Input and Output

Input/Output System

Enables the attachment of input and output devices to the processor

Examples of Input Devices

• keyboards

• mice

• track balls

• touch screens

• musical instruments

• cameras

• environmental sensors

Examples of Output Devices

• displays

• printers

• speakers

• environmental actuators

Examples of Input-and-Output Devices

• network interfaces

  • ethernet

  • WIFI

  • bluetooth

• disks

• audio cards

• MIDI devices

Connecting I/O Devices

I/O Challenge 1 - Speed-Gap Challenge

I/O devices are often mechanical, and therefore run orders of magnitude slower than the CPU

So how can we ensure that the CPU is not slowed down when it interacts with I/O devices (e.g. when fetching data from the hard disk)?

• We can’t afford to waste CPU cycles

(with very slow CPUs and very fast devices the problem can be reversed - any difference in performance is a problem)

I/O Challenge 2 - Device Diversity Challenge

Devices are extremely diverse, e.g

• Diversity of data-access modes

• Device-specific operations

• I/O protocol

Diversity of Data-Access Models

• Read-only or write-only or read-and-write

• Access by the individual byte or by the block/ by the stream

• Access randomly (like a disk) or sequentially (like a tape)

Device-Specific Operations

Change the resolution (only for screens), set the time (only for clocks), focus (only for cameras)

I/O Protocol

• We may need to be concerned with potential data transfer errors (e.g. from electrical noise, wireless transmission errors)

• Synchronous or asynchronous

Device Drivers

These are software plug-ins inside the operating system 

Their job is to abstract over device diversity by “grouping” sets of “somewhat similar” types of device; e.g.

• Many devices can be made to “look like a mouse” to software 

• All hard disks are “essentially the same”

Example Functions of Device Drivers

• Registering a device with the OS and initialising it

• Initiating data transfers to or from a device

• Monitoring status events from a device

• Managing device/ system shutdown

  • Ensuring that the OS doesn’t “stop” until all unwritten data is stored, and the device is left in a safe state)

Traditional Two-Fold classification of Device Types (and Device Driver Types)

• character devices

• block devices

Character Devices

Send and receive one byte at a time 

Classic example: the keyboard

Block Devices

Send and receive a multi-byte block at a time

Classic example: the hard disk:

Traditional Two-Fold Classification of Processor Support for I/O

• Isolated I/O

• Memory-Mapped I/O

Isolated I/O

• The processor provides:

• Dedicated physical pins for the connection of I/O devices, and

• And dedicated instructions for doing I/O operations

• Suited to simple devices

  • Having only a fixed set of special I/O instructions does not help much with device diversity

• Example Intel x86 instructions

  • IN destination_register, port_address

  • OUT source_register, port_address

• Port addresses are typically 8 bits: narrower than main memory addresses

Memory-Mapped I/O

• Here, devices sit within the CPU’s linear memory address space

  • E.g. we might access the next keyboard character by reading from memory address 0x40000040

• Simple, flexible programming model

• Downside is that it adds complexity to devices

  • They need to understand larger addresses and to work at memory speeds