1. Input/Output (I/O) Important OS function – control I/O
Should present device independent
interface.
Interested in programming aspects,
not how it works internally.
Block devices – info is in fixed size
blocks (i.e. 512 bytes to bytes)
each block has an address.
Blocks can be read and written
independently.
Disks are an example.

2. Input/Output (I/O) Character devices – works with streams
of chars, not blocks.
Solaris disk example:
/dev/dsk/ disk devices are block while
/dev/rdsk/ disk devices are character.

3. Input/Output (I/O) I/O controllers have some control
registers the OS can write into.
- to make a device perform some
operation.
- can also read to get status
Many devices have a data buffer
(i.e. Video RAM)

4. Input/Output (I/O) One way to write or read device port
is to have instructions r/w a control
register – separate I/O and memory
Another way is to have memory-mapped
I/O where the control registers are
assigned an address space.
Could also have a hybrid approach.

5. Direct Memory Access (DMA)
Having the cpu request data from an
I/O controller is a waste of its time.
A DMA controller has access to the
system bus independent of the cpu.
Cycle stealing – DMA competes with cpu
for bus and will delay the cpu.

6. I/O methods Programmed I/O – uses polling or
“busy waiting”. Checks the status of
device in a loop.
Wastes the cpu.
OK in an embedded system where the
cpu has nothing else to do.
Interrupt-driven I/O – Allow cpu to
context switch to another job and
receive an interrupt when I/O is done.

7. I/O methods I/O with DMA – cpu not involved
DMA controller takes care of things
allowing cpu to do other things
The DMA controller can write directly
to memory w/o the cpu.

8. More I/O Device drivers handle much of the I/O.
Need a driver for each type of device.
- present a device independent view.
Modules can be loaded dynamically
rather than compiled into kernel.
Block and character devices
major device number locates driver
minor device number is parameter to
driver for with unit.
/dev files protected with permission bits.

9. Buffering More efficient to buffer data than to
interrupt after each character.
User buffers and kernel buffers.
When kernel buffer fills copy to user buffer
Double buffering – have two kernel buffers
start filling second while copying full
buffer to user space.