$6\mathrm{.}18$ Consider a system consisting of processes $P_1,P_2,$dots,$P_n,$ each of which has

a unique priority number. Write a monitor that allocates three identical

line printers to these processes, using the priority numbers for deciding

the order of allocation.

Answer: Here is the pseudocode:

monitor printers $\{$

int num$\_$avail $=3$;

int waiting$\_$processes $[$MAX$\_$PROCS$]$ ;

int num$\_$waiting;

condition $c$;

void request$\_$printer $($int proc$\_$number$)\{$

if $($num$\_$avail $>0)\{$

num$\_$avail$--$;

return;

$\}$

waiting$\_$processes $[$num$\_$waiting$]=$ proc$\_$number;

num$\_$waiting$++$;

sort $($waiting$\_$processes$)$;

while $($num$\_$avail $==0$ &&

waiting$\_$processes $[0]!=$ proc$\_$number$)$

c$.$wait $()$ ;

waiting$\_$processes $[0]=$

waiting$\_$processes $[$num$\_$waiting$-1]$ ;

num$\_$waiting$--$ ;

sort $($waiting$\_$processes$)$ ;

num$\_$avail$--$;

$\}$

void release$\_$printer$()\{$

num$\_$avail$++$;

c$.$broadcast $()$ ;

$\}$

$\}$