$,$ Question $2[20$ Marks$]$

$($a$)$ In our kemel, context switching begins when the current running process calls tswitch$(),$ which changes the execution to a new process. Some people refer to this as a co$-$routine call, not a complete context switching. To soothe such critics, we may implement tswitch $()$ as

$```$

$\_$tswitch:

INT $100$

ret

$```$

When the x$86$ CPU executes the INT $100$ instruction, it first saves the CPU's $[$FLAG$,$ CS$,\backslash (\backslash$mathrm$\{$PC$\}]\backslash )$ registers onto the $($current$)$ stack. Then, it loads the contents of the memory locations $[400,402]$ into CPU's $[$PC$,$ CS$],$ causing it to execute from the newly loaded $[$PC$,$ CS$].$ Corresponding to INT $100,$ the instruction IRET pops $3$ items into the CPU's $[$PC$,$ CS$,$ FLAG$]$ registers. If the memory contents of $[400,402]=[$SAVE$,0$x$1000].$ Then, after INT $100,$ the CPU would execute SAVE in our kernel.

Justify whether it is necessary to implement context switching this way,

$($b$)$ Assume that a process P has executed the following function calls:

Draw a diagram showing the stack contents of $\backslash ($ P $\backslash )$ when execution is at the label HERE in function $3().$ Clearly show the stack frames of each function together with locations of FP and SP$.$

Question $3[20$ Marks$]$

Assume: In our kernel, P$1$ is a Casanova process, which hops from segment to segment. Initially, P$1$ runs in the segment $\backslash (0\backslash$times $2000\backslash ).$ By a hop $(\backslash ($ u $16\backslash )$ segment$)$ syscall, it enter kemel to change segment. When it returns to Umode, it returns to an IDENTICAL Umode image but in a different segment, e$.$g$.\backslash (0\backslash$times $4000\backslash ).$

$($a$)$ Devise an algorithm for the hop $()$ syscall.

$($b$)$ When a Casanova process tries to change its Umode segment, the target segment may already be occupied by another process. If so$,$ the original process must be evicted to make room for the Casanova process. Devise a technique that would allow the Casanova process to move into a segment without destroying the original process in that segment.