§4.4 Input and Output Devices

Douglas Wilhelm Harder, M.Math. LELDepartment of Electrical and Computer Engineering

University of Waterloo

Waterloo, Ontario, Canada

ece.uwaterloo.ca

dwharder@alumni.uwaterloo.ca

© 2012 by Douglas Wilhelm Harder. Some rights reserved.Based in part on Gary Nutt, Operating Systems, 3rd ed.

2

Outline

This topic looks at input and output devices– I/O Devices

• Categories

– Device controllers– Direct Memory Access– Memory-mapped I/O

I/O Devices

3

I/O Devices

I/O devices communicate with a CPU through the system bus– Input devices signal the CPU that it is ready to transmit data

• The CPU retrieves and deals with that data• Often the data is from registers into main memory

– Similarly, the CPU can signal that it has data ready to transmit to output devices

• Devices retrieve the data• Again, often the data that is being transmitted had to be copied from

main memory into registers before it was sent on

I/O Devices

4

I/O Devices

I/O Devices

Inputkeyboardmousetouch-sensitive screenmicrophone

Outputterminalgraphicalspeakerprinter

Storagehard drivesUSB drivesCD ROM

Communicationserial and parallel portsnetwork cards (including wireless)infrared transmitter/receivers

User: Bilby

5

I/O Devices

Internal devices are connected directly to

Communications connections usually pass through some port:– Ethernet– USB– Firewire (IEEE 1394) and thunderbolt– VGA and DVI– SCSI– Parallel, PS/2, Serial and TRS

I/O Devices

6

I/O Devices

More and more common these days is to see devices with a network connection (wireless or landline)– Significantly greater costs– Significantly greater flexibility

It’s useful that we’re moving from IPv4 to IPv6 soon– 232 IP addresses aren’t enough– IPv6 allows for 2128 IP addresses

• Again, the cost of quadrupling the IP address size is insignificant to the increase in hardware

I/O Devices

7

I/O Devices

Each device has a controller that connects it to the CPU via the system bus– Each controller provides the standard interface

Most data buses today are either 32-bit or 64-bit– This means four or eight bytes can be loaded or read at once– The size of the data bus describes the length of a word

I/O Devices

8

Device Controllers

Applications communicate through a common interface presented by the device driver– The driver communicates with a device controller– The controller is doubly hardware dependent

I/O Devices

9

Device Controllers

Most device controllers will have hardware implementations for basic functionality– The controller must interface with the interface between devices– Examples: PSI Express

I/O Devices

10

Device Controllers

Each controller will have registers including:– A command register

• What the device should do next

– Data registers• Information passed

to or copied fromthe device

– Status register• The current state

of the device

I/O Devices

11

Device Controllers

Often, two flags within the command register are often busy and done – When both are 0 (false), data can be copied to the registers and

a command can besent to the device

– A new command setsthe busy flag to 1: thedevice now eitherreads from or writesto the data registers

– When the commandis complete, the doneflag is set to 1 and the busy flagis reset to 0

– If both are 1, the state is intermediate

I/O Devices

12

Device Controllers

If an unrecoverable error occurs during the command, an error code will be copied to a different part of the status register

I/O Devices

13

Device Controllers

Another common feature is a buffer– The CPU can continue reading data from an internal buffer while

the controller is busy with the next command

I/O Devices

14

Direct Memory Access

Another common issue is copying data between a device and main memory– By default, all data must be copied through the CPU– This can be exceptionally expensive

I/O Devices

15

Direct Memory Access

Direct Memory Access (DMA) allows for direct transfer between a device and main memory– The device driver provides an address in main memory– The DMA controller facilitates the copy data to or from starting at

this memory location– The CPU can now work in parallel so long as the system bus is

not required

I/O Devices

16

Memory-Mapped I/O

Instructions for communicating with devices could be through CPU instructions– Copy status register to the CPU data register

test CPU_register, device_address

– Send commands to the command register input device_address output device_address

– Copy values between data registers copy_in CPU_register, device_address,

controller_register copy_out CPU_register, device_address,

controller_register

I/O Devices

17

Memory-Mapped I/O

An alternate model is to explicitly associate specific memory locations with each device– Each register is associated with a memory location– Writes to those memory locations copies to the data

to the corresponding– Changes to registers are copied to main memory

Benefits:– This allows a smaller instruction set– Address registers can address device registers

I/O Devices

18

Memory-Mapped I/O

Consider the original MS DOS

– It dedicated the top 384 KiB of memory available to an 8088 tosystem memory including memory-mapped I/O

– In this case, the mapping is permanent

I/O Devices

19

Summary

This topic looks at input and output devices– I/O Devices

• Input, output, storage, communication

– Device controllers– Direct Memory Access– Memory-mapped I/O

I/O Devices