Subject: Re: [stella] 6502 question|
From: Nick S Bensema <nickb@xxxxxxxxxxxx>
Date: Fri, 4 Apr 1997 07:18:17 -0700 (MST)
> >Question for you 6502 gurus (both of you =) ). >Suppose I have a number between $90 and $9F stored in $80. What's the >fastest way to get the number at the address in $80 into the accumulator? >(that is, if the value in location $80 equals $94, I want the accumulator >to have the value in $94.) > >It looks like I need to zero Y and then do LDA ($80),Y which is indirect >indexed for 5 cycles plus clearing Y, or zero X and then LDA ($80,X) which >is indexed indirect for 6 cycles plus clearing X. These would work on the >zero page, right? Would the "zero page-ness" decrease the number of cycles >as it does for direct addressing? It gets worse. $81 must read 0 so you don't inadvertently read the RIOT or ROM. Here's an idea: LDX $80 LDA $00,X You can also LDY $00,X this way too. Or LDX $00,Y if I'm not mistaken. Hope this helps. -- Archives available at http://www.biglist.com/lists/stella/archives/ E-mail UNSUBSCRIBE in the body to stella-request@xxxxxxxxxxx to be removed.