align with offset in dasm

[Kylearan]

How can you specify "align" with an offset in dasm?

 

Take a skipDraw routine for example, where you have to make sure the graphics data starts at least n bytes after a page boundary. How can you do this in dasm without resorting to a hard-coded "org" address?

 

[+SpiceWare]

I suspect your starting point will have a min and max number of bytes it could be from from the start of a page (max because you want to avoid a page boundary crossing). If so I'd use a couple IF statements, something like this (untested):

MIN_OFFSET = 20
MAX_OFFSET = 256 - SIZE_OF_DATA

...


; make sure sprite data starts at correct location within page
  IF ( (* & $FF) > MAX_OFFSET)
    align 256
  ENDIF
 
  IF ( (* & $FF) < MIN_OFFSET)
    ds (MIN_OFFSET - (* & $FF))
  ENDIF

; sprite data
FirstImage:
  .byte %11111111
  ...
LastImage:
  .byte %11111111
  ...
SIZE_OF_DATA = * - FirstImage

---

 

The * is the current address, so (* & $FF) returns the current offset from the start of the current page. The order of the IF statements is important.

[Kylearan]

That looks like a very nice solution, thanks a lot! I think I'll put in a safeguard for SIZE_OF_DATA so that it throws an error if it gets too big and then make a macro out of it, and will be reconciled with dasm once again.