2015/10/14 10/8 OS作業

10/8 OS作業
1.(I)Caches are useful when two or more components need to ex-change data, and the components perform transfers at differing speeds.
(II)Caches solve the transfer problem by providing a buffer of intermediate speed between the components.
(III)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.
(IV)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)
(b) the cache is affordable, because faster storage tends to be more expensive.

2.(I) 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 polling. A trap can be used to call operating system routines or to catch arithmetic errors.
(II)Interrupt:Hardware generated interrupt ; Trap:Software generated interrupt
(IV)User programs create traps for debugging purposes. A trap can be used to call the OS routines or to catch arithmetic errors.

3.(I)CPU has to implement 4 basic functions during instruction cycle: fetch, decode, execute, store and device controller stops and starts the activity of the peripheral device.CPU is the brain of computer,device controller receives other equipment signal
(II)interprets commands before executing them.

4.It stops what it is doing and immediately transfers execution to a fixed location.The fixed location contains the starting address where the service routine executes
then resume the interrupted computation.