Subject: RE: [stella] 6502 tutorial From: "Andrew Davie" <adavie@xxxxxxxxxxxxxxxxx> Date: Wed, 23 Dec 1998 08:29:05 +1100 |
The sample code in that tutorial has a 16-bit count down loop, copied below. ; ; A 16-bit count down loop ; start LDY #$FF ; load Y with $FF loop1 LDX #$FF ; load X with $FF loop2 DEX ; X = X - 1 BNE loop2 ; if X not zero goto loop2 DEY ; Y = Y - 1 BNE loop1 ; if Y not zero goto loop1 RTS ; return There are two loops here, X will be set to 255 and count to zero for each time Y is decremented. The net result is to count the 16-bit number Y (high) and X (low) down from $FFFF = 65535 to zero. In both cases, the #$FF should be changed to #0 to ACTUALLY achieve a 65535 loop count. As it is, it will only iterate 255*255 = 65025 times. Pedantically, A --???????? _????????????? Andrew Davie ??? _? *? (_|?? _???????? adavie@xxxxxxxxxxxxxxxxx <mailto:adavie@xxxxxxxxxxxxxxxxx> ?? (/_??? _? , | )??????? ICQ 3297382 (boofly)?? ?,???? ,~' L_|\??? _| (?? ,-'??????? \? (_|???? http://www.comcen.com.au/~adavie? + ?? (???? GMT+10 \?? _????? /slide? = Slide Rule Trading Post \/? \??? __??? @/? (?????? /weird? = Weird Computing Machines ? _? L,~'? "\__/? _??????? /slide/calculator/soviet.html = ?(_|????????? v? (/_???????? Museum of Soviet Calculators ????? _|?????????????????? /javaslide/javaslide.html = ???? (_| *? \/\/???????????? Interactive Java Slide Rule > -----Original Message----- > From: owner-stella@xxxxxxxxxxx [mailto:owner-stella@xxxxxxxxxxx]On > Behalf Of John Saeger > Sent: Wednesday, December 23, 1998 4:19 AM > To: stella@xxxxxxxxxxx > Subject: Re: [stella] 6502 tutorial > > > > -----Original Message----- > From: Tennessee Carmel-Veilleux <veilleux@xxxxxxxxx> > > > > I once downloaded a 6502 assembly tutorial that was located at the > >'Incridible 6502 page'. It was an excellent tutroial called "Assembly > >language in one step" if I remember. However, I know someone who > would like > >to learn 6502 assembly but the site where this tutorial was is down and I > >lost the digital copy. All that remains is the hardcopy i made > but I don't > >want to xerox and send 55 pages halfway around the world :) Would anyone > know > >of a good 6502 assembly tutorial I could suggest to him, or even better, > >would anyone still have that "Assembly in one step" tutorial ? > > I just happen to have a copy :-) > > John > > -- Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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