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b. The program code below involves IO. The first line of an interrupt (IRQ) service routine is also shown. Describe the purpo
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PORTC   EQU $1003 //Assign Address $1003 to label PORTC (I/O port C reg)
DDRC    EQU $1007 //Assign Address $1007 to label DDRC(Data Direction Reg C)
STACK   EQU $4FFF //Assign Address $4FFF to label STACK(Initial Stack Pointer
IRQVECT   EQU $FFF2 //Assign Address $FFF2 to label IRQVECT(Interrupt service routine vector)

ORG    $1000 //main origin

START SEI //Set global interrupt flag

LDS      #STACK   //initialize stack pointer
LDD      #INTSERV //Load D with #INTSERV
STD       IRQVECT //Store D to IRQVECT
LDAA   #$0F //put $0F into A
STAA   DDRC //to make PORTC pin PC7-PC0 as output and other pin as input
STAA   PORTC //Write from Reg A to Port C
CLI //Enable IRQ
....more code

ORG       $2000 //interrupt origin

INTSERV LDAB    PORTC //put PORTC value into Register B
       ...

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