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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 399
368
INPUT/OUTPUT
CHAP. 5
Instead what is done is to create a special process, called a
daemon
, and a spe-
cial directory, called a
spooling directory
.
To print a file, a process first generates
the entire file to be printed and puts it in the spooling directory.
It is up to the dae-
mon, which is the only process having permission to use the printer’s special file,
to print the files in the directory.
By protecting the special file against direct use by
users, the problem of having someone keeping it open unnecessarily long is elimi-
nated.
Spooling is used not only for printers.
It is also used in other I/O situations.
For example, file transfer over a network often uses a network daemon.
To send a
file somewhere, a user puts it in a network spooling directory.
Later on, the net-
work daemon takes it out and transmits it. One particular use of spooled file trans-
mission is the USENET News system (now part of Google Groups).
This network
consists of millions of machines around the world communicating using the Inter-
net. Thousands of news groups exist on many topics. To post a news message, the
user invokes a news program, which accepts the message to be posted and then
deposits it in a spooling directory for transmission to other machines later. The en-
tire news system runs outside the operating system.
Figure 5-17 summarizes the I/O system, showing all the layers and the princi-
pal functions of each layer. Starting at the bottom, the layers are the hardware, in-
terrupt handlers, device drivers, device-independent software, and finally the user
processes.
I/O
request
Layer
I/O
reply
I/O functions
Make I/O call; format I/O; spooling
Naming, protection, blocking, buffering, allocation
Set up device registers; check status
Wake up driver when I/O completed
Perform I/O operation
User processes
Device-independent
software
Device drivers
Interrupt handlers
Hardware
Figure 5-17.
Layers of the I/O system and the main functions of each layer.
The arrows in Fig. 5-17 show the flow of control. When a user program tries to
read a block from a file, for example, the operating system is invoked to carry out
the call. The device-independent software looks for it, say, in the buffer cache.
If
the needed block is not there, it calls the device driver to issue the request to the
hardware to go get it from the disk. The process is then blocked until the disk oper-
ation has been completed and the data are safely available in the caller’s buffer.
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