FIT2100 Lecture Notes - Lecture 10: Virtual Memory, Multiple Buffering, Cache Replacement Policies
31 May 2018
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I/O Management and Disk Scheduling (Lecture 10)
I/O: Categories
-External devices engage in I/O with computer systems can be grouped into three categories
-Human Readable: suitable for communicating with the computer users (e.g. printers,
terminals, video display, keyboards, mouse)
-Machine Readable: suitable for communicating with electronic equipment (e.g. disk drivers,
USB keys, sensors, controllers, actuators)
-Communication: suitable for communication with remote devices (e.g. modems, digital line
driver)
Describe the organisation of the I/O function
I/O Function: Evolution
1. Processor directly controls a peripheral device
2. A controller or I/O module is added
3. Same configuration as step 2, but now interrupts are employed
4. The I/O module is given direct control fo memory via DMA
5. The I/O module is enhanced to become a separate processor, with a specialised instruction
set tailored for I/O
6. The I/O module has a local memory of its own and is, in fact, a computer in its own right
Discuss various techniques used for performing I/O and I/O buffering
I/O: Techniques
-Programmed I/O
-The processor issues and I/O command on behalf of a process to an I/O module — that
process then busy waits for the I/O operation to be completed before proceeding
-Basic operations of Programmed I/O:
-Processor requests and I/O operation
-I/O module performs the operation
-I/O module sets status bits
-Processor check status bits periodically
-I/O module does not inform or interrupt processor directly
-Processor may wait or come back later to check
-Interrupt-driven I/O
-The processor issues and I/O command on behalf of a process
-Non-blocking — the processor continues to execute instructions from the process that issued
the I/O command
-Blocking — the next instruction the processor executes is form the OS, which will put the
current process in a blocked state and schedule another process
-Basic operations of interrupt-driven I/O:
-Processor requests a read operation
-I/O modules gets data fro the I/O device while processor does other work
-I/O module interrupts processor when data is ready for transfer
-Processor requests the data from I/O module
-I/O module transfers the data to processor
-Direct Memory Access (DMA)
-DMA module controls the exchange of data between main memory and an I/O module (over
the system bus)
-More efficient than interrupt-driven or programmed I/O
-Basic operations of DMA:
-Processor sends DMA a request for the transfer of a block fo data
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