1. Operating Systems Chapter 5: Input/Output Management
Instr: Yusuf Altunel
IKU Department of Computer Engineering

2. Content Principles of I/O hardware Principles of I/O software
I/O Devices
Device Controllers
Direct Memory Access
Interrupts
Principles of I/O software
Programmed I/O
Interrupt Driven I/O
I/O Using DMA

3. Introduction OS Functions The I/0 codes represent
to control all the computer’s I/O (Input/Output) devices
issue commands to the devices
catch interrupts
handle errors
to provide an interface to the devices
device independence:
the interface should be the same for all devices
The I/0 codes represent
a significant fraction of the total operating system

4. I/O Hardware block devices stores information Common block sizes:
in fixed-size blocks,
each one with its own address.
Common block sizes:
range from 512 bytes
to 32,768 bytes.
it is possible
to ‘read or write data blocks
independently of all the other ones.
Disks are the most common block devices

5. I/O Hardware character devices delivers or accepts
a stream of characters,
no block structure is available.
It is not addressable
does not have any seek operation.
Examples:
printers,
network interfaces,
mouse, …

6. Some device, network, and data base rates
Speed
Some device, network, and data base rates

7. Device Controllers I/O devices have components: device controller
mechanical component
electronic component
device controller
the electronic component
can handle multiple devices
Controller's tasks
convert serial bit stream to block of bytes
perform error correction
make data available
to transfer it to main memory

8. Memory-Mapped I/O Devices addressed independent of the memory
Devices are addressed in common address space
A hybrid approach

9. Bus Architecture (a) A single-bus architecture
(b) A dual-bus memory architecture

10. Direct Memory Access CPU must be able DMA (Direct Memory Access)
to address the device controllers
to exchange data with them
DMA (Direct Memory Access)
memory address register
The memory address
to be used in DMA operation
a byte count register
The amount of bytes to transfer
one or more control registers to identify
the I/O port to use
the direction of the transfer
unit: bytes or words
the number of bytes to transfer in one burst

11. DMA Transfer
Operations of DMA Transfer

12. Interrupts When an I/O device finishes its job interrupts can be
it causes an interrupt
by asserting a signal on a hits line
This signal is detected by the interrupt controller
a chip on the main board
interrupts can be
enabled or disabled by the operating system
If no other interrupts are pending
process the interrupt immediately.
If another one is in progress
the device is just ignored for the moment
it continues to assert
until it is serviced by the CPU
Pending: Askıda

13. How an Interrupt Happens
Connections between devices and interrupt controller actually use interrupt lines on the bus rather than dedicated wires

14. Handling Interrupts To handle the interrupt,
the controller puts a number
on the address lines
for the address of the device
The interrupt signal causes
the CPU to stop what it is doing
and start doing something else.
The number on the address lines
is used as an index into a table
called the interrupt vector
to fetch a new program counter.
program counter
points to the interrupt service procedure.

15. I/O Software Device independence Uniform naming Error handling
programs can access any I/O devices
without specifying device in advance
floppy,
hard drive,
CD-ROM
...
Uniform naming
device name must be machine-independent
Error handling
handle erorrs as close to the hardware

16. I/O Software Synchronous vs. asynchronous transfers Buffering
blocked transfers
interrupt-driven transfers
Buffering
data coming from a device
cannot be stored in final destination
Sharable vs. dedicated devices
disks are sharable
tape drives can not be shared

17. I/O Types
Programmed I/O
Interrupt-driven I/O
I/O using DMA

18. Programmed I/O user process to print string “ABCDEFGH” on the printer:
assemble the string
in a buffer in user space
acquire the printer for writing
by making a system call to open it
If the printer is currently in use
fail and return an error code
or will block until the printer is available
make the system call
to print the string on the printer

19. Programmed I/O Operating System copies the buffer checks to see
to an array in kernel space
checks to see
if the printer is available
If not, it waits until it is available
copies the first character
to the printer’s data register
if the printer is ready
to accept another one
when ready,
it prints the next character

20. Steps in Printing a String

21. Interrupt-Driven I/O to allow the CPU In case of system call
do something else
while waiting for device
In case of system call
to print the string is made,
the buffer is copied to kernel space,
as soon as the printer becomes ready
the first character is copied to the printer
cuncerrently., CPU calls the scheduler
to run some other processes
The process printing the string
is blocked
until the entire string has printed.

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

23. I/O Using DMA Interrupts take time A solution is to use DMA
wastes CPU time
A solution is to use DMA
let the DMA controller
feed the characters
to the printer one at time
without bothering the CPU
DMA is programmed I/O
the DMA controller is doing all the work
instead of the main CPU

24. Disadvantages DMA controller is If then much slower than the main CPU.
the DMA controller is
not capable of driving the device at full speed
or the CPU usually has nothing to do
while waiting for the DMA interrupt
then
interrupt-driven I/O
or even programmed I/O
may be better

25. Input/Output Management
End of Chapter 5
Input/Output Management