Question

b. The program code below involves IO. The first line of an interrupt (IRQ) service routine is also shown. Describe the purpose of each line of code and insert suitable comments EQU $1003 EQU $1007 EQU $4FFF IRQVECT EQU SFFF2 PORTC DDRC STACK ORG $1000 START SEI #STACK LDS LDD #INTSERV STD IRQVECT #S0F LDAA STAA DDRC PORTC STAA CLI More code ORG $2000 INTSERV LDAB PORTC [8 marks

Answer 1

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
       ...