Re: [stella] Death Derby, new build

Subject: Re: [stella] Death Derby, new build
From: "B. Watson" <atari@xxxxxxxxxxxxxx>
Date: Sun, 7 Oct 2001 03:43:05 -0400 (EDT)
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...



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