A Computer serves no useful purpose without the ability to receive information from an outside source and to transmit results in meaningful forms. Therefore a number of peripheral devices are to be attached with the computer.
- Peripherals Devices :
- I/O Devices : Keyboard, Mouse, Printer, Screen
- Some Screen types : CRT, LCD, LED, TFT
- Some Printer types : Daisy Wheel, DOT Matrix, Ink Jet, Laser.
These are few of the peripheral devices. Peripherals are electromechanical and electromagnetic devices of some complexity. This makes very slow data transmission as compared to CPU.
Input Output Interface
- Provides a method for transferring information between internal and external I/O devices.
- Purpose of the communication link is to resolve the differences that exist between the central computer and each peripheral :
- Signal conversion
- Data transfer rates and synchronisation
- Data codes and formats
- Controlling the operating modes of various peripherals.
I/O bus and Interface module :
- I/O bus has data lines, address lines and control lines.
- Each peripheral has an interface.
- Each interface
- decodes address and control
- interprets control and provides signals
- synchronisation for the data flow.
Processor puts the address of the peripheral on the address bus and sends a control message.
The peripheral matching the address responds only after its interface detects the cell.
A control command/instruction might be:
- Control Command
- Status Command
- Data Output
- Data Input
I/O versus Memory Bus
In addition to communicating with I/O, the processor must communicate with the memory unit.
- Memory bus contains : address, Data and Control lines.
- Three ways that computer buses can be used to communicate with memory and I/O are:
- Use two separate buses, one for I/O and the other for memory.
- Use one common bus for both but different control lines.
- Use one common bus for both and use same control lines.
Separate buses for I/O and Memory
- We have a separate I/O processor (IOP).
- IOP provides a separate pathway for the transfer of information between external devices and internal memory.
- Memory communicates with the IOP and CPU with memory bus, IOP communicates with I/O devices using I/O bus.
- This is called ISOLATED I/O.
- One bus for data and address but, different READ/WRITE lines.
- CPU has distinct input/output instructions, each associated with the interface register address.
- If an address is put on the lines, the control will distinguish between Memory and I/O transfers
- This is called MEMORY MAPPED I/O.
- CPU has a single set for READ/WRITE signals and does not distinguish between memory and I/O.
- The interface register is treated as a memory location, thus it has to have a different address other than memory.
- CPU manipulates the I/O data residing in the interface registers as if they are memory words.
- Memory type instructions are used to access I/O data.
- Advantage is that Load and Store instructions among others are used to interface I/O registers as well.