Input/Output Flashcards
(25 cards)
What is an interrupt ?
A hardware signal
What is ISR ?
Interrupt service routine
When is ISR executed ?
In response to an interrupt
Who sends an interrupt to whom ?
I/O device to processor
What is the bus control line dedicated to interrupts called ?
Interrupt - Request line
What does an interrupt-acknowledge signal do ?
Informs the device that the interrupt signal has been received
What is interrupt latency ?
Delay between an interrupt being received and an ISR starts
What increases interrupt latency ?
Saving and restoring registers
What is the full form of DMA ?
Direct Memory Access
What is DMA used for ?
Transfer large blocks of data at high speed
What is the advantage of DMA ?
No interference from processor
What does DMA do ?
Transfer block of data directly
To what does DMA place the block of data in ?
External device and main memory
What controls DMA ?
DMA controller
What information does the program need to tell the DMA controller before calling it ?
- Starting address
- Number of words in block
- Direction of transfer
How does the DMA controller inform the processor that the instruction has been executed ?
It sends an interrupt signal
Can the program that requested DMA transfer continue while DMA executes ?
No, it can’t
Can the processor do any other work ?
Yes, it can execute other programs
In an OS, what kind of programs request I/O operations ?
Application programs
What does the OS do when a DMA operation starts to the application program that requested it ?
It puts the application in blocked state
When does the application program go back to the runnable state ?
After the DMA controller sends an interrupt request indicating it’s end
What is a bus master ?
Device that can initiate data transfers on a bus at any given time
What are the two main approaches to bus arbitration ?
Centralised and distributed
What happens in centralised bus arbitration ?
Single bus arbiter performs arbitration
