1.Caches are useful w.hen two or more components need to ex-change data, and the components perform transfers at differing speeds. Caches solve the transfer problem by providing a buffer of intermediate speed between the components. If the fast device finds the data it needs in the cache, it need not wait for the slower device. The data in the cache must be kept consistent with the data in the components.
If a component has a data value change, and the datum is also in the cache, the cache must also be updated. This is especially a problem on multiprocessor systems where more than one process may be accessing a datum.
A component may be eliminated by an equal-sized cache, but only if:
(a) the cache and the component have equivalent state-saving capacity (that is, if the component retains its data when electricity is removed, the cache must retain data as well),
and
(b) the cache is affordable, because faster storage tends to be more expensive.
2. An interrupt is a hardware-generated change-of-flow within the system. An interrupt handler is summoned to deal with the cause of the interrupt; control is then returned to the interrupted context and instruction.
A trap is a software-generated interrupt. An interrupt can be used to signal the completion of an I/O to obviate the need for device pooling.
A trap can be used to call operating system routines or to catch arithmetic errors.
3. 1.Difference:
CPU starts an I/O operation, and continuously polls (checks) that device until the I/O operation finishes.
Device controller contains registers for communication with that device.
Input register, output register — for data
Control register — to tell it what to do
Status register — to see what it’s done
2.Similarity:
CPU and device controllers all use a common bus for communication
4.
1.A device asserts the interrupt signal at a hardwired interrupt level.
2.The processor registers the interrupt and waits to finish the current instruction execution.
3.Once the current instruction execution is completed, the processor initiates the interrupt handling by saving the current register contents on the stack.
4.The processor then switches to supervisor mode and initiates an interrupt acknowledge cycle.
5.The interrupting device responds to the interrupt acknowledge cycle with the vector number for the interrupt.
6.Processor uses the vector number obtained above and fetches the vector.
7.The address found at the vector is the address of the interrupt service routine (ISR) for the interrupting device.
8.After the ISR routine has performed its job, the ISR executes the "return from interrupt" instruction.
9.Execution of the "return from interrupt" instruction results in restoring the processor state. The processor is restored back to user mode.
Interrupt Handler
Setps in simple words:
1. Stop the running process.
2. Save the status of the process.
3. OS will search Interrupt vector according to Interrupt ID。
4. Get ISR（Interrupt Service Routine）original address.
5. Execute ISR.
6. After Executing ISR, go back to the status before stopping the process.