OCR GCSE Computer Science Paper 1 Revision Guide
1.1 – Systems Architecture
Architecture of the CPU
Definition = responsible for carrying out instructions in a computer program
Purpose of the CPU = Fetch-Decode-Execute Cycle
1. Data copied from PC to MAR
2. CU fetched data stored at address and copies it to MDR
3. PC increments
4. CU decodes
5. Instruction is executed
Common Components of the CPU
ALU (Arithmetic Logic Unit) – carries out all arithmetic calculations and logical decisions
CU (Control Unit) – coordinates activity of the CPU, fetch and decode instructions
Cache Memory – holds data used repeatedly or likely to access = faster to access than RAM
Registers – memory locations in the CPU which hold data temporarily and can be accessed
very quickly
Von Neumann Architecture
MAR (Memory Address Register) – stores the address of data to be fetched or sent to
MDR (Memory Data Register) – stores data fetched from or to be sent
PC (Program Counter) – stores the address of next instruction and increments
Accumulator – stores the results of calculations made by ALU
CPU Performance
1. Clock speed – faster the clock speed the more instructions completed per second
(GHz)
2. Number of cores – more cores = more instructions processed simultaneously
3. Cache memory – more cache = more data accessed faster
Embedded Systems
Definition – a computer system that is designed for a dedicated function as part of a bigger
system (e.g. washing machines, microwave ovens)
Purpose
Manufactured as a single chip
More robust and reliable than general purpose computers
Characteristics
Designed to perform a limited set of tasks to reduce size and improve performance
Programs uploaded at manufacturing stage
Low power consumption
Small in size
Low cost
Limited options to modify
, 1.2 – Memory and Storage
Primary Storage
A computer needs primary storage for data it needs to access quickly – including start-up
instructions, the operating system and programs that are running.
Two main types of storage – RAM and ROM.
RAM
Main memory
Volatile – loses contents when power is turned off
RAM holds the operating system and any applications and data currently in use by
the computer
The CPU can access RAM quickly
Read or written to
The more RAM = the more programs and data run at the same time
ROM
Non-volatile – keeps contents
Storage for data and instructions needed to start up the computer
Read-only – information is written at manufacturing stage
Virtual Memory
Secondary storage that acts as an extension of RAM.
Stores data from a running program not in use
Relatively slow access – slows down performance
Adding more RAM = less virtual memory used = better performance
Secondary Storage
Needed to store files and programs
Non-volatile
Low cost
High capacity
Reliable
Magnetic Storage
Magnetic storage mostly uses hard disk drives (HDDs)
Made of a stack of magnetic disks that rotate
Reliable and cost effective
Provides high-capacity storage at low cost
Solid State Storage
Uses a technology called flash memory – used in portable devices and in the form of solid-
state drives
1.1 – Systems Architecture
Architecture of the CPU
Definition = responsible for carrying out instructions in a computer program
Purpose of the CPU = Fetch-Decode-Execute Cycle
1. Data copied from PC to MAR
2. CU fetched data stored at address and copies it to MDR
3. PC increments
4. CU decodes
5. Instruction is executed
Common Components of the CPU
ALU (Arithmetic Logic Unit) – carries out all arithmetic calculations and logical decisions
CU (Control Unit) – coordinates activity of the CPU, fetch and decode instructions
Cache Memory – holds data used repeatedly or likely to access = faster to access than RAM
Registers – memory locations in the CPU which hold data temporarily and can be accessed
very quickly
Von Neumann Architecture
MAR (Memory Address Register) – stores the address of data to be fetched or sent to
MDR (Memory Data Register) – stores data fetched from or to be sent
PC (Program Counter) – stores the address of next instruction and increments
Accumulator – stores the results of calculations made by ALU
CPU Performance
1. Clock speed – faster the clock speed the more instructions completed per second
(GHz)
2. Number of cores – more cores = more instructions processed simultaneously
3. Cache memory – more cache = more data accessed faster
Embedded Systems
Definition – a computer system that is designed for a dedicated function as part of a bigger
system (e.g. washing machines, microwave ovens)
Purpose
Manufactured as a single chip
More robust and reliable than general purpose computers
Characteristics
Designed to perform a limited set of tasks to reduce size and improve performance
Programs uploaded at manufacturing stage
Low power consumption
Small in size
Low cost
Limited options to modify
, 1.2 – Memory and Storage
Primary Storage
A computer needs primary storage for data it needs to access quickly – including start-up
instructions, the operating system and programs that are running.
Two main types of storage – RAM and ROM.
RAM
Main memory
Volatile – loses contents when power is turned off
RAM holds the operating system and any applications and data currently in use by
the computer
The CPU can access RAM quickly
Read or written to
The more RAM = the more programs and data run at the same time
ROM
Non-volatile – keeps contents
Storage for data and instructions needed to start up the computer
Read-only – information is written at manufacturing stage
Virtual Memory
Secondary storage that acts as an extension of RAM.
Stores data from a running program not in use
Relatively slow access – slows down performance
Adding more RAM = less virtual memory used = better performance
Secondary Storage
Needed to store files and programs
Non-volatile
Low cost
High capacity
Reliable
Magnetic Storage
Magnetic storage mostly uses hard disk drives (HDDs)
Made of a stack of magnetic disks that rotate
Reliable and cost effective
Provides high-capacity storage at low cost
Solid State Storage
Uses a technology called flash memory – used in portable devices and in the form of solid-
state drives
