> how-to's > how to access a structure

A structure is a collection of variables, a logical block! For example, a high score can be a structure. It contains a name and the number of points, so how does this structure look? We suggest you to use the following type: {structurename}.{variable/membername} equ {structureposition}
For example our highscore would look like this

highscore.pname equ 0 ; Pointer to the name highscore.wscore equ 4 ; Word, Number of points sizeof_highscore equ 6 ; Size of the highscore structure

To calculate the position of your structure, the first index is zero, the next will be the size of the first index, the third [size of first index + size of the second index], ... The last name (sizeof_) should be the size of the structure
Example of a bigger structure

lemming.x_position equ 0 ; X Position, Word lemming.y_position equ 2 ; Y Position, Word lemming.work equ 4 ; Current work, Word lemming.subroutine equ 6 ; Lemming function, Pointer lemming.time equ 10 ; counter, Word sizeof_lemming equ 12

 

>> access of a structure in assembler

Only problem is the access. You should use pointers to a structure (addressregister), because then you can simply add the value of the position to it. It sounds difficult, but please look at the following example:

; Simple access example
lemming.x_position      equ   0         ; X Position, Word
lemming.y_position      equ   2         ; Y Position, Word
lemming.work            equ   4         ; Current work, Word
lemming.subroutine      equ   6         ; Lemming function, Pointer
lemming.time            equ   10        ; counter, Word

sizeof_lemming          equ   12
; ProcessLemming
; Input         a0      pointer 
ProcessLemming:
   move.w  lemming.x_position(a0),d0    ; Load x position
   move.w  lemming.y_position(a0),d1    ; Load y position
   move.l  lemming.subroutine(a0),a1    ; Pointer to the routine
   jsr     (a1)                         ; Call the routine
   rts

_main:
   lea     LemmingStructure(PC),a0
   bsr     ProcessLemming
   rts

; Space for 5 lemmings!
LemmingStructure        ds.b   5*sizeof_lemming