Flashcards
SCC 211- Operating Systems -Input and Output
Cards in this deck (16)
- 1How are IO devices visible to the processor in a Memory mapped I/O?They are visible as normal memory locations.
They can be easily accessed using C pointers.
All IO devices are connected to the Memory Bus which in all is also connected to the Processor. - 2How is Memory mapped by devices in the Memory mapped IO system?- Each device will divide information to the computer by presenting a set of memory and that memory will be pressed to the memory space which will visible to the processor.
- Video card that is actually taking data from memory and putting it on a screen
- A programming language can access data on memory - 3What is the problem with too much memory on the memory space of I/O devices?- The problem with this is that , as the memory increases and the speed is fast, the demand on the memory bus will be more in demand. Which can be expensive (Speed).
- 4Define how an isolated I/O works and Advantages, Disadvantages, Limitations of this method?Separate bus for I/O devices
- This bus can operate slower than the memory bus
– Easier interfacing: distinct from high-speed memory bus
– Accessed via special instructions
• Generally not supported by compilers (must use assembler) - 5Why do we use Assemble to access content on the Memory bus?There is no natural way of accessing the IO bus. To access things on this bus, Assembly (registers etc)
- 6Describe the process of a Thread in a Synchronous I/O?In synchronous file I/O, a thread starts an I/O operation and immediately enters a wait state until the I/O request has completed.
- 7Describe the process of a Thread in an Asynchronous I/O?A thread performing asynchronous file I/O sends an I/O request to the kernel by calling an appropriate function. If the request is accepted by the kernel, the calling thread continues processing another job until the kernel signals to the thread that the I/O operation is complete. It then interrupts its current job and processes the data from the I/O operation as necessary.
- 8Disadvantages of Synchronous I/O?Code ‘spins’ on busy flag* awaiting status change
– Very wasteful of processor time
• Very difficult to interleave useful code
– Need automated mechanism - 9Describe in full how an interrupt is discovered in an Interrupt driven/ Asynchronous I/O?- Processor will go through an operation
- We have the processor, with in a device driver, initiate some IO
- IO device does something to retrieve/handle data
- While that is going on, the processor can run some other program
- Once the IO is complete and the data transfer, the wire can be signalled that something has happened
- At that point, the processor would go on to a fixed operation called the interrupt handler
- If you get one of these interrupts, run the interrupt handler
- We schedule the processing of data which has come in from IO, resume operations. - 10Describe in full, how a sub-routine call is handled?○ Processor gets the signal, I have to handle this interrupt
○ Where does it find the address . In memory
□ We take starting address in memory
□ When process gets memory, gets memory location, finds address and that’s where the interrupt is.
.We push the registers on the processor
. Whatever code was running, it will have no idea that the interrupt will be run
□ It will have that state in a registers
□ When we return, those registers will have to be in the state the program will expect them to be in
. You can push all the registers
□ Will have a parameter which can run a C function
□ In assembly
□ Noice
□ Pop it
□ Re enable interrupt
□ Restart main program - 11Problem if devices share the same interrupt line?No idea which generated interrupt
• Back to polling
It could be more than one
It will be expensive
Not efficient
May not want all devices on interrupt line - 12What happens to devices poll after an interrupt?– Must poll each device
for data ready/
state change - 13What is an ISR?ISR is an interrupt service routine which is a callback function in micro controller firmware, an operating system, or a device driver whose execution is triggered by the reception of an interrupt.
- 14Disadvantages with multiple service routines?Handy to have all disks on 1 interrupt line
Slow devices on another
Multiple interrupt lines
Multiple service routine calls - 15Advantages of Multiple Interrupt lines?• Narrows search when subsequently polling
• Groups similar devices - 16Summarize how the processor handles interrupt line?- Depending on which single interrupt line
- The processor will choose which interrupt will go to and it will then call that specific call
- We can keep our interrupt routines separate, or roll them all together
- If we roll them, call(push) all values
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- Flashcards on SCC 211- Operating Systems -Input and Output, created by Ayo Olutobi on 25/01/2017.
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- Ayo Olutobi
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