The CPU and the fetch-decode-execute cycle

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Lesson notes

What the CPU does

The central processing unit (CPU) is the "brain" of a computer. It fetches instructions from memory, decodes them and executes them.

Components

ALU (arithmetic logic unit): performs calculations and logical comparisons.
Control unit (CU): directs the operation of the CPU, controlling the flow of data and instructions.
Registers: very fast storage inside the CPU: program counter (PC) (address of the next instruction), memory address register (MAR), memory data register (MDR), accumulator (ACC) (stores results).
Cache: small, very fast memory close to the CPU holding frequently used data.

CPU components

The fetch-decode-execute cycle

Fetch: the PC's address is copied to the MAR; the instruction at that address is fetched into the MDR; the PC increments.
Decode: the control unit decodes the instruction.
Execute: the instruction is carried out (the ALU may calculate).

Fetch, decode and execute

Performance

Clock speed (GHz: cycles per second), number of cores (more can run tasks in parallel), and cache size affect speed.

Multiple processor cores

Von Neumann architecture

Instructions and data are stored in the same memory. Embedded systems (e.g. washing machine controller) are dedicated computers.

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Practice questions

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1.What does CPU stand for?
Easy
- Acentral processing unit
- Bcentral program unit
- Ccomputer processing unit
- Dcore processing utility
2.What does ALU stand for?
Easy
- Aarithmetic logic unit
- Ball logic unit
- Canalogue logic unit
- Darray logic unit
3.What does the control unit do?
Easy
- Adirects the flow of data and instructions
- Bperforms calculations
- Cstores files
- Ddisplays output
4.What is a register?
Easy
- Aa small, fast storage location in the CPU
- Ba hard drive
- Ca network card
- DRAM
5.What is cache memory?
Easy
- Asmall, very fast memory close to the CPU
- Bslow storage
- Ca hard drive
- Da printer
6.What does the PC hold?
Easy
- Athe address of the next instruction
- Bthe result
- Cthe current data
- Dthe clock speed
7.What does the MAR hold?
Easy
- Athe address of the memory location being accessed
- Bthe data
- Cthe instruction
- Dthe result
8.What does the MDR hold?
Easy
- Adata transferred to or from memory
- Bonly a memory address
- Conly the clock frequency
- Donly the result of every addition

More questions

9.What does the accumulator hold?
Easy
- Athe results of calculations
- Bthe next address
- Cthe clock
- Dthe cache
10.What is clock speed measured in?
Easy
- Ahertz or gigahertz
- Bbytes
- Cpixels
- Dmetres
11.What is a core?
Easy
- Aa processing unit within a CPU
- Ba type of memory
- Ca hard disk
- Da motherboard
12.During a normal sequential fetch in the simplified CPU model, how is the PC updated?
Medium
- Ait increments to the next address
- Bit is erased
- Cit doubles
- Dit stays the same
13.Which stage involves the ALU performing a calculation?
Medium
- Aexecute
- Bfetch
- Cdecode only
- Dnone
14.Why can a larger useful cache improve performance?
Medium
- Amore frequently used data can be accessed quickly
- Bit adds more cores
- Cit increases clock speed
- Dit stores files
15.Why do more cores not always double the speed?
Medium
- Asoftware may not use all cores
- Bcores are slow
- Ccores use more memory
- Dcores are broken
16.What is an embedded system?
Medium
- Aa computer built into a device for a dedicated task
- Ba desktop PC
- Ca server
- Da laptop
17.What is von Neumann architecture?
Medium
- Ainstructions and data share the same memory
- Bseparate memory for data and instructions
- Cno memory
- Donly cache
18.A 3 GHz CPU performs:
Medium
- A3 billion cycles per second
- B3 million
- C3 thousand
- D3 cycles
19.In the accumulator-based CPU model used here, which register holds an addition result?
Medium
- Aaccumulator
- BPC
- CMAR
- DMDR
20.What is the correct order?
Medium
- Afetch, decode, execute
- Bexecute, fetch, decode
- Cdecode, execute, fetch
- Dfetch, execute, decode
21.Why might a simple dedicated embedded controller use a less powerful processor than a general-purpose desktop?
Medium
- Athey perform simple, dedicated tasks
- Bthey are expensive
- Cthey are fast
- Dthey are general-purpose
22.What are the three stages of the cycle?
Easy
- Afetch, decode, execute
- Binput, process, output
- Cread, write, delete
- Dstart, stop, pause
23.What is the clock?
Easy
- Aa timer that synchronises CPU operations
- Ba wall clock
- Ca storage device
- Da program
24.Which group describes components within the simplified CPU model used here?
Medium
- AALU, control unit, registers and cache
- Bmonitor, mouse, keyboard
- Chard drive, SSD
- Drouter
25.What is meant by 'multicore'?
Medium
- Aa CPU with more than one core
- Bmore than one computer
- Cmore than one program
- Dmore memory
26.The ALU performs calculations.
Easy
True or false?
27.The PC holds the address of the next instruction.
Easy
True or false?
28.Cache is slower than RAM.
Easy
True or false?
29.Clock speed is measured in hertz.
Easy
True or false?
30.The control unit performs calculations.
Medium
True or false?
31.Which are CPU registers?
Medium
- APC
- BMAR
- CMDR
- DACC
- EHDD
32.Which affect CPU performance?
Medium
- Aclock speed
- Bnumber of cores
- Ccache size
- Dsoftware efficiency
- Escreen colour
33.Describe what happens during the fetch stage of the fetch-decode-execute cycle.
Medium

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