Attention:

• All 6502 registers have to be saved before and restored after the interrupt (before the RTI instruction).
• an example to save registers and variables in the language "Action!" Interrupts in ACTION
• The interrupt routine must end with an RTI (return from interrupt).

Example for saving and restoring the processor registers#

     ;we need to push to stack, first A, then X, then Y
 PHA ;push ACCU to Stack
 TXA ;transfer X to ACCU
 PHA ;push ACCU (now X) to Stack
 TYA ;transfer Y to ACCU
 PHA ;push ACCU (now Y) to Stack
     ;
     ;... routine goes in here ...
     ;
     ;now we need to pull from stack first Y, then X, then Y
 PLA ;pull Y from STACK to ACCU
 TAY ;transfer ACCU to Y
 PLA ;pull X from STACK to ACCU
 TAX ;transfer ACCU to X
 PLA ;pull ACCU from STACK
 RTI ;end interrupt routine with ReTurn from Interrupt

Example for setting your interrupt routine#

• Set bit 7 in the display list line you want the interrupt to occur
• clear bit 7 of NMIEN to disable the interrupt
• set VDSLST to your routine
• set bit 7 of NMIEN to enable the interrupt

see also: Display List Topics, NMIEN, Display List Instruction table