Operating System Software
- acts as an interface between user and computer hardware
- program which controls running of other programs
- e.g. Windows, DOS, DFS (BBC operating system), MacOS, OS2, Unix
Utility Program
- devised to perform operations on files or data
- e.g. PK Zip
Library Program
- part of library facilities of a computer
- e.g. searches for lost files / restoring corrupted files
Applications Software
- written to perform specific tasks such as payroll and stock control
- e.g. Word, Excel
Bespoke Software
- individual program written for a particular company
Generic Software
- includes all common application packages such as word processing, desktop publishing, spreadsheet, database, CAD
Processor
- controls transmission of data from input devices to memory
- processes data held in main memory
- control transmission of information from main memory to output devices
Internal Memory - cache, ROM and some RAM
Computer Architecture
Fetch-Execute Cycle
- Clock ticks
- Fetch tick - fetches next instruction
- Decode - decodes it
- Execute tick - executes instruction
- Detect - checks for interrupts
Von Neumann Stored Program Concept - programs are stored in memory, each instruction after the last in consecutive memory locations. Any deviation is dealt with in a special way.
CPU
- contains control unit and ALU
Control Unit
- contains buses and registers which allow processor to carry out fetch-execute cycle
ALU
- Arithmetic/Logic Unit
- Arithmetic e.g. 'Add'
- Logic e.g. 'Compare'
- contains circuits which perform many different operations
Buses
- channels (set of parallel wires) within computer which carry data between independent components
- shared transmission medium, therefore only one device can transmit along a bus at any one time
Address Bus
- carries address of memory or I/O port
- unidirectional
- WIDTH DETERMINES MAXIMUM POSSIBLE MEMORY CAPACITY OF SYSTEM
Control Bus
- carries control signals
- bi-directional
- read address location from data bus, write from data bus to address location (in memory)
- ensures access to and use of data and address buses by different components of the system
- interrupt request, interrupt ACK, bus, reset
Data Bus
- carries data
- bi-directional
- INCREASING WIDTH INCREASES SYSTEM'S SPEED AND PERFORMANCE
Word
- maximum number of bits that a processor can handle simultaneously
- size measured by number of digits it contains
- consists of a predetermined number of characters / bits to be processed as an entity
- WORD SIZE DETERMINES SPEED
Structure of internal buses influenced by:
- amount of memory which needs to be addressed (requires set no. of bits)
- word length (data bus need to be able to transport enough bits)
RAM
- storing programs which are currently running
- volatile (loses all information once machine is switched off)
ROM
- non-volatile
- contents permanently etched into memory chip at manufacturing stage
- contains bootstrap loaded (instructs computer to load OS software as soon as computer is switched on)
Cache
- used to improve speed of computer
- intermediate store between CPU and main memory
- stores most recently/frequently used data
- 1Kb - 512Kb