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Input/Output

Chapter 5

5.1 Principles of I/O hardware5.2 Principles of I/O software5.3 I/O software layers5.4 Disks5.5 Clocks5.6 Character-oriented terminals5.7 Graphical user interfaces5.8 Network terminals5.9 Power management

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I/O Device

• I/O devices can be divided into two categories: – A block devices is one that stores information in

fixed-size blocks.– A character device delivers or accepts a stream of

characters, without regard to any block structure.

• Some devices do not fit in: clocks, memory-mapped screens.

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Principles of I/O Hardware

Some typical device, network, and data base rates

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Device Controllers

• I/O devices have components:– mechanical component – electronic component

• The electronic component is the device controller or adapter.– may be able to handle multiple devices– On PCs, it often takes the form of a printed circuit card

that can be inserted into an expansion slot.

• Controller's tasks– convert serial bit stream to block of bytes– perform error correction as necessary– make available to main memory

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Memory-Mapped I/O• Each controller ha a few registers that are used for

communicating with the CPU. The operating system can command the device by writing into these registers and learn the device’s state by reading from these registers.

• Many devices have a data buffer that the operating system can read and write. Two approaches exist:– Each control register is assigned an I/O port number.

– All the control registers are mapped into the memory space. This is called memory-mapped I/O.

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Memory-Mapped I/O

• Separate I/O and memory space• Memory-mapped I/O – PDP-11• Hybrid - Pentium

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Memory-Mapped I/O• Advantages of memory-mapped I/O:

– An I/O device driver can be written entirely in C – No special protection mechanism is needed to keep

user process from performing I/O. – Every instruction that can reference memory can

also reference control register.

• Disadvantages of memory-mapped I/O:– Caching a device control register would be

disastrous (not reflect current device status change).– All memory modules and all I/O devices must

examine all memory references.

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Memory-Mapped I/O

(a) A single-bus architecture(b) A dual-bus memory architecture

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Direct Memory Access (DMA)• Direct Memory Access (DMA) is a

capability provided by some computer bus architectures that allows data to be sent directly from an attached device (such as a disk drive) to the memory on the computer's motherboard.

• DMA operations:1. CPU program the DMA controller2. DMA requests transfer to memory3. Data transferred4. The disk controller sends an acknowledgement

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Direct Memory Access (DMA)

Operation of a DMA transfer

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Interrupts Revisited

• The interrupt vector is a table holding numbers on the address lines specifying devices.

• Precise interrupt:– The PC (Program Counter) is saved in a known place.

– All instructions before the one pointed to by the PC have fully executed.

– No instruction beyond the one pointed to by the PC has been executed.

– The execution state of the instruction pointed to by the PC is known.

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Interrupts Revisited

How interrupts happens. Connections between devices and interrupt controller actually use interrupt lines on the bus rather than dedicated wires

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Principles of I/O SoftwareGoals of I/O Software

• Device independence– programs can access any I/O device – without specifying device in advance

· (floppy, hard drive, or CD-ROM)

• Uniform naming– name of a file or device a string or an integer– not depending on which machine

• Error handling– handle as close to the hardware as possible

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Goals of I/O Software

• Synchronous vs. asynchronous transfers– blocking transfers vs. interrupt-driven– Most physical I/O is interrupt-driven.

• Buffering– data coming off a device cannot be stored in

final destination

• Sharable vs. dedicated devices– disks are sharable– tape drives would not be

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I/O Execution

• There are three ways that I/O are performed:– Programmed I/O

• Disadvantage: tying up the CPU full time until all the I/O is done.

– Interrupt-driven I/O• Interrupts might waste time.

– I/O using DMA• Slower than CPU

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Programmed I/O

• Steps in printing a string– String in the user buffer

– A System call to transfer the string to the kernel.

– String printed

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Programmed I/O

Writing a string to the printer using programmed I/O

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Interrupt-Driven I/O

• Writing a string to the printer using interrupt-driven I/O– Code executed when print system call is made– Interrupt service procedure

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I/O Using DMA

• Printing a string using DMA– code executed when the print system call is made– interrupt service procedure

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I/O Software Layers

• I/O Software in four layers:– Interrupt handlers– Device drivers– Device-independent operating system software– User-level I/O software

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I/O Software Layers

Layers of the I/O Software System

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Interrupt Handlers

• Interrupt handlers are best hidden– have driver starting an I/O operation block until

interrupt notifies of completion

• Interrupt procedure does its task– then unblocks driver that started it

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Interrupt Handlers

• Steps must be performed in software after interrupt completed

1. Save registers not already saved by interrupt hardware2. Set up context for interrupt service procedure3. Set up stack for interrupt service procedure4. Acknowledge interrupt controller, reenable

interrupts5. Copy registers from where saved6. Run service procedure 7. Set up MMU context for process to run next8. Load new process' registers9. Start running the new process

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Device Driver

• The device driver is the device-specific code for controlling the I/O device attached to a computer.

• Current operating systems expect drivers to fun in the kernel.

• Operating systems usually classify drivers into:– Block devices– Character devices

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Device Drivers

• Logical position of device drivers is shown here• Communications between drivers and device controllers goes over the

bus

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Device-Independent I/O Software

Functions of the device-independent I/O software

Uniform interfacing for device drivers

Buffering

Error reporting

Allocating and releasing dedicate devices

Providing a deice-independent block size

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Device-Independent I/O Software

(a) Without a standard driver interface – a lot of new programming effort

(b) With a standard driver interface

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Buffering

• Buffering is a widely-used technique. If data get buffered too many times, performance suffers.

• Classes of I/O errors:– Programming errors

– Actual I/O errors

• Some I/O software can be linked with user programs.– Spooling is a way of dealing with dedicated I/O devices in a

multiprogramming system.

– A spooling directory is used for storing the spooling jobs.

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Device-Independent I/O Software

(a) Unbuffered input(b) Buffering in user space(c) Buffering in the kernel followed by copying to user space(d) Double buffering in the kernel

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Device-Independent I/O Software

Networking may involve many copies

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User-Space I/O Software

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

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Disks

• Disks come in a variety of types:– Magnetic disks (hard disks and floppy disks)– Arrays of disks– Optical disks

• CD-ROMs

• CD-Recordables

• CD-Rewritables

• DVD

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DisksDisk Hardware

Disk parameters for the original IBM PC floppy disk and a Western Digital WD 18300 hard disk

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Disk Hardware

• Physical geometry of a disk with two zones• A possible virtual geometry for this disk

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Disk Hardware

• Raid levels 0 through 2 • Backup and parity drives are shaded

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Disk Hardware

• Raid levels 3 through 5• Backup and parity drives are shaded

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Disk Hardware

Recording structure of a CD or CD-ROM

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Disk Hardware

Logical data layout on a CD-ROM

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Disk Hardware

• Cross section of a CD-R disk and laser– not to scale

• Silver CD-ROM has similar structure– without dye layer– with pitted aluminum layer instead of gold

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Disk Hardware

A double sided, dual layer DVD disk

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Disk Formatting

A disk sector

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Disk Formatting

An illustration of cylinder skew

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Disk Formatting

• No interleaving• Single interleaving• Double interleaving

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Disk Arm Scheduling Algorithms

• Time required to read or write a disk block determined by 3 factors

1. Seek time

2. Rotational delay

3. Actual transfer time

• Seek time dominates

• Error checking is done by controllers

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Disk Arm Scheduling Algorithms

Shortest Seek First (SSF) disk scheduling algorithm

Initialposition

Pendingrequests

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Disk Arm Scheduling Algorithms

The elevator algorithm for scheduling disk requests

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Error Handling

• A disk track with a bad sector• Substituting a spare for the bad sector• Shifting all the sectors to bypass the bad one

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Stable Storage

Analysis of the influence of crashes on stable writes

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ClocksClock Hardware

A programmable clock

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Clock Software (1)

Three ways to maintain the time of day

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Clock Software (2)

Simulating multiple timers with a single clock

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Soft Timers

• A second clock available for timer interrupts– specified by applications– no problems if interrupt frequency is low

• Soft timers avoid interrupts– kernel checks for soft timer expiration before it

exits to user mode– how well this works depends on rate of kernel

entries

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Character Oriented TerminalsRS-232 Terminal Hardware

• An RS-232 terminal communicates with computer 1 bit at a time

• Called a serial line – bits go out in series, 1 bit at a time

• Windows uses COM1 and COM2 ports, first to serial lines

• Computer and terminal are completely independent

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• Central buffer pool• Dedicated buffer for each terminal

Input Software (1)

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Input Software (2)

Characters handled specially in canonical mode

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Output Software

The ANSI escape sequences• accepted by terminal driver on output• ESC is ASCII character (0x1B)• n,m, and s are optional numeric parameters

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Display Hardware (1)

Memory-mapped displays• driver writes directly into display's video RAM

Parallel port

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Display Hardware (2)

• A video RAM image – simple monochrome display– character mode

• Corresponding screen– the xs are attribute bytes

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Input Software

• Keyboard driver delivers a number– driver converts to characters

– uses a ASCII table

• Exceptions, adaptations needed for other languages– many OS provide for loadable keymaps

or code pages

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Output Software for Windows (1)

Sample window located at (200,100) on XGA display

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Output Software for Windows (2)

Skeleton of a Windows main program (part 1)

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Output Software for Windows (3)

Skeleton of a Windows main program (part 2)

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Output Software for Windows (4)

An example rectangle drawn using Rectangle

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Output Software for Windows (5)

• Copying bitmaps using BitBlt.– before– after

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Output Software for Windows (6)

Examples of character outlines at different point sizes

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Network TerminalsX Windows (1)

Clients and servers in the M.I.T. X Window System

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X Windows (2)

Skeleton of an X Windows application program

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The SLIM Network Terminal (1)

The architecture of the SLIM terminal system

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The SLIM Network Terminal (2)

Messages used in the SLIM protocol from the server to the terminals

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Power Management (1)

Power consumption of various parts of a laptop computer

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Power management (2)

The use of zones for backlighting the display

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Power Management (3)

• Running at full clock speed

• Cutting voltage by two – cuts clock speed by two, – cuts power by four

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Power Management (4)

• Telling the programs to use less energy– may mean poorer user experience

• Examples– change from color output to black and white

– speech recognition reduces vocabulary

– less resolution or detail in an image