Computer science April mock revision
1.Purpose of the CPU – fetch-decode-execute
2.Common CPU components and their functions
3.Von Neumann architecture
4.Common characteristics of cpu
5.Embedded systems
The purpose of a CPU is to fetch, decode and execute instructions
Fetch:
• Fetch the next instruction from the main memory (ram)
• Brings it back to CPU
Decode:
• Inspect the instruction to find out what needs to be done
Execute:
• Carry out instruction :
• Going back to main memory and fetching data
• performing a calculation
• Storing information back in main memory
Arithmetic logic unit – performs arithmetic calculations and makes logical decisions
Control unit – sends signals to control how data moves around the cpu
Cache – provides fast access to frequently used instructions and data
Registers – tiny, super fast pieces of onboard memory inside the cpu, each with a specific
purpose
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A CPU’s overall performance can be affected by several factors. Three are:
Clock speed:
• Measured in hertz
Cache size:
• Tempory storage of data and instructions being read from and written to ram
• Stores copies of recent data and instructions
• Quicker to read from than main memory
• Saves time
Number of cores:
• Complete copy of cpu
• a quad core processor has four separate processing units
Embedded system: a computer system with a dedicated function within a larger mechanical system
Typical properties:
• Low power consumption
• Small size
• Rugged operating ranges
• Low cost per unit
Examples of:
• Traffic lights
• Domestic appliances
• Factory equipment
• Engine management systems
• Hospital equipment
1. Embedded systems
2. Need for primary storage
3. Ram and rom
4. Virtual memory
5. Need for secondary storage
6. Common types of storage
7. Suitable storage devices and storage media
8. The units of data storage
9. Converting data into binary to be processed
10. Data capacity and calculating data capacity
requirements
11. Converting between denary and 8-bit binary
12. Adding two 8-bit binary integers
13. Converting between denary and 2-digit hexadecimal
14. Binary shifts
15. Representing characters and character sets
16. Representing images
17. Representing sound
18. Compression
PRIMARY STORAGE CONSISTS OF:
• RANDOM-ACCESS MEMORY
• READ ONLY MEMORY
• REGISTERS
• CACHE
IT HOLDS DATA AND INSTRUCTIONS THAT THE CPU NEEDS
TO ACCESS WHILE THE COMPUTER IS RUNNING
THE CPU CAN ACCESS DATA FAR MORE QUICKLY FORM
PRIMARY MEMORY THAN SECONDARY STORAGE
RANDOM-ACCESS MEMORY (RAM):
• HOLDS PROGRAMS AND DATA CURRENTLY IN USE BY THE CPU
• VOLATILE
• READ-AND-WRITE
• LARGE COMPARED TO ROM
READ-ONLY MEMORY (ROM):
• HOLDS STARTUP INSTRUCTIONS FOR THE COMPUTER, KNOWN AS
THE BOOTSTRAP
• IN EMBEDDED SYSTEMS, PROGRAMS MAY BE STORED IN ROM
• NON-VOLATILE
• READ-ONLY
• SMALL COMPARED TO RAM
• VIRTUAL MEMORY IS REQUIRED WHEN THERE IS NOT ENOUGH
PHYSICAL RAM TO STORE OPEN PROGRAMS
• VIRTUAL MEMORY IS STORED ON HARD DISK
• PROGRAMS ARE TRANSFERRED OUT TO VIRTUAL MEMORY FROM
RAM WHNE THEY ARE NOT CURRENTLY EXECUTING
• PROGRAMS ARE TRANSFERRED BACK TO RAM FROM VIRTUAL
MEMORY WHNE THEY ARE REQUIRED
SECONDARY STORAGE IS REQUIRED BECAUSE ROM IS READ-ONLY AND RAM IS
VOLATILE. SECORNDARY STORAGE IS USED FOR:
• STORAGE OF PROBLEMS AND DATA WHEN THE COMPUTER IS POWERED OFF
• MODIFIABLE, SEMI PERMINANT STORAGE OF DATA
• BACKING UP OR ARCHIVING DATA
OPTICAL(CD-R/RW, DVD-R/RW, BLUE RAY):
• LOW CAPACITY COMPARED TO OTHER TPES OF STORAGE
• SLOWER ACCESS SPEEDS
• THIN,LIGHTWEIGHT,PORTABLE
MAGNETIC(HARD DISK DRIVE/TAPE):
• HIGH STORAGE CAPACITY
• FASTER ACCESS SPEEDS
• MOVING PARTS THAT WILL EVENTUALLY FAIL
• HARD DISKS PERFORM BETTER IF THEY ARE DEFRAGMENTED
SOLID-STATE(SSD, MEMORY STICK, FLASH MEMORY CARD):
• MEDIUM STORAGE CAPACITY
• FASTEST ACCESS SPEEDS
• NO MOVING PARTS SO THEY ARE VERY RELIABLE
• NO NOISE
• LOW POWER USE
• EXPENSIVE
THE QUESTION OF WHICH STORAGE DEVICE COMES
DOWN TO:
• CAPACITY
• SPEED
• PORTABILITY
• DURABILITY
• RELIABILITY
• COST
UNIT
BINARY VALUE
BIT
0 OR 1
NIBBLE
4 BITS
BYTE
8 BITS
KILOBYTE
1024 BITS
MEGABYTE
1024^2 BITS
GIGABYTE
1024^3 BITS
TERABYTE
1024^4 BITS
PETABYTE
1024^5 BITS
• CALCULATING TEXT FILE SIZES: (BITS PER CHARACTER) X (NUMBER OF CHARACTERS)
• CALCULATING IMAGE FILE SIZES: (COLOUR DEPTH) X (IMAGE HEIGHT PX) X (IMAGE WIDTH PX)
• CALCULATING SOUND FILE SIZES: (SAMPLE RATE) X (DURATION IN SECONDS) X (BIT DEPTH)
A character set is a defined list if characters recognized by a computer. Each
character is represented by a unique binary number. Well known character sets
include:
• ASCII: a 7-bit character set with 128 characters
• Extended ASCII: an 8-bit character set with 256 character
• Unicode: a 24-bit character set with over 16million characters
Images can be stored in binary as bitmaps or vectors:
• Bitmap vectors are constructed from colored squares called pixels
• Vector images store the mathematics required to draw shapes
Each pixel of a bitmap is stored in binary. The number of bits required for each
pixel depends on the number of colours:
• 1 bit has two possible values; 0 and 1. therefore, 1-bit can store 2 colors; black or white
• 2 bits have 4 possible values; 00, 01, 10 and 11. that means 2 bits can store 4 colors.
The numbers of colors can be calculated a 2^n, where n is the number of bits required for each
pixel
Compression is used to reduce the number of bits in a file, making it smaller in size and
increasing its speed of transfer
Lossy compression:
• Some data is lost and cannot be
recovered
• Greatly reduces file size
• Reduces quality of images/sounds
• Cannot be used with text or
executable files
Lossless compression:
• No data is lost, but encoded
differently
• Files are recovered to their
original state
• Can be used with all types of
data
• Less effective at reducing file
size
• Most suitable for text
documents and executable
files
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Types of networks
Factors that effect network performance
Client server and peer to peer
Hardware used to connect Lan
The internet
Star and mesh network topologies
Modes of connection – wired and wireless
Wireless encryption
Local area network:
• Covers single site within small
geographical area
• Hardware owned by
organization using it
Wide area network:
• Connects lans together over
a large geographical area
• Infrastructure is leased from
telecommunication
companies who own and
manage it
Bandwidth – amount of data can be sent and
received
Number of users – to many users or devices on the
same network can cause it to slow down is there is
insufficient bandwidth
Transmission media – wired connections offer higher
bandwidth than wireless connections
Error rate – less reliable connections increase the
occurrence of errors during data transfer
Latency – the delay between data being transmitted
and a users device receiving it, latency is caused by
bottlenecks in network infrastructure