The following are some events that occur after a device controller issues an interrupt while process $$L$$ is under execution.

$$(P)$$ The processor pushes the process status of $$L$$ onto the control stack.
$$(Q)$$ The processor finishes the execution of the current instruction.
$$(R)$$ The processor executes the interrupt service routine.
$$(S)$$ The processor pops the process status of $$L$$ from the control stack.
$$(T)$$ The processor loads the new PC value based on the interrupt.

Which one of the following is the correct order in which the events above occur?

On a non-pipe-lined sequential processor, a program segment, which is a part of the interrrupt service routine, is given to transfer $$500$$ bytes from an $${\rm I}/O$$ device to memory. Initialize the address register Initialize the count to $$500$$
$$LOOP:$$ Load a byte from device Store in memory at address given by address register $$$\eqalign{ & Increment\,\,\,the\,\,\,address\,\,register \cr & Decrement\,\,\,the\,\,count \cr & If\,\,\,count!\,\,\, = 0\,\,\,go\,\,\,to\,\,\,LOOP \cr} $$$

Assume that each statement in this program is equivalent to a machine instruction which takes one clock cycle to execute if it is non- load/store instruction. The load-store instructions take two clock cycles to execute.

The designer of the system also has an alternate approach of using the $$DMA$$ controller to implement the same transfer. The $$DMA$$ controller requires $$20$$ clock cycles for initialization and other overheads. Each $$DMA$$ transfer cycle takes two clock cycles to transfer one byte of data from the device to the memory.

What is the approximate speed up when the $$DMA$$ controller based design is used in place of the interrupt driven program based input-output?

A device with data transfer rate $$10$$ $$KB/sec$$ is connected to a $$CPU.$$ Data is transferred byte-wise. Let the interrupt overhead be $$4$$ $$\mu \sec $$. The byte transfer time between the device interface register and $$CPU$$ or memory is negligible. What is the minimum performance gain of operating the device under interrupt mode over operating it under program controlIed mode?

Consider the disk drive with the following specifications $$16$$ surfaces, $$512$$ tracks/surface, $$512$$ sectors/track, $$1$$ $$KB/sector$$, rotation speed $$3000$$ $$rpm.$$ The disk is operated in cycle stealing mode whereby whenever one byte word is ready it is sent to memory; similarly, for writing, the disk interface reads a $$4$$ byte word from the memory in each $$DMA$$ cycle. Memory cycle time is $$40$$ $$nsec$$. The maximum percentage of time that the $$CPU$$ gets blocked during $$DMA$$ operation is