io devies

Input/output organization

Introduction• The basic features of a computer is its ability to exchange

data with other devices.

• Input to the computer from various components from or outside system.

• Output may also the same.

Accessing I/O Devices

Accessing I/O Devices• All input/output devices can be connected to a computer

using bus it enables all devices to exchange information.

• It consists of three sets of lines used to carry address, data and control signals.

• Each i/p / o/p device is assigned a unique set of addresses.

• When the processor request either a read (or) write operation. Then, the requested data placed on data line and corresponding address transferred over the address line and activate control line based on the request.

• When the i/o devices and the memory share the same address space, the arrangement is called Memory Mapped I/O.

• For example if DATAIN is the address of the input buffer associated with the keyboard, the instruction

» Move DATAIN, R0

• Reads the data from DATAIN and stores them into R0.

• Move R0 ,DATAOUT,

Sends the contents of register R0 to location DATAOUT, which may be the output data buffer.

• Both the data and status registers are connected to the data bus and assigned unique addresses.

• The address decoder, the data and status registers and the control circuitry coordinate to create a interface circuit.

• Input/ output devices operate at speeds that are vastly different form that of the processor speed.

• Program controlled I/O helps the processor to synchronize the speed with i/o device by which frequently check the status flag.

•

Commonly used mechanism• Interrupts

• Direct memory access.

address range (hexadecimal) size device

0000 – 7FFF 32 KiB RAM

8000 – 80FF 256 bytes general purpose I/O

9000 – 90FF 256 bytes sound controller

A000 – A7FF 2 KiB

video controller/text-mapped display RAM

C000 – FFFF 16 KiB ROM

Thus we might end up with a memory map like so: