Q$4.(6$pts$)$ Timer Interrupts The following Thumr$-2$ code shows an interrupt handler upon receiving a SysTick interrupt. You don't have

to change it at all. Basically this handler invokes the timer$\_$update$()$ routine. Your task is to implement

timer$\_$update$(($

The timer$\_$update$()$ function uses the following four parameters.

The timer$\_$update$()$ function reads the value of the SECOND$\_$LEFT address, decrements the value by $1$

$($second$),$ checks the value, branches to $\_$timer$\_$update$\_$done if the value hasn't reached $0,$ otherwise it needs

to stop the timer and to invoke a user function whose address is maintained in the USR$\_$HANDLER address.

To stop the timer, write STCTRL$\_$STOP to the address of STCTRL$.($Don$\text{'}$t forget to save back a

decremented value into SECOND$\_$LEFT.$)$

; Code the rest.

$\_$timer$\_$update$\_$done

MON pc$,r$; return to SysTick$\_$Handler

The following C program defines sig$\_$handler$()($lines $2-4)$ that is invoked upon receiving a SysTick interrupt

and that changes alarmed from $1$ to $2.$ The main$()$ function $($lines $6-16)$ initializes alarmed to $1($line $8),$

schedules sig$\_$hanlder$()$ to be invoked upon a SysTick interrupt $($line $9),$ and starts SysTick to count down for

$10$ seconds $($line $10).$ The main$()$ function falls into a while$()$ loop $($lines $11-14),$ jumps sig$\_$handler$()$ upon

receiving a Sys Tick interrupt, and gets out of the while$()$ loop as alarmed eventually becomes $2.$

The following Thumr$-2$ code shows an interrupt handler upon receiving a Sys Tick interrupt. You don't have

to change it at all. Basically this handler invokes the timer$\_$update$()$ routine. Your task is to implement

timer$\_$update$().$

The timer$\_$update$()$ function uses the following four parameters.