04. Computer Systems

Hardware

the physical components that make up a computer system.

Software

the instructions that are executed by a computer and control the computer’s hardware.

AND gate (A.B)

• A and B must be 1 for the output to be 1.

OR gate (A+B)

• A or B must be 1 for the output to be 1.

• If both A and B are 1, then the output will still be 1.

NOT gate (Ā)

• the output is the opposite of what is inputted.

• if 1 is inputted, 0 will be the output.

• likewise, if 0 is inputted, 1 will be the output.

XOR gate (A⊕B)

• either A or B must be 1 for the output to be 1.

• if both A and B are 1, then the output will be 0.

Application software

programs that perform user-oriented or end-user tasks.

Examples of application software

• web-browsers

• spreadsheets

• games

• video editing software

• audio editing software

System software

manages the computer system resources, and acts as a platform to run applications.

Examples of system software

• OS

  • manage the computer’s hardware and create an environment for applications to run.

• utility software

  • programs that allow the user to maintain the computer system/perform system-oriented tasks.

Functions of OS

• managing memory

  • controls which parts of memory are being used by the process

• managing processors/processes

  • schedules which processes are to be executed

• managing applications

  • control what data applications can access

• managing I/O (hardware) devices

  • allows devices to send and receive data

• managing security

  • protect the computer system against malware

• managing storage

  • load data and programs into computer’s memory

Examples of utility software

• defragmentation

• backup

• encryption

• compression

High-level languages

• human-readable, therefore easier to develop and debug

• one statement equates to multiple machine code commands

• often portable - can run on different types of CPU

• must be translated into machine code in order for the CPU to execute

Low-level languages

• provide exact control over the CPU, allowing for maximum efficiency

• difficult to read by humans

• will only run on one type of CPU

Machine code and assembly language

• both are low-level languages.

• there is a 1:1 mapping between machine code instructions and assembly language mnemonics.

Machine code

• machine code instructions are in binary form.

• for any program to be processed by a processor, it must be translated into machine code.

• machine code is not portable across different processor architectures.

Translator

• special type of software that translates computer programs from human-readable source code to machine code which the CPU can run.

• translators are essential, as CPU can only execute instructions that are in binary (machine code).

• 3 types of translators:

  • compilers

  • interpreters

  • assemblers

Assemblers

• translate low-level assembly language into machine code.

• translate the whole program in one go.

Compilers

• translate the whole program in one go.

• stand-alone program does not require the compiler/source code to run.

• produce a list of syntax errors at the end of translation.

• do not execute the programming code - instead create an executable file.

Interpreters

• translate the program one line at a time.

• program requires interpreter to be present on the computer to run.

• execute the programming code as it’s being translated

• stop execution as soon as an error is encountered.

von Neumann architecture

ALU

• used during the fetch-execute cycle to:

  • execute mathematical instructions

  • execute logical instructions

  • compare values held in registers

Control unit

• contains a decoder that interprets the instructions loaded from memory and sends control signals to the appropriate components within the computer.

• during the FEC, the control unit decodes the fetched instructions and controls the fetching of data and writing from/to memory.

• contains the CPU’s clock, which allows the CPU to synchronise operations and set the overall clock speed of the CPU, which controls the number of instructions carried out each second.

Registers

• special purpose, small units of memory found within the CPU.

• each register is used for a specific purpose in the FEC, holding a specific piece of data needed by the CPU to control and perform operations.

Bus

• a collection of wires through which data and signals are transmitted from one component to another

Cache memory

• very fast memory found within or close to the CPU.

• unlike registers, cache memory can store either data or instructions to and is much faster but much smaller in capacity than RAM.

• during the FEC, cache memory is used to store frequently used instructions and data.

Main memory

• ROM & RAM.

• used to store data and instructions for the CPU to process.

• programs are permanently stored in secondary storage, and must be loaded into main memory for the CPU to process and execute.

• main memory is directly accessible by the CPU.

Clock speed’s impact on CPU performance

• the faster the clock, the more instructions can be processed per second

CPU cores’ impact on CPU performance

• each CPU core has its own ALU, control unit and L1/L2 cache.

• multi-core CPUs can therefore process multiple instructions simultaneously, leading to a potential increase in CPU performance.

• the programs must be optimised for parallel-processing.

Cache memory’s impact on CPU performance

• increased cache size allows more frequently used instructions to be stored in cache memory

• this makes the CPU’s retrieval of frequently used instructions quicker, allowing it to process and therefore execute the operations commanded by the instructions quicker

• this improves CPU performance

• however, cache memory is quite expensive on a cost/GB basis

Fetch-execute cycle

• fetch: the next instruction to be processed by the CPU is fetched from main memory.

• decode: the instruction to be carried out is identified by the control unit, and control signals are sent by it to the appropriate component to perform the operation.

• execute: the operation is carried out by the appropriate component within the CPU.

RAM

• volatile memory

• temporarily stores program instructions and data while they are being processed by the CPU

• much faster than secondary storage, but more expensive on a cost/GB basis

• OS is copied into RAM when the computer starts up

ROM

• permanently stores program instructions and data, but cannot have new data written to it

• non-volatile

• made from flash memory - cannot be altered without being ‘re-flashed’

• stores the BIOS (Basic Input-Output Sequence) that the CPU executes as soon as it is turned on

What does BIOS contain?

instructions that load the Operating System from secondary storage into RAM for the computer to continue booting up

Why is secondary storage required?

• to permanently store software and data

• to provide “virtual memory” when RAM’s capacity is maximised

The structure of magnetic hard disc drives

• hard discs contain many platters which spin at very high speeds

• the surface of these platters contain many tiny magnetised regions, the magnetic orientations of which represent 1s and 0s

• each platter is divided into tracks and sectors

• the intersections of tracks and sectors are called blocks

How is data read from magnetic hard disc drives?

• as the platters spin, the magnetic read-write head is moved to the correct track on the disc where data should be written to or read from

• data is read by the read/write head one block at a time, and is transferred to an I/O controller on the motherboard via a data cable

Advantages of magnetic hard disc drives

• large capacity

• cheap per gigabyte of storage

• long-lasting and reliable if not dropped or overused

• portable HDDs can be used to back up and transport large quantities of data

Disadvantages of magnetic hard disc drives

• relatively slow access times

• susceptible to wear and tear due to moving parts

• can get damaged if moved or dropped when in use - not very robust

• can be noisy

How do solid state drives work?

• data is stored electronically in flash memory cells, which use floating gate transistors to trap or release electrons

• the presence or absence of electrons represents 1s and 0s, allowing data to be stored without power

• a controller chip manages how data is read, written and erased

Advantages of SSDs

• faster than HDDs due to lack of moving parts

• more robust - no moving parts

• more portable - smaller and use less power than HDDs

• reasonably large capacity (up to 1 TB)

Disadvantages of SSDs

• smaller capacity than HDDs

• more expensive on a cost/GB basis

• limited read/write cycles before becoming unreliable

How does optical storage work?

• use laser light to retrieve data from the surface of optical media (e.g. CDs, DVDs)

• surface is covered by pits & lands

• when the disc spins, the transitions between pits and lands are encoded as 1s, and no transitions are encoded as 0s

Advantages of optical storage

• cheap to mass produce

• faster access times than HDDs

Disadvantages of optical storage

• prone to scratches that can damage the disc surface and corrupt data

• slower data transfer times than HDDs and SSDs

Cloud storage

• remote storage accessed via the Internet

• data is stored in HDDs and SSDs in remote data centres all around the world

• more secure and reliable than the other secondary storage media because backup, server management, firewall and anti-virus is all handled by the Cloud provider

Advantages of cloud storage

• files and applications can be accessed from any Internet-connected device in the world

• files on the Cloud can easily be shared by sending the recipient a URL

• security and backup is managed by the host company

Disadvantages of cloud storage

• a reliable Internet connection is required to access your files and applications

• Cloud data can be targeted by hackers more easily than if it was stored locally

• ongoing subscription fee may become expensive

Embedded systems

• computer systems with a specific functions that are built into physical products, devices or machines

• embedded systems' programs are stored in ROM, so there is no need for secondary storage

• e.g. control system in a washing machine

Non- embedded systems

• general purpose computer systems that perform many tasks and have replaceable software

• e.g. PCs, smartphones