modules 218–239).

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Learning
Duo
Where is a register located?
Inside RAMInside the CPU
Matches
Registers are large storage areas in RAM that hold whole programs
↓
Registers are very small and are inside the CPU, not in RAM. They hold one item of data or one address, not whole programs.
Applying
Meanings
Term
register
Meaning
A very small, very fast storage location inside the CPU (not in RAM) that holds one item of data or one address, such as the PC, MAR, MDR, CIR or ACC.
Practice
PC holds the next instruction address
Testing
Gaps
MAR ←
Check Answer
Spots
3PC ← PC + 1
4MDR ← CIR

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Quick Questions

1. Where is a register located?
A. Inside RAM
B. Inside the CPU ✓
A register is a tiny storage location built into the CPU itself. RAM is a separate component outside the CPU.
2. Which statement about registers is correct?
A. Registers are faster to access than RAM ✓
B. Registers are slower to access than RAM
Registers sit inside the CPU, so the CPU can reach them much faster than RAM.
3. How much can a register hold at one time?
A. One item of data or one address ✓
B. A whole program and all its data
A register is very small. It holds a single piece of data or a single address. Programs are stored in RAM.
4. Which register holds the address of the next instruction to be fetched?
A. Accumulator (ACC)
B. Program counter (PC) ✓
The PC holds the address of the next instruction. The ACC holds the results of calculations from the ALU.
5. Which register holds the result of a calculation done by the ALU?
A. Accumulator (ACC) ✓
B. Memory address register (MAR)
The ACC stores the ALU's results. The MAR holds an address of a location in RAM.

Match the Pairs

register → A very small, very fast storage location inside the CPU that holds one item of data or one address
MAR → A special-purpose register that holds the address in RAM about to be read from or written to
A register is inside the CPU, so it is much faster to access than RAM, which is outside the CPU → CPU: [PC] [MAR] [MDR] [CIR] [ACC] | RAM (outside)
Special-purpose registers each hold a specific thing, such as the next instruction address or the current instruction → PC = next instruction address, CIR = current instruction
Registers are large storage areas in RAM that hold whole programs → Registers are very small and are inside the CPU, not in RAM. They hold one item of data or one address, not whole programs.
The MDR holds the address of the next instruction to be fetched → The MDR holds data or an instruction fetched from or written to RAM. The PC holds the address of the next instruction.

Key Term

register
A very small, very fast storage location inside the CPU (not in RAM) that holds one item of data or one address, such as the PC, MAR, MDR, CIR or ACC.

Fill in the Blanks

1. MAR ← PC
Hint: Type the register that holds the address of the next instruction to be fetched.
2. MDR ← RAM[MAR]
Hint: Type the register that temporarily holds data or an instruction just read from memory.
3. ACC ← ACC + MDR
Hint: Type the register that stores the results of ALU calculations.
4. Register location: inside the CPU
Hint: Type the component of the computer where registers are found, rather than in RAM.
5. Register speed: faster than RAM
Hint: Type the comparison word that describes how quickly registers work compared with RAM.

Spot the Error

Question 1
A MAR ← PC
B MDR ← RAM[MAR]
C PC ← PC + 1
D MDR ← CIR
Line D — The instruction is copied from the MDR into the CIR, which holds the current instruction. The line should be CIR ← MDR.
Question 2
A MAR ← 100
B MDR ← RAM[MAR]
C ACC ← MAR
Line C — The MAR holds an address, not data. The data fetched from RAM is in the MDR, so the line should be ACC ← MDR.
Question 3
A MDR ← PC
B MDR ← RAM[MAR]
C CIR ← MDR
Line A — The PC holds an address, and addresses are sent to the MAR. The MDR holds data from RAM. The line should be MAR ← PC.
Question 4
A location ← "RAM"
B speed ← "very fast"
C holds ← "one item"
Line A — Registers are inside the CPU, not in RAM. The line should be location ← "CPU".
Question 5
A IF register = PC THEN
B holds ← "data from RAM"
C ENDIF
Line B — The program counter holds the address of the next instruction to fetch. Data read from RAM is held in the MDR, not the PC.

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