Buffers And Spoolers
Higher Computing Systems - Peripherals Infosheet 3.18
Produced by S Lambert, R Simpson and HSDU for City of Edinburgh Council, 2004
In general a buffer or spooler will be used when a fast acting part of a system is exchanging data with a
slow acting device. Then buffer or spooler is used to store data until the data can be dealt with. This
ensures that the processor is not held up by slow communications to and from peripherals.
Buffers
Buffers are a piece of memory used to store information temporarily.
Most interfaces have buffers to store blocks of data while in transfer
between the processor and the peripheral. The buffer must also keep the
status of the peripheral so that the processor knows when it is able to
send or receive data and when problems arise.
Example
Printing might involve (if the file is too big for the buffer to handle in one go)
• the processor checking with the interface to see if the printer is ready
• the interface sending back an OK
• the processor sending data to the interface
• the interface saying its buffer is full
• the interface sending data to the printer
• the interface saying to the processor that it is ready to accept more data.
These steps will be repeated until all the data is sent.
Peripheral Buffers
Most peripherals have buffers of their own (device based) and some interfaces and
peripherals often have their own processors to control their operation. Some laser printers
have a better processor and more memory than the computer its connected to.
Spoolers
Another approach to ensuring the processor is not held up by slow
peripherals is to send data to be spooled. This means that the data is sent
to fast backing storage usually a disc. This is preferred to buffering when
large amounts of data is to be sent or if the peripheral is shared across a
network. In a spooled system, data is sent to backing store and fed to the
peripheral in the gaps between processing actions (an example of this is
background printing). In this way, it is very often possible for the
peripheral to operate continually during a program and for the action of the
computer also to appear continuous.
Memory-Mapped I/O
Memory-mapped I/O is when each interface is treated by the processor as one or more memory locations
and the data transfers are accomplished by normal read and write operations. This means that there is no
need for special input and output instructions.
Isolated I/O
Isolated I/O is when special instructions have to be known to the processor to enable communication
between the processor and the peripheral. Although these are easily distinguishable from normal memory
instructions, they reduce the number of other types of instructions within the processor instruction set
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