IGCSE Computer Science 0478 — 3.1 Computer Architecture Hangman

The 33 key terms

1input, process, store, output

A digital system takes in data (input), works on it (process), keeps it (store) and shows a result (output).

2CPU

The central processing unit is the 'brain' of a digital system. It carries out program instructions one after another and processes data. Its speed affects how fast tasks run.

3RAM

Random Access Memory: fast, temporary memory that holds the data and programs currently in use. It is volatile, so everything in it is lost when the power is turned off.

4microprocessor

A CPU built as an integrated circuit on a single chip. It is only the processor, not the whole computer, and is found in PCs, phones and embedded devices.

5Von Neumann architecture

A computer design where instructions and data share the same main memory (RAM) and the CPU fetches them one at a time. This is the stored program concept, named after John von Neumann.

6ALU

Arithmetic logic unit: the part of the CPU that does calculations (add, subtract) and logic decisions (AND, OR, NOT, comparisons). Results are held in the accumulator.

7Control Unit (CU)

The part of the CPU that decodes instructions and sends control signals to coordinate the ALU, registers, memory and other parts during the fetch-decode-execute cycle.

8register

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.

9Program counter (PC)

A register in the CPU that holds the address of the next instruction to be fetched. It increments after each fetch and changes when the program jumps.

10MAR

Memory Address Register: a CPU register that holds the address of the memory location about to be read from or written to. It holds an address, not data, and sends it along the address bus.

11MDR

Memory Data Register: a CPU register that temporarily holds the data or instruction just fetched from RAM, or data about to be written to RAM. It holds data, not addresses.

12CIR

Current Instruction Register: holds the instruction copied from the MDR while the control unit decodes it and the CPU executes it. It stores the instruction itself, not an address.

13Accumulator (ACC)

A register in the CPU that holds data being worked on and the results of ALU calculations, storing intermediate results before they are saved back to memory or output.

14bus

A set of wires that carries data, addresses or control signals between the CPU, RAM and other components. More wires means more bits can be carried at once.

15address bus

A one-way bus that carries the address of a memory location from the CPU (sent from the MAR) to RAM. It carries addresses, not data.

16data bus

A set of wires that carries data and instructions in both directions between the CPU and RAM. It connects to the MDR.

17control bus

A set of wires that carries control signals, such as read and write, from the control unit to other components so they work together. It carries signals, not data or addresses.

18fetch-decode-execute cycle

The CPU repeats three stages for every instruction: fetch it from RAM, decode what it means, then execute it. The cycle then starts again with the next instruction.

19MAR <- PC

At the start of fetch, the address held in the PC is copied into the MAR. It then travels along the address bus to RAM. The PC still holds the address at this point, because it is copied, not moved.

20MDR <- RAM[MAR]

Fetch step: RAM finds the instruction at the address in the MAR (address bus), a read signal comes via the control bus, and the instruction travels on the data bus into the MDR.

21PC ← PC + 1; CIR ← MDR

End of fetch: the PC is increased by 1 so it points to the next instruction, and the instruction held in the MDR is copied to the CIR, ready to be decoded.

22Decode

The fetch-decode-execute stage where the control unit reads the instruction in the CIR and works out the operation to perform and the data (operand) it needs.

23execute

The stage of the fetch-decode-execute cycle where the decoded instruction is carried out, e.g. fetching data, calculating in the ALU, storing in the ACC, or changing the PC.

24register trace

Recording the contents of the PC, MAR, MDR, CIR and ACC, and the bus used, at each step as one instruction goes through the fetch-decode-execute cycle.

25Clock speed (GHz)

How many billions of cycles per second a CPU can run. A higher GHz usually means faster single tasks, but it is not the only measure of speed.

26CPU core

One processing unit inside a CPU. A dual-core, quad-core or octa-core CPU has 2, 4 or 8 cores, so it can work on separate tasks at the same time.

27cache

A small amount of very fast memory in or near the CPU that stores frequently used instructions and data, so the CPU can get them faster than from RAM.

28CPU performance

How fast a CPU runs programs. More cores, a bigger cache or a faster clock speed can raise it, but each has limits: software must split tasks, cache is costly, and a faster clock makes more heat.

29instruction set

The list of all the commands (such as load, add, store and jump) that a CPU design can process. The control unit decodes each instruction using it.

30machine code

The binary (0s and 1s) form of instructions that the CPU can run directly. Each instruction has an opcode and an operand. Programs must be translated into it to run.

31embedded system

A computer system built into a larger device to perform one dedicated function, usually running a fixed program. Examples: a washing machine controller or a microwave timer.

33general-purpose computer

A computer, such as a PC or laptop, that can do many different jobs by running different programs, and the user can install new software.

35fetch-decode-execute

A full-mark CPU answer uses exact names for registers, buses and units, and gives the fetch, decode and execute steps in order, e.g. PC to MAR, then MDR, then CIR.