Uppsala University Operating Systems

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Uppsala UniversityOperating Systems
Input Output Brahim Hnichhttp//www.csd.uu.se/b
rahim/os1.html
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Overview I

• Principles of I/O hardware
• I/O devices
• device controllers
• direct memory access (DMA)
• Principles of I/O software
• goals
• interrupt handlers
• device drivers
• device dependent I/O software
• user space I/O software
• Disks
• disk hardware
• disk arm scheduling,
• Summary

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Principles of I/O Hardware

• Different view points
• Programming I/O devices
• NOT designing, building, maintaining I/O devices
• Here only necessary background to understand how
  to program I/O devices

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I/O Devices

• Block devices store information in fixed size
  blocks, each one with its own address
• common block sizes 128 Bytes- 1024 Bytes
• it is possible to read or write each block
  independently of all other ones
• example disks
• Character devices delivers or accepts a stream
  of characters, without regard to any block
  structure
• it is not addressable and does not have any seek
  operation
• examples terminals, line printers, paper tapes,
  punch cards, network interfaces, mice, and most
  other devices that are not disk-like
• Clocks or memory mapped screens
• Device drivers low-level software hiding the
  device specific details to the file system (deals
  with abstract block devices)

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Device Controllers

• Device mechanical component of I/O unit
• Device controller (adapter) electronic component
  of I/O unit (often printed circuit card), and the
  OS deals with that
• Communication between CPU and controllers
• micro- and mini-computers single-bus model
• mainframes multiple buses and specialized I/O
  computers (I/O channels)
• Memory mapped I/O registers of the controller
  (to communicate with the CPU) are part of the
  regular memory address space

Disk drives
printer
Controller-device interface
CPU
Memory
Disk controller
Printer controller
Other Controllers
System bus
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Direct Memory Access (DMA)

• Many controllers for block devices support DMA
• a DMA transfer is done entirely by the controller

Drive
CPU
Memory
Disk controller
Buffer
Count
DMA registers
Memory address
Count
System bus

• Interleaving

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0
7
0
7
0
7
0
7
0
5
0
1
1
1
1
6
4
3
3
2
No interleaving
Single interleaving
Double interleaving
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2
2
2
5
1
6
6
7
3
4
3
4
3
4
3
4
5
2
1
4
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Principles of I/O Software I

• Goals
• organize software as a series of layers
• device independent
• uniform naming
• error handling
• synchronous (blocking) versus asynchronous
  (interrupt-driven) transfers
• sharable versus dedicated devices
• I/O software is structured in four layers
• interrupt handlers
• device drivers
• device-dependent OS software
• user level software

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Principles of I/O Software II

• Interrupt handlers
• at a very low level of the OS
• (block) unblock processes (by using semaphores,
  condition variables,)
• Device drivers
• device dependent code
• each device driver handles one device type or one
  class of closely related devices
• issues commands to the device controller(s),
  i.e., to the device registers
• only the device driver knows about sectors,
  tracks, cylinders, heads, arm motion, interleave
  factors, motor drives, head setting times, and
  all other mechanics of making a disk work
  properly
• accepts request from the device-independent
  software above

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Principles of I/O Software III

• Device-independent I/O software (functions that
  are common to all devices)
• uniform interfacing for the device drivers
• device naming (major and minor device number)
• device protection
• providing a device-independent block size
• buffering
• storage allocation
• allocating and releasing dedicated devices
• error reporting
• User-space I/O software (small part)
• libraries linked together with user programs
  (system calls)
• spooling system daemon, spooling directory

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Principles of I/O Software IV

• Layers of I/O system and main functions of each
  layer

I/O reply
layers
User processes
Make I/O call format I/O spooling
I/O request
Device-independent software
Naming, protection, blocking, buffering,
allocation
Device drivers
Setup device registers checks status
Interrupt handlers
Wakeup driver when I/O completed
Hardware
Perform I/O operation
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Disks

• Properties and advantages over main memory
• storage capacity available is much larger
• price per bit is much lower
• information is not lost when power is turned off
• Hardware
• platters, cylinders, tracks, sectors, arms, heads
• overlapped seeks

Head
Arm
Actuator
Platter
Outer track
Sector
Inner track
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Disk Arm Scheduling Algorithms I

• The time to read or write a disk block is
  determined by 3 factors
• the seek time the time to move the arm to the
  proper cylinder (most important)
• the rotational delay the time for the proper
  sector to rotate under the head
• the actual transfer time
• Three algorithms
• FCFS (first come, first served) the disk driver
  accepts the request one at a time and carries
  then out in that order
• SSF (shortest seek first)
• Elevator algorithm

PENDING REQUESTS
SSF
INITIAL POSITION
X
X
X
X
X
X
X
Time
0 5 10 15
20 25 30
35 39
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Disk Arm Scheduling Algorithms II
Elevator Algorithm
PENDING REQUESTS
INITIAL POSITION
X
X
X
X
X
X
X
Time
0 5 10 15
20 25 30
35 39
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Summary

• Input / output important but often ignored topic
  (substantial fraction of OS is concerned with
  I/O)
• Hardware and software
• Next Lectures?
• Deadlocks
• Unix