Transcript What is a Device Driver?

NETW 3005
I/O Systems
Reading
• For this lecture, you should have read
Chapter 13 (Sections 1-4, 7).
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This Lecture: I/O systems
• Hardware: ports, buses, controllers
• Application I/O interface
• Kernel I/O services.
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Central issues
• How are I/O commands actually
implemented?
• How does the O/S manage the wide
range of I/O devices that are available?
– Disks, CD/DVD drives, tapes …
– Modems, network cards …
– Screens, keyboards, mice, joysticks …
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What is a Device Driver?
Each device has its own set of specialized commands
that only its driver knows.
A device driver is a system program that converts the system
calls of the operating system into device specific commands .
printers, video adapters, network cards, sound cards
A device that controls the transfer of data from a computer to
a peripheral device and vice versa.
Example
Graphics controller
SCSI ( Small Computer system Interface) Controller–
IDE ( Integrated Drive Electronics) Controller-
I/O Hardware – some background
• An I/O device is linked to a machine via
a port.
• The link is normally a set of wires called
a bus.
• At the end of the link, there’s a device
controller, (basically, a processor.)
• A signal on a bus is basically a temporal
sequence of voltages for each wire.
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I/O Hardware
bus
device
controller
CPU
I/O
device
voltage
port
time
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CPU
monitor
graphics
controller
IDE disc
controller
disc
disc
cache
memory/
controller
memory
PCI bus
expansion
bus interface
SCSI bus
The PCI bus
disc
disc
disc
keyboard
disc
disc
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parallel port
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serial port
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Device controller registers
• How does the CPU give commands to a
device via a bus?
– A device controller has a number of
registers for holding signals.
– The CPU can effectively write to and read
from these.
– Some registers hold data; some hold
control signals.
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Device controller registers
• How can the CPU specify which device
it’s giving commands to?
– Each different port in the system has an
address range.
– The CPU can issue I/O instructions to
particular addresses.
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Device I/O Port Locations on PCs
(partial)
Some example controller registers
• A controller might have the following
registers:
– data-out: contains a byte of data.
– status: contains a busy bit (indicating the
controller’s state).
– command: contains a command-ready bit
(indicating CPU has sent some data) and a
write bit (indicating direction of data flow).
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Programmed I/O and handshaking
For each byte to be transferred to device:
1. The CPU reads the busy bit until it is clear.
2. The CPU sets the write bit and writes a byte to
the data-out register.
3. The CPU sets the command-ready bit
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Programmed I/O and handshaking
4. The controller reads the command-ready
bit until it is set. Then it sets the busy bit.
5. The controller reads the write command
from the command register and then
copies the data-out register to the device.
6. The controller clears the command-ready
bit and then the busy bit.
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Polling and its Problems
• The step where the CPU cycles waiting
for the busy bit to be clear is known as
polling or busy waiting.
• If the CPU is polling, then its wastes
CPU time.
• An alternative to polling is interrupts.
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Interrupts
• The CPU hardware contains a wire called the
interrupt request line.
• The CPU tests this line after each instruction
it executes.
• If a signal is detected, the CPU saves its
current state and transfers control to interrupt
handler .
• Interrupt handler process the interrupt and
transfers the control to the CPU.
• If devices uses interrupt to communicate,
then CPU doesn't need to poll.
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Types of interrupt
• Errors (exceptions)
– divide by zero, invalid memory access, …
• Device-generated interrupts
– device has read a byte; modem buffer full.
• Software interrupts (traps)
– system call, context switch
• Some of these are more urgent than others.
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Interrupt priority levels
• One way of dealing with differing degrees of
urgency is to provide two interrupt request
lines:
– maskable interrupts – device-generated
interrupts and traps. These can be disabled.
– nonmaskable interrupts: for signalling error
messages. Never disabled.
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Direct Memory Access
• Programmed I/O (sending data byte-bybyte to a device) is laborious.
• For large chunks of data, direct
memory access (DMA) is used.
• A DMA controller is a kind of processor
that transfers data between I/O devices
and memory without involving main
processor (CPU)
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Direct Memory Access
•
1.
2.
3.
4.
DMA operation needs the
channel number (which I/O device requires the DMA),
a beginning address in memory for the transfer,
the number of bytes to transfer, and
the direction of transfer (I/O to memory, or vice-versa)
•
The processor starts the DMA activity with an ordinary
I/O write to a control register in the DMA controller, then
continues its work as usual
•
When DMA activity is finished, DMA controller interrupts
the processor
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What is a Device Driver?
Each device has its own set of specialized commands
that only its driver knows.
A device driver is a system program that converts the system
calls of the operating system into device specific commands .
printers, video adapters, network cards, sound cards
A device that controls the transfer of data from a computer to
a peripheral device and vice versa.
Example
Graphics controller
SCSI ( Small Computer system Interface) Controller–
IDE ( Integrated Drive Electronics) Controller-
The kernel-device interface
• How does the O/S manage the huge
range of possible I/O devices?
• Clearly, we don’t want to rewrite the
kernel every time we add a new device.
• We therefore need to impose a
standard interface protocol on devices.
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The kernel-device interface
• The method for doing this is to
encapsulate device-specific information
in special kernel modules called device
drivers.
• These device drivers translate system
calls into device-specific commands.
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Device drivers and controllers
• There’s a one-to-one mapping between
device drivers and device controllers.
– Device drivers are software.
– Device controllers are hardware.
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Types of I/O device
• I/O devices vary widely, along several
dimensions.
– Character stream versus block.
– Sequential versus random access.
– Synchronous versus asynchronous.
– Shareable versus dedicated.
• OSs work with a small set of device types
which can execute a standard set of
commands.
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Character devices
character devices relate to devices through
which the system transmits data one character
at a time. (Get and Put)
Example
Key boards
serial modem
Block devices
• Block devices correspond to devices through
which the system moves data in the form of
blocks.
• Example : addressable devices
hard disks, CD-ROM drives, or memory-regions.
• The standard commands necessary for a block
device are:
– Read block,
– Write block,
– Seek block (for random-access block devices).
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Network devices, e.g. sockets
A socket is one endpoint of a two-way communication link
between two programs running on the network.
• The standard commands for a socket:
– Create socket
– Connect local socket to remote socket
– Listen for remote applications to local socket
– Send information to a socket
– Receive information from a socket
• Select – monitors a set of sockets.
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Ip
address
+port
Of
server
00000111110000
0000001111111
Blocking and nonblocking I/O
• There’s an important distinction between
blocking and nonblocking I/O system calls.
– If a process issues a blocking I/O system call, it
waits until the I/O is completed.
– If a process issues a nonblocking I/O call, it
waits for a fixed interval, and then returns.
– If a process issues an asynchronous I/O call,
the I/O operation occurs in full, but the process
doesn’t wait for it.
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Differences?
• In nonblocking I/O, you know how long
the I/O is going to take, but you don’t
know if all the data will be transferred.
• In asynchronous I/O, you know all the
data will be transferred but you don’t
know how long it will take, so you don’t
know where you’ll be in your program
when it comes back.
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Examples?
• Blocking I/O
– disk reads. (The standard situation.)
• Non-blocking I/O
– reads from a mouse, joystick, real-time video.
• Asynchronous I/O
– printing something.
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Kernel I/O services
• So far, we have demonstrated the use
of device drivers for the kernel.
• But we haven’t really explained why
there should be an I/O subsystem.
• Why not just interface straight to the
device drivers?
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Kernel I/O services
• In fact, there are several services
performed by the I/O subsystem before
the device drivers are called.
– I/O Scheduling,
– I/O Buffering,
– Spooling.
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I/O scheduling
• At any one time, there might be several
requests for I/O waiting to be serviced.
• We could just do these in first-comefirst-served order.
• Disadvantages with this scheme?
– Let’s say the disk arm is currently near the
beginning of a disk, and there are 5 I/O
requests: 3 for places near the beginning
and 2 for places near the end.
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I/O buffering
• A buffer is used to provide communication
between two processes with different
speeds.
• In the consumer/producer problem, assume
that the consumer process is faster than the
producer process.
– Producer fills up the buffer, switches to a
second buffer and signals the consumer;
– Consumer reads full buffer, then waits.
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Spooling
• A spool is a buffer that holds output for a
device that cannot accept interleaved data
streams - e.g. a printer.
• Rather than sending data straight to the
printer, the O/S stores the data in buffers.
• The printer can then process the output
from one buffer at a time.
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