Consider a computer that reads data from a network card. The CPU clock frequency is $4$ GHz$.$ The CPU uses a polling strategy to access data from the network card, and, on each access, $64$ words are transferred from the network card to memory through the processor $($one word at a time$).$

$($a$)$ Assuming $80$ CPU cycles of overhead for the polling procedure $($calling the polling routine, accessing the device, and returning$),$ and $4$ CPU cycles for each word transfer, what percentage of CPU time is spent in polling the network card if we want to achieve the maximum card$-$to$-$memory data transfer rate $($i$.$e$.$ with the processor doing nothing other than polling and transferring data$)?$ What is this maximum transfer rate in bytes per second?

$($b$)$ If we want to limit the CPU time spent on polling and transferring data to $0\mathrm{.}30\%,$ how frequently should the CPU be polled $($polls per second$)?$

$($c$)$ For the polling rate obtained in part $($b$),$ what is the rate of data transfer from the network card to the memory $($in bytes per second$).$