Summary GCSE OCR Computer Science

1.1 Systems architecture
F-D-E cycle
Fetch
1. Copy memory address from PC to MAR
2. Copy instructions from address in MAR to MDR
3. Increment PC to point to next instruction
Decode
1. Instruction in the MDR is decoded by the CU
2. CU can then prepare for next instruction
Execute
1. Instruction is performed this could be by;
a. Loading data from memory
b. Doing a calculation
c. Halting program


Embedded systems…
..a computer system that has a dedicated function within a larger mechanical system
Characteristics;
 Low power consumption
 Small size
 Rugged operating system
 Low cost per unit
Examples;
 Traffic lights
 Coffee machines
 Hospital instruments
 Digital clocks
 Washing machines


Common CPU components
 Cache - a piece of temporary memory that stores recently used instructions. It can refer to a
part of the RAM, storage disk or CPU
 ALU - a component of the CPU that performs arithmetic and logic calculations
 CU - decodes instructions and sends signals to control how the data moves around the CPU
 Registers - a small piece of memory inside the processors that can hold one instruction or
address

Effect of common characteristics of their performance
 Cache size
 Cache is very fast memory and stores regularly used data so if your cache size if bigger the
less time the processor has to wait for instructions to be fetched because they are already
stored in the cache therefore this improves performance
 Clock speed
 The clock speed of the CPU is measured in hertz. It indicates the number of FDE cycles that
run per second. If your clock speed increases the faster your computer will run
 3gHz = 3 billion cycled per second
 Number of cores

,  A core is a small central processing unit within a CPU. More cores allows the ability to run
multiple instructions at the same time. It does not necessarily mean speed will increase
because the cores need to communicate with each other which takes time, disrupting
performance


Key Terms
 Memory - part of a computer that stores data
 CPU - the brain of a computer that processes program instructions

Von Neumann architecture
 All Von Neumann architecture is classed as registers
 MAR - stores single address of data to be fetched from or the address of where it is stored
 MDR - holds data fetched from or to be written to the memory
 PC - holds single address of next instruction
 Accumulator - holds the results of calculations



1.2 Memory and storage
1.2.1 Primary storage (memory)
Primary storage is the immediate memory the computer has access too. It gives fast access to the
CPU

Need for primary storage
Primary storage consists of;
 RAM
 ROM
 Registers and cache
Its holds the data and instructions which the CPU needs access to while a computer is running. The
CPU can access data from primary storage much faster than from secondary storage

RAM and ROM
RAM ROM
 Holds the operating system,  Holds the first instructions
programs and data in use by the when the computer is first
CPU when the computer is turned on, known as the
running bootstrap
 Read AND write  Programs may be stored
 Volatile (contents lost when in ROM in embedded
turned off) systems
 Large in comparison to ROM  Non volatile
 Read only
 Small in comparison to
RAM

Virtual memory
An area of secondary storage used by the operating system as extra RAM.

How does it work?

,Virtual memory enables data that is in RAM and not currently being used to be transferred to the
hard disk (where it is held). This frees up room in RAM for other programs and data.

When the data on the hard disk is needed again, any other unused data is transferred to the hard
disk before the original data is transferred back to RAM.

It takes time however to move data between the HDD and the RAM (switching). This disrupts
performance.

1.2.2 Secondary storage
Secondary storage - data storage which is used to store data so that the computer can be switched
off.
Long term, non-volatile storage

Why is it needed?
Secondary storage is needed because ROM is read only and RAM is volatile
Its needed for;
 Storage of programs and data when the power is turned off
 Semi-permanent storage of data that can change
 Backup of data files
 Archive of data files

Common types of storage/ advantages + disadvantages
Storage type Examples Advantages Disadvantages
Optical Blu-ray  Cheap per  Scratched easily
DVD-R GB  Slowest average
 Robust/ read/write speed
portable  Lowest average
 No capacity
damage
by water
or shocks
Magnetic Hard disk  Highest  Slow when finding
drive average specific data stored
Tapes capacity on it
 Lowest
average
cost per
GB
 Fast
read/write
speed
Solid state Solid state  Fastest  Most expensive
drive read/write  Vulnerable to
Memory speed sudden power loss
sticks  Good or electromagnetic
portability fields
 Very
reliable -

, no moving
parts


Which storage device?
1. Capacity
2. Speed
3. Portability
4. Durability
5. Reliability
6. Cost




1.2.3 Units
Why is data stored in binary format?
With just 2 states electronic components it means its;
 Easier for the computer to process
 Takes up less space
 Cheaper
 More reliable

Units of data;
 Nibble (4 bits)
 Byte (8 bits)
 Kilobyte (1,000 bytes or 1 KB)
 Megabyte (1,000 KB)
 Gigabyte (1,000 MB)
 Terabyte (1,000 GB)
 petabyte (1,000 TB)

Converting between units