OCR Computer Science Paper 1

PC

Program counter, contains the address of the next instruction

CIR

Current Instruction Register: stores the address of the next instruction currently being executed and decoded

MAR

Memory Address Register, holds relevant memory address (to read from or write to)

MDR

Memory Data Register, stores data being transferred to and from memory, acts as a buffer

ALU

Arithmetic and Logic Unit, does all mathematical calculations and makes all logical decisions

Accumulator

A storage register in the ALU that holds data temporarily while the data is processed and before it is transferred to memory.

RISC

Reduced Instruction Set Computer: Only simple instructions taking one clock cycle are executed - currently more widely used than CISC
Advantages: allows for pipelining, execution will be quicker or as fast as CISC
Disadvantages: Compiler has to do more work, more RAM required

CISC

Complex Instruction Set Computer: Large instruction set is used to minimise lines of code required - mainly used for microcontrollers and embedded systems
Advantages: compiler has very little work to do, not much RAM required
Disadvantages: Building in specialised instructions can be inefficient when only ~20% get used per program

GPU

Graphics Processing Unit - dedicated graphics processor on a graphics card, used to render images stored in the RAM of the graphics card.

Multi-core CPU

A CPU with more than one core on the same chip - makes use of parallel processing for optimum efficiency

What affects processor performance?

- The number of processor cores: more cores -\> better performance
- The processor's clock speed: faster clock speed -\> better performance
- Amount/type of cache memory: more/better cache memory -\> better performance

RAM

Random Access Memory - stores programs and data currently in use. Volatile

ROM

Read Only Memory - holds information which must be permanently in memory eg. BIOS

What are the functions of an operating system?

- Providing a user interface
- Memory management
- Interrupt service routines
- Processor scheduling
- Backing store management
- Input and output management

What are the types of memory management?

Paging: Available memory is divided into fixed size pages, process in memory can be held in non-contiguous pages, mapped to logical addresses by a page table
Segmentation: Divides address space logically into segments of varying length

What is virtual memory?

A technique that uses a portion of the computer's hard disk as an extension of RAM - swaps files in and out of RAM by a process called disk thrashing

What is a distributed operating system?

Form of parallel processing system which splits tasks over multiple computers - OS coordinates so that the system appears to the user as a single system

What is a multitasking operating system?

An OS which allows each user to run more than one job at a time.

What is a multi-user operating system?

Allows multiple users to use a computer system by allocating each one a slice of processor time - uses a scheduling algorithm

What does a mobile operating system comprise of?

Mobile OSes are made up of two separate operating systems:
A main system operating the user interface/application software
A low-level real-time operating system which is hardware-specific

What is an embedded operating system?

An OS on a chip instead of on a hard drive, designed to do simple things adapted for specific hardware/functions

What is a real-time operating system

Processes data as it comes in - in real time

Open Source Software

Program code made publicly available for free; it can be copied, distributed, or changed without the stringent copyright protections of proprietary software products.

Closed Source Software

Also know as proprietary software, computer software licensed under exclusive legal right of the copyright holder with the intent that the licensee is given the right to use the software only under certain conditions, and restricted from other uses, such as modification, sharing, studying, redistribution, or reverse engineering.

Utility program

A small program that performs many of the general housekeeping tasks for the computer, such as system maintenance and file compression - Disk defragmentation, automatic backup/updating, virus checking, compression software

Scheduling Algorithms

- Round robin
- Shortest remaining time
- Shortest job first
- First come, first served
- Multi-level feedback queues

Shortest remaining time

A scheduling algorithm that deals with each user or task based on a calculating an estimated time remaining to complete.

Shortest job first

A scheduling algorithm that deals with each user or task based on the getting the smaller ones out of the way.

Multi-level feedback queues

This algorithm is designed to give preference to short jobs, give preference to input/output bound processes and separate processes into categories based on their need for the processor.

Round Robin

Processes are dispatched on a first in first out (FIFO)
basis, with each process in turn being given a limited amount of CPU time, called a time slice.

First come first served

Processes are carried out by the CPU in the order in which they arrive

BIOS

Basic input/output system contains programs to load the hardware, stored in ROM

Device drivers

Utility software used by the operating system to communicate with peripheral devices.

Virtual machines

A software implementation of a computer system, allowing one physical computer to run several "virtual computers", each with their own independent operating system and application software.

Low-level language

A programming language that is designed to be easy for a computer to execute; also called machine language or assembly language.

High-level language

A programming language like Python that is designed to be easy for humans to read and write.

Interpreter

Converts a program written in a higher level language into a lower level language and executes it, one line at a time

Compiler

Translates a high-level source program into machine instructions readable by a processor and saves it to disk. The program executes when run.

Assembler

Translates an assembly-language program into machine code

Stages of compilation

lexical analysis - syntax analysis - code generation - optimisation

Lexical analysis

Comments and whitespace are removed
Variables, and subroutines stored in symbol table
Which also holds data such as scope and data type
Code is converted to a series of tokens

Syntax analysis

The code is checked to ensure it follows the rules of the language. Where it breaks the rules of the language errors are generated.
If no rules are broken then it's passed on to the next stage

Code generation and optimisation

Here the object code (accept machine code) is created.
(i.e. the binary that is executed by the processor)
This code may be inefficient: may contain unnecessary instructions or groups of instructions that can
be replaced by simpler ones (point of optimisation)

Linker

Combines together a number of separate object code files.

Loader

Loads an executable program into main memory

Library programs

A collection of compiled routines that other programs can use

Waterfall lifecycle

Sequential stages, each stage must be completed for the next to follow - suitable for small projects requiring supervision, drawback that no user involved

Spiral model

Uses structured steps but develops a program iteratively, constantly improving upon a prototype, allows for productive user input as they can comment on a prototype

Agile modelling

Prioritises user participation: keeps model simple and relies on rapid user feedback to make rapid incremental changes, allows for productive user input as they can comment on a prototype

Extreme Programming

Produces frequent project releases in short development cycles, improving productivity and introducing checkpoints, good for large projects to present bogging down

Rapid Application Development (RAD)

A development method that uses workshops/focus groups and an iterative approach to rapidly produce a high-quality system, good for large projects to present bogging down

Procedural Programming

Method of programming that uses step-by-step instructions (encompasses structured programming) supported by python/pascal

Object-oriented programming

A style of programming that involves representing items, things, and people as objects rather than basing the logic around actions, supported by Java, Python, Delphi

Declarative programming

Problem is described, language implementation decides how to solve it eg. SQL

Functional Programming

A programming paradigm that uses functions to create programs. Supported by Haskell, Python, Java

ADD

Add the contents of the memory address to the accumulator

SUB

Subtract the contents of the memory address from the accumulator

STA

Store the accumulator value in the memory address given

LDA

Load the contents of the given memory address into the accumulator

BRA

Branch- use the address given as the address of the next instruction

BRZ

Branch to the given address if accumulator is zero

BRP

Branch to the given address if the accumulator value is positive (or zero)

INP

Input into the accumulator

OUT

Output contents of the accumulator

HLT

Stops the execution of the program

DAT

Indicates a location containing data

Addressing modes

-Immediate addressing
-Direct addressing
-Indirect addressing
-Indexed addressing

Immediate Addressing

The operand is the actual value to be operated on

Direct Addressing

The operand holds the memory address of the value to be operated on

Indirect Addressing

The operand is the location holding the address of the relevant data

Indexed Addressing

Modifies the address in the address field by the addition of a number held in a special-purpose registers, called an index register, before the address is used.

Classes

A description of what data looks like and can do: an object is an instance of a class

Methods

Things that objects are coded to do

Attributes

The data associated with an object

Inheritance

Where a class retains the methods and attributes of its parent class as well as having its own

Polymorphism

When subclasses redefine a method or attribute originally inherited from a superclass

Encapsulation

The process of keeping an object's attributes private so they can only be accessed and changed via public methods

Lossy Compression

data compression techniques in which some amount of data is lost - redundant information is eliminated

Lossless Compression

a data compression algorithm that allows the original data to be perfectly reconstructed from the compressed data - no data is lost, file size is still made smaller

Run Length Encoding

A compression algorithm that represents an image in terms of the length of runs of identical pixels - lossless

Dictionary-based compression

Uses a dictionary to replace repeated phrases with shorter binary strings - lossless

Symmetric Encryption

An encryption method whereby the same private key is used to encode and to decode the message

Asymmetric Encryption

a type of encryption based on algorithms that require two keys -- one of which is secret (or private) and one of which is public (freely known to others).

Hashing

transforming plaintext of any length into a short code called a hash

Uses of Hashing

-Passwords
-Digital signatures
-Digital certificates
-Hash tables

Flat file database

Database which consists of just one table.

Primary Key

A field (or group of fields) that uniquely identifies a given entity in a table

Secondary Key

A field that has some identifying information, but typically does not uniquely identify a record with complete accuracy.

Foreign Key

A primary key of one table that appears as an attribute in another table and acts to provide a logical relationship between the two tables

Composite Key

Two or more fields that collectively define the primary key by unique combinations of their values.

First Normal Form (1NF)

A table that has a primary key and in which there are no repeating attributes or groups of attributes

Second Normal Form (2NF)

A table in first normal form in which there are no partial dependencies (only relevant if the primary key is a composite primary key)

Third Normal Form (3NF)

A table in 2NF where all attributes are dependent on the key, the whole key and nothing but the key

SELECT

SELECT field(s)
FROM table(s)
WHERE criteria
ORDER BY ascending/descending

CREATE TABLE

CREATE TABLE name (
field datatype
)

ALTER TABLE

ALTER TABLE name
ADD field datatype

ALTER TABLE name
DROP COLUMN column

ALTER TABLE name
MODIFY COLUMN name modification

INSERT INTO

INSERT INTO table_name (columns)
VALUES (values)

UPDATE

UPDATE table
SET field \= new value
WHERE condition

ACID

Atomicity, Consistency, Isolation, Durability

Atomicity

Requires that a transaction be processed completely or not at all