Computer architecture

Computer Science · Hardware. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The CPU processes the instructions and data in a computer. A microprocessor is a CPU built on a single integrated circuit (chip).

In the Von Neumann model, programs and data are stored together in memory. The CPU repeats one cycle again and again: fetch the next instruction from memory, decode it, then execute it. Small, very fast stores inside the CPU called registers hold the addresses and data needed during the cycle, and buses carry them between the CPU and memory.

How fast a CPU works depends on its clock speed, its number of cores and the size of its cache.

Rules to remember

PC: address of the next instruction. MAR: address being read from or written to. MDR: data or instruction just fetched. CIR: instruction being decoded. ACC: results from the ALU
ALU: does arithmetic and logic. CU: decodes instructions and sends control signals
Address bus: one direction, CPU to memory. Data bus and control bus: both directions
Fetch: PC → MAR, PC increases by 1, instruction → MDR → CIR. Then the CU decodes and the instruction is executed
Clock speed: cycles per second. More cores: more instructions at the same time. Larger cache: more often-used data kept close to the CPU
Instruction set: the list of all machine code commands a CPU can carry out
Embedded system: a computer built into a device to do one dedicated job, e.g. washing machine, traffic lights

Common mistake

Students confuse the PC and the MAR. The PC holds the address of the next instruction; the MAR holds the address in use right now.

Worked example

The PC holds 200. One instruction is fetched. What do the PC and the MAR hold after the fetch stage?

The address in the PC, 200, is copied to the MAR.
The PC is increased by 1, so it now holds 201, ready for the next instruction.
The instruction stored at address 200 travels along the data bus to the MDR and is copied to the CIR. The MAR still holds 200.

Answer: The PC holds 201 and the MAR holds 200.

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