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Computer Systems - AQA Computer Science Cheat Sheet by [deleted]

Hardware and Software

Hardware is the physical components of a computer system, whereas software is the programs that are run using the hardware. Generally, one is useless without the other.

Software Classi­fic­ation

Operating Systems
Software loaded by the computer following the initial boot-up which controls both the hardware and software, incuding the processor, memory, I/O devices, and security.
Windows 10, macOS
Systems software
Software that controls the operation of hardware in a computer.
Applic­ation software
Programs designed for the user to use to perform a specific task(s).
Internet Explorer, Microsoft Word
Utility Programs
A type of systems software that manages the computer's resources
Defrag­men­tation, file encryption

Truth Tables for Logic Gates

Logic Gate
Truth Table
x ­ ­ y ­ ­ z
0   0   0
1   0   0
0   1   0
1   1   1
x ­ ­ y ­ ­ z
0   0   0
1   0   1
0   1   1
1   1   1
x ­ ­ z
1   0
0   1
Here, 'x' and 'y' are the inputs, while 'z' is the output.

Logic Circuits

Truth Tables for Logic Circuits

A   B   C­ ­ ­ ­D   E   Z
0   0   0­ ­ ­ ­1   0   1
0   0   1­ ­ ­ ­1   0   1
0   1   0­ ­ ­ ­1   0   1
0   1   1­ ­ ­ ­1   1   1
1   0   0­ ­ ­ ­0   0   0
1   0   1­ ­ ­ ­0   0   0
1   1   0­ ­ ­ ­0   0   0
1   1   1­ ­ ­ ­0   1   1
This is an example of a truth table for the logic circuit above.

Embedded Systems

An embedded system is normally built into a larger device. It has a small number of specific functions. An example could be a satnav in a car. Non-em­bedded systems, on the other hand, carry out multiple functions and are not built into anything else

Systems Archit­ecture

Central Processing Unit (or CPU)
Processes data and instru­ctions and make decisions. It is composed of many components (e.g. the CU, the ALU).
Control Unit (or CU)
Controls the operation of the ALU, memory and I/O devices; tells them how to respond to instru­ctions it has fetched and decoded.
Arithmetic Logic Unit (or ALU)
Carries out arithmetic and logical operat­ions. Results are stored in a register.
A crystal that vibrates at high speeds, dictating how many times a second the fetch-­dec­ode­-ex­ecute cycle can be carried out.
Transports data and instru­ctions around the CPU.
Storage areas in the CPU.
Main Memory
Any form of memory directly accessible by the CPU (excl. cache, regist­ers).
Small, fast memory that is close to the CPU. Stores data that is used freque­ntly.
The AQA specif­ication is only interested in the Von Neumann Archit­ecture, which is also called the Princeton Archit­ecture. Unlike in Harvard archit­ecture, Von Neumann stores data and instru­ctions together.

Von Neumann Archit­ecture

Fetch-­Dec­ode­-Ex­ecute Cycle

The fetch-­dec­ode­-ex­ecute cycle is a process carried out by the CPU to process an instru­ction. In the 'fetch' stage, the instru­ction is loaded into the main memory. The computer then decodes and 'under­stands' the instru­ction and executes the instru­ction. This final stage may include calcul­ation being carries out by the ALU.

Secondary Storage

What is secondary storage?
Any storage that isn't under the direct control of the CPU. It stores data and inform­ation when the computer is off.
What is RAM?
Random acess memory. Is volatile, so all data is lost once the power supply is switched off. Stores programs and data currently in use. Can be written to and read from.
What is ROM?
Read only memory. Stores programs needed to boot up the computer. Can only be read from. Non-vo­latile. Generally smaller in memory capacity.

Types of Storage

Magnetic storage uses magnets to record data on rotating metal plates. The most common example is a hard drive.
1. Large storage capacity
2. Decent read/write access
3. Cheap
1. Moving parts result in wear and tear
2. Data can be lost/a­ltered by magnets
Optical storage uses a lens and a light beam to read and write data onto a disk (e.g. CD, DVD).
1. Light and portable
2. Cheap
3. Durable
4. Some formats (e.g. CD-R) mean that data cannot be overwr­itten
1. Small storage capacity
2. Specific drive needed to read/write data to/from the disk
Solid State
Solid-­state uses flash memori­es/­ele­ctrical circuits to store data. It is commonly used in USBs and SD cards.
1. Very quick read/write speed
2. Compact
3. No moving parts
3. Robust
1. Expensive
2. Limited number of times that data can be written to
Data is stored (using normal magnet­ic/­solid state storage) at a remote location and is accessed via the Internet. Popular examples are Google Drive and Dropbox.
Advant­ages: 1. Can be accessed from anywhere in the world, granting increased flexib­ility
2. Can be accessed by multiple users at once
3. Users don't need to buy additional hardware
1. Requires an Internet connection
2. No control over the data in terms of security

Factors Affecting CPU Perfor­mance

Clock Speed (Hz)
Dictates the number of fetch-­dec­ode­-ex­ecute cycles run per second.
Number of Processor Cores
Having multiple cores allows a CPU to process multiple instru­ctions simult­ane­ously.
Cache Size
Allows the CPU to store more instru­cti­ons­/data that are regularly used, reducing the time taken to process an instru­ction.
Cache Type
L1 cache is faster than L2 and L3 cache.


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