Transcript I/O Managment

Computer Studies (AL)
I/O Management
Reference
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Silberschatz, Galvin, Gagne “Operating
System Concepts 6th edition”, 2003, Wiley
Stallings, “Operating Systems Internals and
Design Principles”, 2001, Prentice Hall
Content
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Overview
Roles of device driver
Spooling
Remark
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Polling, Interrupt, DMA, have already
discussed in previous chapter. Please refer
to the notes of interrupt handling.
Role of OS in I/O
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The role of the OS in computer I/O is to
manage and control I/O operations and I/O
devices
Development direction of I/O-devices
technology
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Standardization of software and hardware
interface
Variety of I/O devices
Device driver
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The basic I/O hardware elements, such as ports,
buses and device controllers accommodate a wide
variety of I/O devices.
To encapsulate the details and oddities of
different devices, the kernal of an OS is
structured to use device-driver modules
The device drivers present a uniform deviceaccess interface to the I/O subsystem calls
provide a standard interface between the
application and the operating system
E.g. Remember I/O control layer?
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In layered file system, we have discussed I/O
control layer.
It consists of device drivers and interrupt handlers
to transfer information between the main memory
and the disk system.
The device driver can be thought of as a translator.
Its input consists of low-level commands. Its
output consists of low-level hardware-specific
instructions that are used by the hardware
controller, which interfaces the I/O device to the
rest of the system.
Keyword
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Device communicates through port
A bus is a set of wires used to connect
devices in a computer system
A controller is a collection of electronics
that can operate a port, a bus or a device
Buffering
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A buffer is a memory area that stores data
while they are transferred between two
devices or between a device and an
application
Why buffer?
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1. Cope with a speed mismatch between the producer and consumer
of a data stream
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E.g. buffer is created in main memory to accumulate the bytes
received from the modem
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Since the disk write is not instantaneous and the modem still
needs a place to store additional incoming data two buffers are
used.
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After the modem fills the first buffer, the disk write is requested.
The modem then starts to fill the second buffer while the first
buffer is written to disk.
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By the time the modem has filled the second buffer, the disk
write from the first one should have completed, so the modem
can switch back to the first buffer while the disk writes the
second one.(double buffering)
Why buffer?
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2. To adapt between devices that have different data-transfer sizes
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E.g. Networking: buffers are used widely for fragmentation and
reassembly of messages.
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Large message: fragmented to small packets.
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Receive side: use buffer place them
Why buffers?
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3. Support copy semantics for application I/O.
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E.g. Suppose that an application has a buffer of data
that it wishes to write to disk (use write() system call)
On that time, if the application changes the content of
buffer, an error may occur! (since the change occurs
AFTER the system call)
With copy semantics, OS guarantee the copy
semantics is for the write() system call. That is, the
disk write is performed from the kernal buffer, so that
subsequent changes to the application buffer have no
effect
Caching
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A cache is a region of fast memory that holds
copies of data.
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E.g. the instructions of the currently running process
are stored on disk, cached in physical memory, and
copied again in the CPU’s secondary and primary
caches.
The difference between a buffer and a cache is
that a buffer may hold the only existing copy of a
data item, whereas a cache, by definition, just
holds a copy on faster storage of an item that
resides elsewhere.
Spooling and Device reservation
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A spool is a buffer that holds output for a device,
such as a printer, that cannot accept interleaved
data streams.
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E.g. many print jobs want to use the printer.
Spooling is one way that OS can coordinate
concurrent output.
Each application’s output is spooled to a separate
disk file. When an application finishes printing,
the spooling system queues the corresponding
spool file for output to the printer.
Summary – transform I/O to hardware Operations