Fundamentals of Hardware
•Computer system
•Internal & external components
•Memory & stored program concept
•The Processor
•Arithmetic / Logic Unit (ALU)
•Introduction to Buses
Computer system
•Internal components are contained in the CPU
–‘CPU’sometimes used to mean actual processor unit used to carry out fetching, decoding and executing of instructions.
–OR the unit housing the processing unit, plus memory & controllers
•Internal components:-
–The processor
–Main memory
–I / O controllers, some input/output only, some both input & output (Processor receives and transmits data through a part of an I/O controller called an I/O port)
–Buses
•External components (peripherals)
–keyboard
–mouse
–printer
–disk drives
The Processor
•The processor executes instructions and controls the operation of all components.
•The processor executes instructions by repeating 3 operations:-
•Fetch
–cause next instruction to be fetched from main memory;
•Decode
–translate the program instruction into commands that the computer can process;
•Execute
–cause the instruction to be executed;
Main Memory
(Primary Memory)
•Main memory holds the current programs that are being executed
•Main memory also holds the current data being processed
•Main memory is temporary memory –loses its contents when computer turned off
•Data transferred to main memory from secondary memory when needed
Buses
•A set of parallel wires connecting two or more components
•CPU connected to main memory by three separate buses -the data, address and control buses
•When CPU wishes to access a particular memory location it sends the address to memory on the address bus.
•Data in location is then returned to the CPU on the Data bus.
•Control signal sent along the control bus.
•The data, address and control buses are all system buses
•Each bus is a shared transmission medium, only ONE device can transmit along the bus at any one time
Control Bus
•Bi-directional bus
•Data and address buses sharedby all components
•Control lines ensure access to and use of the address buses by components doesn’t lead to conflict
•Used to transmit command, timing and specific status informationbetween system components
•Timing signals indicate validity of data and address info.
•Command signals specify operations to be performed
•Specific status signals indicate the state of data transfer requests or status of a component request to gain control of system bus
Typical control lines
•Memory Write:causes data on data bus to be written into addressed location
•Memory Read:causes data from addressed location to be placed on data bus
•I/O Write:causes data on data bus to be output to the addressed I/O port
•I/O Read:causes data from the addressed I/O port to be placed on the data bus
•Transfer ACK:indicates that data have been accepted from or placed on the data bus
•Bus Request:indicates a component needs to gain control of system bus
Address Bus
•When processor wishes to read a word (say 8, 16 or 32 bits) of data from memory
•It first puts address of desired word on address bus
•Width of address bus determines the max. possible memory capacity of system
•If address bus consisted of only 8 lines, then max. capacity (inbinary) would be 256 i.e. 28-each line can carry 1 bit.
•More realistic min. bus width 20 lines giving capacity of 220=1Mb
•Address bus also used to address all other peripherals
I/O Controllers
(Interface Cards)
•Peripherals cannot be connected directly to the processor
•Each device works in different ways
•Processor controls and communicates with peripherals through an I/O or device controller
•I/O controllers can operate both input and output transfers of bits e.g floppy disk controller
•Some controllers operate in one direction either as an input controller, e.g. keyboard controller or as output controller, e.g. VDU controller
I/O Controllers (contd.)
•Controller is an electronic circuit board consisting of 3 parts:-
–Interface allowing connection of the controller to the system or I/O bus;
–Set of data, command and status registers;
–Interface allowing connection of the controller to the cable connecting the device to the computer
•Interface is a standardised form of connection defining signals, pins/sockets and voltage levels.
–E.g. USB interface enabling serial transmission of data between computer and printers
–Printer would also contain an USB interface for compatability
