Input/Output The messiest part of O.S.

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Input/OutputThe messiest part of O.S.

Chap11

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Speed difference of I/O Hardware

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Device Controllers

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

• The electronic component is the device controller– may be able to handle multiple devices

• 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 (1)

• Separate I/O and memory space• Memory-mapped I/O• Hybrid

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Memory-Mapped I/O (2)

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

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I/O Architecture

• Several types of bus,such as ISA, EISA,PCI and MCA, are currently in use.

• data bus – the pentium has 64 bit wide data bus• Address bus – • Control bus – a group of lines that transmit control

information to the connected circuits. The Pentium uses control lines to specify, for example, whether the bus is used to allow data transfers between a processor and the RAM, or alternatively, between a processor and an I/O device.

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CPUCPU

I/O busI/O bus

I/O PortI/O Port I/O PortI/O Port

I/O InterfaceI/O Interface

I/O ControllerI/O Controller

PC’s I/O architecturePC’s I/O architecture

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PC’s I/O architecture

• When a bus connects to the CPU to an I/O device, it is called an I/O bus.

• 80x86 use 16 out of the 32 address lines to address I/O devices and 8,16, or 32 out of the 64 data lines to transfer data

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I/O ports

• Each device connected to the I/O bus has its own I/O address, which are usually called I/O ports

• PC provides up to 65536 8-bits I/O ports. Two consecutive 8-bit ports may be regarded as a single 16-bit port.

• Four assembly instructions in,ins,out,outs allow CPU to read and write into an I/O port

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Memory mapped I/O ports in PC

• PC’s device I/O ports may also be mapped into address of physical address space.

• O.S. can use mov instruction that operate directly on memory.

• Modern hardware devices are more suited to mapped I/O, since it is faster and can be combined with DMA

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I/O and registers

• The I/O ports of each device are actually structured into a set of specialized registers

• CPU write commands to control registers• CPU reads a value that represents the internal

state of the device from status register• CPU fetch data from a device by reading input

register• CPU push data to device by writing bytes into

output registers

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CPUCPU

Control registerControl register

Status registerStatus register

input registerinput register

Output registerOutput register

Device’sDevice’sI/O InterfaceI/O Interface

Specialized I/O ports

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• To lower costs, the same I/O port is often used for different purposes. e.g., some bits describe the device state and others are used to specifycommands.e.g., the same I/O ports may be used as an input register or an output register

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What is an I/O interface?

• An I/O interface is a hardware circuit inserted between a group of I/O ports and the corresponding device controller.

• It acts as an interpreter that translates the values in the I/O ports into commands and data for the device.

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Types of I/O interfaces

• Custom I/O interfaces– devoted to one specific hardware device– usually, the device controller is on the

same card of I/O interfaces.

• General-purpose I/O interface– used to connect several different

hardware devices

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Custom I/O interfaces

• Keyboard interface• Graphic interface • Disk interface• Network interface

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General –purpose I/O interface

• Parallel port– traditionally connected to printers, but can also be

connected to removable disks, scanners

• Serial port– one bit at a time– use UART chip (Universal Asynchronous Receiver and

Transmitter) to string out the bytes into a sequence of bits.

• Universal serial bus (USB)• PCMCIA interface• SCSI (Small Computer System Interface) Interface

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Device Controller

• It interprets the high-level commands received from the I/O interface and force the device to execute specific actions by sending proper sequence of electrical signals

• It converts and properly interprets the electrical signals received from the device and modifies (through the I/O interface) the value of the status register

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I/O shared memory

• Several hardware device sometimes include their own memory, which is often called I/O shared memory. E.g. graphic cards.

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PC’s mapping addressesof I/O shared memory

• For device connected to ISA bus– mapped into physical address from

0xa0000 to 0xfffff: a hole between 640K-1MB. These are reserved page frames

• For some old devices using VESA Local bus– 0xe00000 to 0xffffff : 14MB-16MB– going out of production

• For device connected to PCI bus– is mapped into very large physical address, well above the end

of RAM’s physical address. Much simpler to handle• AGP (Accelerated Graphics Port)

– use a special hardware circuit GART (Graphics Address Remapping Table)

– GART enables AGP to sustain much higher data transfer rate.

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Direct Memory Access (DMA)

Operation of a DMA transfer

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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 (1)

• 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 (2)

• Synchronous vs. asynchronous transfers– blocked transfers vs. 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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Programmed I/O (1)

Steps in printing a string

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Programmed I/O (2)

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

Layers of the I/O Software System

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Interrupt Handlers (1)

• 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

• Steps must be performed in software after interrupt completed

1. Save regs not already saved by interrupt hardware

2. Set up context for interrupt service procedure

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Interrupt Handlers (2)

3. Set up stack for interrupt service procedure4. Ack 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 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 (1)

Functions of the device-independent I/O software

Uniform interfacing for device drivers

Buffering

Error reporting

Allocating and releasing dedicate devices

Providing a device-independent block size

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(a) Without a standard driver interface(b) With a standard driver interface

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(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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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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UNIX I/O (1)/Device Driver

Some of the fields of a typical cdevsw table

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UNIX I/O (2)

The UNIX I/O system in BSD

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Streams

An example of streams in System V

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Disk Hardware (1)

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

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Disk Hardware (2)

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

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Disk Hardware (3)

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

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Disk Hardware (4)

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

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Disk Hardware (5)

Recording structure of a CD or CD-ROM

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Disk Hardware (6)

Logical data layout on a CD-ROM

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Disk Hardware (7)

• 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 (8)

A double sided, dual layer DVD disk

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Disk Formatting (1)

A disk sector

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Disk Formatting (2)

An illustration of cylinder skew

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Disk Formatting (3)

• No interleaving• Single interleaving• Double interleaving

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Disk Arm Scheduling Algorithms (1)

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

1. Seek time2. Rotational delay3. Actual transfer time

• Seek time dominates• Error checking is done by controllers

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Shortest Seek First (SSF) disk scheduling algorithm

Initialposition

Pendingrequests

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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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Skeleton of a Windows main program (part 1)

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Skeleton of a Windows main program (part 2)

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An example rectangle drawn using Rectangle

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• Copying bitmaps using BitBlt.– before– after

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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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• Running at full clock speed

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

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• 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

Input/Output The messiest part of O.S.

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