Systems architecture

Quick links

3.4.1

Hardware and software

3.4.2

Boolean logic

3.4.3

Software classification

3.4.4

Systems architecture

 

Useful links

Syllabus content

Content   Additional Information

Explain the Von Neumann architecture.

   
     

Explain the role and operation of main memory and the following major components of a central processing unit (CPU):

  • • arithmetic logic unit
  • • control unit
  • • clock
  • • bus
  A bus is a collection of wires through which data is transmitted from one component to another. Main memory will be considered to be any form of memory that is directly accessible by the CPU, except for cache and registers.
     

Explain the effect of the following on the performance of the CPU:

  • • clock speed
  • • number of processor cores
  • • cache size
  • • cache type.
   
     
Understand and explain the Fetch-Execute cycle.  

The CPU continuously reads instructions stored in main memory and executes them as required:

  • • fetch: the next instruction is fetched to the CPU from main memory
  • • decode: the instruction is decoded to work out what it is
  • • execute: the instruction is executed (carried out). This may include reading/writing from/to main memory.
     

Understand the differences between main memory and secondary storage.

Understand the differences between RAM and ROM.

  Students should be aware that SSDs use electrical circuits to persistently store data but will not need to know the precise details such as use of NAND gates.
     
Understand why secondary storage is required    
     

Be aware of different types of secondary storage (solid state, optical and magnetic).
Explain the operation of solid state, optical and magnetic storage.

Discuss the advantages and disadvantages of solid state, optical and magnetic storage.

  Students should be aware that SSDs use electrical circuits to persistently store data but will not need to know the precise details such as use of NAND gates.
     
Explain the term 'cloud storage'   Students should understand that cloud storage uses magnetic and increasingly solid state storage at a remote location.
     
Explain the advantages and disadvantages of cloud storage when compared to local storage.    
     
Understand the term 'embedded system' and explain how an embedded system differs from a non-embedded system.   Students must be able to give examples of embedded and non-embedded systems.

 

System architecture

Von Neumann architecture

In 1945, while consulting for the Moore School of Electrical Engineering on the EDVAC project, von Neumann wrote an incomplete set of notes, titled the "First Draft of a Report on the EDVAC". This widely distributed paper laid foundations of a computer architecture in which the data and the program are both stored in the computer's memory in the same address space, which will be described later as von Neumann Architecture (see the drawing below). This architecture became the de facto standard for a long time and is still used today. The alternative architecture was known as Harvard Architecture where the data and program were stored separately.

Here is an interesting slide show that explains the von Neumann architecture and the fetch - execute cycle.

 

Content on this page requires a newer version of Adobe Flash Player.

Get Adobe Flash player

It is essential that you appreciate the fundamentals of the "Fetch-Decode-Execute" Cycle in such a way that you can describe it (with diagrams if required) to an examiner. The CPU moves data and instructions from the IAS to the appropriate registers in the Control Unit. Certain registers have specific functions which you will have seen already; program counter (PC) address register (AR) instruction register (IR) and the accumulator where the results of arithmetic or logic calculations will be found.

Registers:

  • Program Counter (PC) – a counter that keeps track of the memory address of which instruction is to be executed next.
  • Memory Address Register (MAR) – the address in main memory that is currently being read or written.
  • Current Instruction register (CIR) – a temporary holding area for the instruction that has just been fetched from memory.
  • Accumulator (ACC) - is the special register of the computer. A register is a special memory location that allows very fast access. Here, the accumulator is a temporary memory location that stores values of all arithmetic and logical calculations that are being carried out by the CPU. The increments of values occur in the accumulator for programming calculations

Activity 1. Describe in your exercise book or as a series of annotated screenshots of LMC to be pasted into your exercise book the fetch execute cycle.

CPU performance

Activity 2. In your exercise book, research and then explain the effect of the following on the performance of the CPU:

  • clock speed
  • number of processor cores
  • cache size
  • cache type.

Here is a site that explains exactly these points.

Here (Right click on the link and select "save link as") is a worksheet that you can use to record your own notes on the structure of a CPU.

Computer architecture (trends and implications)

The BBC site tells us that computer architecture is a term used to describe the different aspects of hardware that are required to allow a computer system to operate:

  • Processor - Registers, ALU, Control unit
  • Buses - Data Bus, Address Bus
  • Main Memory
  • Cache Memory
  • Processor

The processor is responsible for carrying out the fetch/execute cycle. This involves accessing memory locations to read and write data, either before or after execution by the processor. Modern processors are commonly quad core processors. This means that there are actually four processors that make up the central processing unit. At first there was generally only one core on a processor. This evolved to become dual-core (two processors) and now quad-core. A multi-core central processing unit, including an integrated graphics chipset, a shared level 3 cache, and a memory controller. The more processors present, the greater the number of tasks that can be processed simultaneously. This is known as parallel processing, as two different processors can process different tasks (threads) at the same time. As processor technology evolves it is anticipated that octa-core processors (8 processors) will become more common.

There are three main components to every processor:

  • Registers
  • ALU
  • Control unit
  • Registers

Registers are used to store data, instructions or addresses on the actual processor. A register is simply a storage location on the processor. Data/instructions/addresses that are needed during the fetch/execute cycle are held within registers. Registers offer the fastest access time of any storage location as they are part of the actual processor. There are a variety of registers with different purposes.

Two examples are:

MAR – Memory Address Register. The MAR holds the address of the next item of data that is needed by the processor.

MDR – Memory Data Register. The MDR holds data that is either to be passed to the data bus or has just been received from the data bus.

ALU

The Arithmetic and Logic Unit is used by the processor to carry out operations that require the use of:

  • Logical operations such as AND, OR, NOT
  • Arithmetic operations such as +, -, *, /

Control Unit

The control unit is the term used to describe the part of the processor containing decoding circuitry and a series of independent lines that need to be used to signal the current state of the processor and fetch/execute cycle.

Examples of the control lines are:

  • Byte Enable Line - specifies the instruction size of the data being sent/received
  • Read Line - initiates a memory read operation (from memory to the processor)
  • Write Line - initiates a memory write operation (from the processor to memory)
  • Clock Line - dictates the pace of the fetch/execute cycle
  • Interrupt Line - used to indicate that an interrupt to the fetch/execute cycle is taking place
  • Reset Line - resets the processor to its base state

Activity 3. In your exercise book, research and then explain the control lines in a CPU.

Storage

Understand the differences between main memory and secondary storage.

Understand the differences between RAM and ROM.

Understand why secondary storage is required

Activity 4: Right click on either this PowerPoint or this Powerpoint and select "save link as" and add notes explaining all of the issues raised by the author.

More about storage

Be aware of different types of secondary storage (solid state, optical and magnetic).
Explain the operation of solid state, optical and magnetic storage.

Discuss the advantages and disadvantages of solid state, optical and magnetic storage.

Here are some statements about the CPU of a computer.
State whether each of the following statements is true or false.

  • CPU stands for Central Processing Unit.
  • The CPU fetches and decodes instructions.
  • The speed of a CPU is usually measured in GigaHertz (GHz).
  • If a CPU has many cores, this slows down the computer.
  • The hard disk drive is part of the CPU.

 

Third party storage and embedded systems

Activity 5: Research and explain in your exercise book the term 'cloud storage'.

Explain the advantages and disadvantages of cloud storage when compared to local storage.

Understand the term 'embedded system' and explain how an embedded system differs from a non-embedded system.

Further reading

Here are a few links to web sites and interesting things that you really should read in your own time. There may be a sentence or two in one of these sites that might spark a difficult concept or resolve a confusing idea or may simply be a better explanation that suits you and enables you to fully understand the points that the examiner is trying to make.

The "Little Man Computer (LMC)" and the RISC version

"The true fathers of computing" George Dyson's new book challenges computing's creation myth by highlighting the key role played by John von Neumann.

In the article "Scientific computing infrastructure" the author describes the symbiosis between science and technology when designing computer chips.

Something else about LMC.

Worksheet

3.1 Fundamentals of algorithms

3.2 Programming

3.3 Fundamentals of data representation

3.4 Computer systems

3.5 Fundamentals of computer networks

3.6 Fundamentals of cyber security

3.7 Ethical, legal and environmental impacts of digital technology on wider society, including issues of privacy

3.8 Aspects of software development

Glossary and other links

Glossary of computing terms.

AQA 8520: The 2016 syllabus

Software classification.

Why does a computer need an operating system?

How do operating systems work?

What is Linux?

What is BIOS?

Further reading