On Sat, 6 Oct 2001, Glenn Saunders wrote:
>
> In this build a lot of the stuff is laid out in memory, the sprites et. al.
>
> I actually have a car showing up, but I don't like the routines that handle
> the logic. Too much RAM and cycles.
>
> I tried to use indirect addressing to set up pointers to the start of car
> sprite ROM in order to handle the rotations but I don't think I understand
> how to do this properly. I had to go back to hardcoding it to one car shape.
>
> Please assist :)
>
You're trying to use LDA (addr,X)... you should be using LDA (addr),Y
The difference is, LDA (addr,X) adds X to addr, then uses that value as
a pointer. LDA (addr),y uses addr for a pointer, and adds Y to that
pointer, after dereferencing it.
Say I have this in memory:
Location Value
addr f0
addr+1 a1
addr+2 f2
addr+3 a3
Now if I do this:
ldx #1
lda (addr,x)
...then A ends up containing whatever was in $F2A1. (addr,x) is called
`Pre-indexed indirect' mode, meaning X gets added to the address *before*
it gets used as a pointer.
whereas if I do this:
ldy #1
lda (addr),y
...then A gets whatever was in $A1F1. (addr),y is `Post-Indexed indirect'
mode, meaning Y gets added to the effective address pointed to by the
pointer (addr), after is has been calculated.
There are no (addr),x or (addr,y) modes, either... X is always pre-indexed
and Y is always post-indexed.
Swap the roles of the X and Y registers in the section of
code where you want to use indirect addressing, and use (addr),y and
your problem should go away...
B.
---
If a trainstation is the place where trains stop, what is a workstation?
-
Archives (includes files) at http://www.biglist.com/lists/stella/archives/
Unsub & more at http://www.biglist.com/lists/stella/
