Assembly code for TOF Interrupt

;Program to flash LEDs connected to PORTB every time TCNT overflows

PORTB  =  0x1004
TFLG2  =  0x1025
TMSK2  =  0x1024

         .area CODE (ABS)
         .org   0x00D0
         jmp    tof_isr   ;jump to ISR on TOF int

         .org   0x0100
         lds    #0x01ff   ;load stack pointer
         ldaa   #0x55     ;initialize LEDs
         staa   PORTB
         ldaa   #0x80     ;clear TOF flag
         staa   TFLG2    
         ldaa   #0x80     ;enable TOF int
         staa   TMSK2
         cli              ;enable ints
here:    bra    here      ;do nothing

tof_isr: com    PORTB     ;flash LEDs
         ldaa   #0x80     ;clear TOF flag
         staa   TFLG2
         rti              ;return to main prog

C code for TOF Interrupt

/* Program to flash LEDs connected to PORTB every time TCNT overflows */

#include <hc11.h>

void tof_isr(void);

main()   /* C automatically loads stack pointer */
{
    PORTB = 0x55;           /* Initialize LEDs */

    TOF_JMP = JMP_OP_CODE;  /* jmp tof_isr */
    TOF_VEC = tof_isr;

    TFLG2 = 0x80;           /* Clear TOF flag */
    TMSK2 = TMSK2 | 0x80;   /* Enable TOF int */

    enable();               /* Enable ints */

    while(1) {}             /* Do nothing */
}

/* Exit tof_isr with an RTI instruction */

#pragma interrupt_handler tof_isr

void tof_isr(void)
{
    PORTB = ~PORTB;
    TFLG2 = 0x80;
}