Unit 1 - Components of a computer

The Arithmetic and Logic Unit

• The ALU completes all of the arithmetical and logical operations. Arithmetical operations include all mathematical operations such as addition and subtraction on fixed or floating point numbers. Logical operations include boolean operations such as AND, OR, NOT and XOR.

The Control Unit (CU)

• The Control Unit is the component of the processor which directs the operations of the CPU. It has the following jobs:
• Controlling and coordinating the activities of the CPU
• Managing the flow of data between the CPU and other devices
• Accepting the next instruction
• Decoding the instructions
• Storing the resulting data back in memory

Registers

• Registers are small memory cells that operate at a very high speed. They are used to temporarily store data and all arithmetic, logical and shift operations occur in these registers

Registers

• Program Counter (PC) - Hold the address of the next instruction to be executed
• Accumulator (ACC) - Stores the result from calculations
• Memory Address Register (MAR) - Holds the address of a location that is to be read from or written to
• Memory Data Register (MDR) - Temporarily stores data that has been read or data that needs to be written
• Current Instruction Register (CIR) - Holds the current instruction being executed, divided up into operand and opcode

Buses

• Buses are a set of parallel wires which connect two or more components inside the CPU. There are three buses in the CPU: data bus, control bus and address bus. These buses collectively are called the system bus.
• The width of the bus is the number of parallel wires the bus has. The width of the bus is directly proportional to the number of bits that can be transferred simultaneously at any given time. Buses are typically 8,16,32 or 64 wires wide.

Buses

• Data bus - this is a bi-directional bus (Meaning bits can be carried in both directions). This is used for transporting data and instructions between components.
• Address bus - This is the bus used to transmit the memory addresses specifying where data is to be sent to ore retrieved from. The width of the address bus is proportional to the number of addressable memory locations.
• Control bus - This is a bi-directional bus used to transmit control signals between internal and external components. The control bus coordinates the use of the address and data buses and provides status information between system components .
• The control signals include:
• Bus request - shows that a device is requesting the use of the data bus
• Bus grant - shows that the CPU has granted access to the data bus
• Memory write - data is write into the addressed location using this bus
• Memory read - data is read from a specific location to be placed onto the data bus
• Interrupt request - shows that a device is requesting access to the CPU
• Clock - used to synchronise operations

Assembly language

• Assembly code uses mnemonics to represent instructions, for example ADD represents addition. This is a simplified way of representing machine code. The instruction is divided into operand and opcode in the Current Instruction Register. The operand contains the data or the address of the data upon which the operation is to be performed. The opcode specifies the type of instruction to be executed.

Pipelining

• Pipelining is the process of completing the fetch, decode and execute cycles of three separate instructions simultaneously, holding appropriate data in a buffer in close proximity to the CPU until it is required. While one instruction is being executed , another can be decoded and another fetched.
• Pipelining is aimed to reduce the amount of the CPU which is kept idle. It is separated into instruction pipelining and arithmetic pipelining. Instructions pipelining is separating out the instruction into fetching, decoding and executing. Arithmetic pipelining is breaking down the arithmetic operations and overlapping them as they are performed.

Fetch - Decode - Execute Cycle

• Fetch -
• Address from the PC is copied to the MAR
• Instruction held at the address is copied to MDR by the data bus
• Simultaneously, the contents of the PC are increased by 1
• The value held in the MDR is copied to the CIR
• Decode -
• The contents of the CIR are split into operand and opcode
• Execute -
• The decoded instruction is executed

Factors affecting CPU performance

• There are three factors that affect CPU performance: clock speed, number of cores and the amount and type of cache memory.

Clock speed

• The clock speed is determined by the system clock. This is an electronic device which generates signals, switching between 0 and 1. All processor activities begin on a clock pulse, and each CPU operation starts as the clock changes from 0 to 1. The clock speed is the time taken for one clock cycle to complete.

Number of cores

• A core is an independent processor that is able to run its own fetch -decode-execute cycle. A computer with multiple cores can complete more than one FDE cycle at any given time. A computer with dual cores can theoretically complete tasks twice as fast as a computer with a single core. However, not all programs are able to utilise multiple cores efficiently as they have not been designed to do so, so this is not always possible

Cache Memory

• Cache memory is the CPU's onboard memory. Instructions fetched from main memory are copied to the cache, so if required again, they can be accessed quicker. As cache fills up, unused instructions are replaced.
• Level 1 Cache - Very fats memory cells with small capacity. (2-64KB)
• Level 2 Cache - Relatively fast memory cell, with a medium sized capacity. (256KB-2MB)
• Level 3 Cache - Much larger and slower memory cell.

Von Neumann Architecture

• This architecture includes the basic components of the computer and processor (single control unit, ALU, registers and memory units) in which a shared memory and shared data bus is used for both data and instructions. Von Neumann architecture is built on the stored program concept.
• Advantages of Von Neumann:
• Cheaper to develop as the control unit is easier to design
• Programs can be optimised in size

Harvard Architecture

• Harvard Architecture has physically separate memories for instructions and data, more commonly used with embedded processors. This is useful for when memories have different characteristics, i.e. instructions may be read only, while data may be read-write. This also allows you to optimise the size of individual memory cells and their buses depending on your needs, i.e. the instruction memory can be designed to be larger so a larger word size can be used for instructions.
• Advantages of Harvard Architecture:
• Quicker execution as data and instructions can be fetched in parallel
• Memories can be different sizes, which can make more efficient use of space

Von Neumann vs Harvard Architecture

• Von Neumann:
• Used in conventional processors in PCs, servers and embedded systems with only control functions
• Data and programs share the same memory
• One bus is used to transfer data and instructions
• Programs can be optimsised in size
• Harvard:
• Used in digital signal processing and in embedded systems, mobile communication systems, audio, speech and image processing systems
• Instructions and data are held in separate memories
• Parallel data and instructions buses may be used
• Programs tend to be large

Contemporary Processing

• Contemporary processors use a combination of Harvard and Von Neumann architecture. Von Neumann is used when working with data and instructions in main memory, but uses Harvard architecture to divide the cache into instruction cache and data cache.

Reduced Instruction Set Computers (RISC)

• Small instruction set
• Each instruction is one line of machine code
• Used in common computers
• Pipelining is possible since each instruction takes one clock cycle
• The compiler has to do more work to translate high level code into machine code
• More RAM is required to store the code

Complex Instruction Set Computers (CISC)

• Large Instruction set
• Instructions are built into the hardware
• Used in microcontrollers and embedded systems
• The compiler has less work to translate high level code into machine code
• Less RAM is required since code is shorter
• Many specialised instructions are made even though only a few of them are used

Multi-core and Parallel Systems

• Multi-core CPUs have multiple independent cores that complete separate FDE cycles which results in higher performance
• Parallel systems complete multiple instructions simultaneously using techniques like pipelining, it can be completed using and single core, by using treading.
• Generally multi-core systems perform better in larger projects than parallel systems

Graphics Processing Unit (GPU)

• A graphics processing unit (GPU) is a device which unlike CPUs has lots of independent processors which work in parallel making it very efficient at completing repetitive tasks such as image processing and machine learning. GPUs are a type of co-processor ( a secondary processor designed to supplement the activities of the primary processor)

Input and Output devices

• An input device is one which can be used to put data and information into a computer. For example - Keyboard
• An output device is one which can be used to send information from the computer. For example - Speakers

Optical storage

• Read from and written to using lasers
• Binary information represented by portions of the disk which wither reflect or scatter the incident laser. A pit scatters light and represents 0 and a land reflect light and represents 1
• Pits and lands are written in spiral tracks on the disk's surface

Optical storage - CD

• Stands for Compact Disk
• Use optical technology to store small quantities of information
• Most commonly used for audio files
• Can also be used to store text and digital images
• Small, thin and light so very portable
• Easily damaged by scratches
• Limited storage capacity
• Relatively slow transfer speeds

Optical storage - DVD and Blu-Ray

• DVD:
• Stands for Digital Video Disk
• Higher storage capacity than CDs
• Suited to storing digital videos
• Blu - Ray:
• More than five times as much storage than traditional DVDs
• Useful for storing high resolution films

Magnetic storage

• Represented information using two magnetic states - polarised/unpolarised
• Most common type is hard disk drives
• Magnetic tape also stores information magnetically

Magnetic storage - Hard Disk Drives (HDD)

• Typically have high capacities of between 500GB and 5 TB
• Rotate magnetic platters at high speeds under a read/write head on an actuating arm
• Most will have multiple platters stacked to maximise storage capacity
• Have somewhat slow data transfer speeds
• Many moving parts introduces tendency to be damaged by movement.

Magnetic storage - Magnetic tape

• Long stretches of tape wound onto reels passed through readers
• A space consuming way to store data

Magnetic storage - Floppy disks

• A thin magnetic disk enclosed in plastic to protect the disk from dust and dirt
• Thin size and low weight made them extremely portable
• Typical capacity of 1 MB

Flash Storage

• Fast and compact
• Silicon semiconductors form the logic gates NAND and NOR
• Logic gates used to store electrical charge in one of two states - high/low
• Information stored in blocks, combined to form pages
• Non - volatile
• Generally more expensive per gigabyte

Flash storage - Solid State Drives (SSD)

• Extremely light and portable
• Have no moving parts
• Much more resistant to damage
• Renowned for high data transfer rates
• Expensive, limited life span

RAM and ROM

• Two types of primary storage which store information like code instructions to execute and files which are required by running programs.

Random Access Memory (RAM)

• A type of fast , volatile(loses information when power is lost) main memory
• Used to store data and programs that the computer is currently using
• Speeds up the computers execution
• Higher access speeds than flash memory
• More expensive per GB than secondary storage devices

Read Only Memory (ROM)

• Non volatile and cannot be modified
• Once programmed, the state of the memory cells inside does not change
• Useful for storing fixed sequences of instructions like computers bootstrap routine

Virtual Storage

• Virtual storage is the name given to storing information remotely so that it can be easily accessed by any computer with access to the same system.
• Example of virtual storage is cloud storage services like Google Drive
• Disadvantages - limitations of user's network speeds and high costs