fall 2000m.b. ibáñez lecture 25 i/o systems. fall 2000m.b. ibáñez categories of i/o devices...

Fall 2000 M.B. Ibáñez

Lecture 25I/O Systems


Categories of I/O Devices

• Human readable– used to communicate with the user– video display terminals– keyboard– mouse– Printer

From Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall



• Machine readable– used to communicate with electronic equipment– disk drives– tape drives– controllers– Actuators




• Communication– used to communicate with remote devices– digital line drivers– Modems



Differences in I/O Devices

• Data Transfer Rate

• Application– disk used to store files must have file-

management software– disk used to store virtual memory pages needs

special hardware to support it– terminal used by system administrator may

have a higher priorityFrom Operating Systems. Internals and Design Principles. W. Stallings. Prentice Hall


Differences in I/O Devices• Complexity of control

• Unit of transfer– data may be transferred as a stream of bytes for a

terminal or in larger blocks for a disk

• Data representation– encoding schemes

• Error conditions– devices respond to errors differently

Operating Systems. Internals and Design Principles

. W. Stallings. Prentice Hall


Hardware

• Device

• Port

• Bus

• Controller

• Computer System


Hardware

PCI bus

monitor processor

cache

memoryGraphics controller

Memorycontroller

IDE disk controller

disk disk


Techniques for Performing I/O

• Programmed I/O– process is busy-waiting for the operation to

complete

• Interrupt-driven I/O– I/O command is issued– processor continues executing instructions– I/O module sends an interrupt when done



Techniques for Performing I/O

• Direct Memory Access (DMA)– DMA module controls exchange of data

between main memory and the I/O device– processor interrupted only after entire block has

been transferred



Programmed I/O: Polling

• Controller– Indicates its state

through the busy bit in the status register.

– Sets the busy bit when it is busy working

– Clears the busy bit when it is ready to accept the next command

• Host– Signals its wishes via

the command-ready bit in the command register

– Sets the command ready bit when a command is available to execute


Programmed I/O: Polling

Controller Host

status register

data-out register

command-register

busy bit

write bitready bit

1

INFO

1

=0 ?

0

a

=1?1

a

writecommand?


Simple Interrupt Processing

Device controller orother system hardwareissues an interrupt

Processor finishesexecution of currentinstruction

Processor signalsacknowledgmentof interrupt

Processor pushes PSWand PC onto controlstack

Processor loads newPC value based oninterrupt

Save remainder ofprocess stateinformation

Process interrupt

Restore process stateinformation

Restore old PSWand PC



Interrupt Handler

• A program that determines nature of the interrupt and performs whatever actions are needed

• Control is transferred to this program

• Generally part of the operating system



Multiple InterruptsSequential Order

• Disable interrupts so processor can complete task

• Interrupts remain pending until the processor enables interrupts

• After interrupt handler routine completes, the processor checks for additional interrupts



Multiple InterruptsPriorities

• Higher priority interrupts cause lower-priority interrupts to wait

• Causes a lower-priority interrupt handler to be interrupted

• Example when input arrives from communication line, it needs to be absorbed quickly to make room for more input



Direct Memory Access (DMA)

• I/O exchanges occur directly with memory

• Processor grants I/O module authority to read from or write to memory

• Relieves the processor from the task

• Processor is free to do other things



Six step process to perform DMA transfer

From Operating System Concepts. Silberschatz & GalvinAddison Wsley

fall 2000m.b. ibáñez lecture 25 i/o systems. fall 2000m.b. ibáñez categories of i/o devices...

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