Question

The title is basically it.

I have lights that i need to switch on and off. There is a button that dictates which light it should be. So when the button is pressed or not pressed, i modify a variable that contains the PORT address of that light. To turn a light on i have to store $00 at that address. For instance:

;**********************************************************
;LED1on subroutine
;
;   This will turn LED 1 on, and then return.

LED1on
        LDAA        #$00        ; Load $00 into accumulator a (the value to turn the light on)
        STAA        $PORTA      ; Store the loaded value into PORTA, PORTA is a MACRO that =$0000

        RTS                     ; Return to sender

So what i want to do is have a variable, PoSelect=$0000. And use it instead.

;**********************************************************
;LED1on subroutine
;
;   This will turn LED 1 on, and then return.

LED1on
        LDAA        #$00        ; Load $00 into accumulator a (the value to turn the light on)
        STAA        PoSelect    ; PoSelect is a variable that contains a port address

        RTS

This however just stores the contents of ‘accumulator a’ into the variable PoSelect. What i want to do, is store the contents of ‘accumulator a’ into the address that is stored at the variable PoSelect. Essentially using the variable PoSelect like a pointer.

How do i do that???

Answer 1

Assuming this is HC11, you might want to use indexed addressing, something like:

LDX         PoSelect      ; load address from PoSelect to IX register
STAA        0,X           ; store contents of A register to the address in IX