1. §4.4 Input and Output Devices
Douglas Wilhelm Harder, M.Math. LEL
Department of Electrical and Computer Engineering
University of Waterloo
Waterloo, Ontario, Canada
ece.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

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

4. I/O Devices Communication serial and parallel ports
network cards (including wireless)
infrared transmitter/receivers
Input
keyboard
mouse
touch-sensitive screen
microphone
Output
terminal
graphical
speaker
printer
User: Bilby
Storage
hard drives
USB drives
CD ROM

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

6. I/O Devices
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

7. I/O Devices
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

8. I/O Devices
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

9. I/O Devices
Device Controllers
Most device controllers will have hardware implementations for basic functionality
The controller must interface with the interface between devices
Examples: PSI Express

10. Device Controllers Each controller will have registers including:
I/O Devices
Device Controllers
Each controller will have registers including:
A command register
What the device should do next
Data registers
Information passed to or copied from the device
Status register
The current state of the device

11. I/O Devices
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 be sent to the device
A new command sets the busy flag to 1: the device now either reads from or writes to the data registers
When the command is complete, the done flag is set to 1 and the busy flag is reset to 0
If both are 1, the state is intermediate

12. I/O Devices
Device Controllers
If an unrecoverable error occurs during the command, an error code will be copied to a different part of the status register

13. Device Controllers Another common feature is a buffer
I/O Devices
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

14. I/O Devices
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

15. I/O Devices
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

16. I/O Devices
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

17. I/O Devices
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

18. Memory-Mapped I/O Consider the original MS DOS
I/O Devices
Memory-Mapped I/O
Consider the original MS DOS
It dedicated the top 384 KiB of memory available to an 8088 to system memory including memory-mapped I/O
In this case, the mapping is permanent

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