Processors, Memory, & Buses
Unit 7 Summary - part 1
Processors/CPU are the brains of computers
Input devices send data to the processor.
Storage devices hold data that the processor can use.
Output devices display data from the processor.
Input+Storage+Output together are called peripherals.
CPUs have 3 of these:
1. ALU (Arithmetic and Logic Unit): Just like the name, ALU carries:
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arithmetic processing: finding answers to math problems.
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logical processing: finding true/false answers (if functions).
2. CU (Control Unit):
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CU signals ALU to read data from main memory. Then ALU processes it and writes results back to main memory.
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CU tells peripherals to transfer data to and from main memory.
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CU sends signals in the right order, and waits each action to finish before it sends the next signal.
3. System clock.
(Scroll down to see its explanations).
A CPU works with Main Memory.
Main memory is sometimes also called IAS (Immediate Access Store).
Main memory has so many blocks/locations, each has its own binary number.
There are 3 types of computer buses:
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Address Bus: transmits addresses.
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Data Bus: transmits data.
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Control Bus: transmits control signals, telling the receiver to do something.
Registers are small memory areas.
Signals travel through buses to the registers.
Registers hold the signals to be processed by the CPU.
Buses are also used to connect the processors to hardware.
Advantage:
CPU can access data and instructions faster on Registers than on RAM.
Disadvantage:
A register can only hold one item only (data/instruction).
CPUs only have small numbers of registers.
John von Neumann first proposed a basic structure of a computer model.
His invention is called Von Neumann model or Von Neumann architecture.
Von Neumann architecture has key features:
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A CPU contains ALU and CU.
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Main memory that stores data and instructions (in electronic form).
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External storage that retains data and instructions (in non-electronic form).
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Input and Output devices connected to the processors.
Fetch-Decode-Execute cycle:
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The CU fetches the instruction from main memory (ILS), the instructions are coded in Binary.
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The CU decodes the instruction so it knows what to do.
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The ALU executes the actions.
Memory Types
Main Memory has 2 types
1. ROM (Read-Only Memory):
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ROM is your BIOS, used to load your basic computer the first time you turn on your computer.
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ROM is set up by the computer's factory.
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Just like the name READ-ONLY, you can't change / save anything there.
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ROM is non-volatile: what's stored there can't disappear when the computer is turned off.
2. RAM (Random Access Memory):
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A CPU needs to run fast, so it needs RAM to hold temporary data/instructions.
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Without RAM, CPU will run slowly as it needs to search data on a hard disk.
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RAM is volatile: the data in it disappears when you turn off your computer.
What is a System clock & clock speed?
System clock:
CPU has a timer based on a crystal. CU (Control Unit) uses system clock to give time to instructions to be finished before sending another signal.
2GHz speed means it can transfer 2 billions instructions a second.
Increasing clock speed can make a computer runs a little bit faster; however, there are disadvantages:
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Some components might not work fast enough. When CU gives a new signal and the old one hasn't been finished, it might cause some problems to the computer.
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The computer can overheat.
What is a CPU Cache?
CPU can get data more quickly from registers than from RAM; however, registers can only hold 1 instruction at a time.
To deal with this, computers have a larger cache. Cache copies data/instructions from RAM.
What ARE Processor's CoreS?
Modern processors have more than 1 core
The advantages of having more than 1 core processor:
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More than 1 instruction is carried out at the same time, so the computer is faster.
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Old processors might be linked together with a new processor to increase speed without buying a new motherboard.
The disadvantages of having more than 1 core processor:
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Software must be written to take advantage of the new processor.
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Not all software makes full use of the the extra core.
Oxford AQA IGCSE 2020
05.1.
One of the functions of the operating system is to act as a memory manager.
State two other functions of the operating system. [2 marks]
Function 1
__________________________________________________________________________________
Function 2
__________________________________________________________________________________
Alternative answers:
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Processor management;
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I/O Device management;
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Application management;
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Security management;
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Managing secondary storage
05.2.
Describe what happens at each of the three stages of the Fetch-Execute cycle. [3 marks]
Fetch:
__________________________________________________________________________________
Decode:
__________________________________________________________________________________
Execute:
__________________________________________________________________________________
Alternative answers:
Fetch [1 mark]:
the next instruction is fetched (to the CPU) from (main) memory; 1 mark:
Decode [1 mark]:
the instruction is decoded to work out what it is decoded by control unit; 1 mark:
Execute [1 mark]:
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the instruction is carried out/run/performed/completed
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execution (of arithmetic and logic instructions) performed by ALU / arithmetic and logic unit
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data may need to be read from/written to memory/registers (as a result of execution)
05.3.
The main memory of a computer normally consists of both volatile and non-volatile memory. Explain the term volatile memory. [1 mark]
__________________________________________________________________________________
Answer:
Volatile memory will lose its contents when power turned off
05.4.
Tick (√) one box to indicate which of these is usually stored in the ROM of a desktop computer.
Answer:
D
(Startup instructions to be executed when the computer is turned on)
Oxford AQA IGCSE Specimen Paper
07.
Two factors that can affect the performance of a processor are:
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number of cores
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size of cache memory.
07.1.
Explain how the number of cores affects processor performance. [2 marks]
__________________________________________________________________________________
Answer:
Each core can process instructions independently; More cores mean that more instructions can be processed in parallel
07.2.
Explain how the size of cache memory affects processor performance. [2 marks]
Answer:
Cache memory stores data/instructions that are frequently accessed;
Fetching data/instructions from cache memory is faster that fetching from (main) memory;
Having more cache memory means it is more likely that an instruction/some data that is needed will be in the cache;
07.3.
State one other factor that can affect processor performance. [1 mark]
__________________________________________________________________________________
07.3.
State one other factor that can affect processor performance. [1 mark]
__________________________________________________________________________________
Answer:
Clock speed;
Alternative Answer:
Other factors that would affect processor performance (not just system performance)
07.4.
Processors follow a three-stage cycle to carry out an instruction.
Complete Figure 4 by writing the names of each of the stages of this cycle, in the correct order, in to the correct boxes on the figure. [2 marks]
Figure 4
Answer:
Oxford AQA IGCSE Mock Paper
14.
Figure 8 shows a diagram of some of the main components of a computer. The diagram contains an error.
Figure 8
14.1.
Shade one circle to indicate which of the statements below correctly describes the error in Figure 8. [1 mark]
14.2.
Explain the role of the Clock. [2 marks]
__________________________________________________________________________________
14.3.
The CPU and main memory are connected by buses.
Explain what a bus is. [2 marks]
__________________________________________________________________________________
