INPUT/OUTPUT

INPUT/OUTPUT

1. A.EXTERNAL DEVICES

   • CATEGORIES:
     • Machine readable
       • communicating with equipment
     • Communication
       • communicating with remote devices
     • Human Readable
       • communicating with user
   • Definition
     • Exchanging data between the external environment
       • Connecting to I/O module

2. B.KEYBOARD/MONITOR

   • INTERNATIONAL REFERENCE ALPHABET(IRA)
     • Most common user interaction
       • 128 different characters can be represented

3. C.THREE TECHNIQUES FOR I/O OPERATIONS

   • 1.PROGRAMMED I/O
     • 2.INTERRUPTED-DRIVEN I/O
       • 3.DIRECT MEMORY ACCESS(DMA)
         • -The I?O module and main memory exchange data directly without processor involvement
       • -issues an I/O command, continues to execute other instructions,and is interrupted by the I/O module when the latter has completed its work
     • -data exchanged between processor and I/O module
     • -Processor executes a program that gives it direct control of I/O operation

4. D.I/O Commands

   • Four types of I/O commands
     • 1. Control-activate peripheral(to tell what to do)
       • 2.Test-test various status conditions
         • 3.Read-causes I/O to obtain item and place in an internal buffer
           • 4.Write-causes the I/O module to take item from data bus & transmit to peripheral

5. E.I/O MAPPING SUMMARY

   • 1.MEMORY MAPPED I/O
     • -Devices & memory share an address space
       • -No special commands for I/O
         • *Large selection of memory access commands available
   • 2.Isolated I/O
     • -Separate address space
       • -Need I/O or memory select lines

6. F.Design Issues

   • Two design issues arise in implementing interrupt I/O
     • Reason:
       • 1.There will be multiple I/O modules how does the processor determine which device interrupted
       • 2.Multiple interrupts have occurred how does the processor decide which one to process