6502 illegal opcodes questions
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From: Anders Carlsson on 25 Apr 2006 04:11 Linards Ticmanis <ticmanis(a)gmx.de> writes: > what is "the high byte of the target address of the operand" > in the indirect-y case (93)? I'd suppose it means what it says. If $FB=33, $FC=$71, .Y=0, the result probably is the same as in the absolute case, only it takes one more cycle to perform: AXA ($FB),Y -- Anders Carlsson
From: sicklittlemonkey on 25 Apr 2006 04:13 It might have been naivety for some, but also perhaps the necessity for obfuscation. I'm trying to get hold of original titles that used such opcodes to see whether there was really a need or advantage. Cheers, Nick.
From: Michael J. Mahon on 25 Apr 2006 04:35 sicklittlemonkey wrote: > It might have been naivety for some, but also perhaps the necessity for > obfuscation. I'm trying to get hold of original titles that used such > opcodes to see whether there was really a need or advantage. There are infinitely many ways to make a program obscure without making it hardware accident-dependent. ;-) -michael Music synthesis for 8-bit Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it is seriously underused."
From: Linards Ticmanis on 25 Apr 2006 05:48 Thanks for all your helpful replies, which are going to find their way into an AppleWin improvement soon, if all goes well. I'm still not quite happy with some ugly details that I haven't seen documented too well: 1.) Operation of ARR ($6B) when the decimal flag is SET. 2.) Exact operation of decimal mode ADC and SBC in both the 6502 and the 65C02. I'll take a look at the VICE code first. Do you know of any other GOOD code for this? Best wishes, -- Linards Ticmanis
From: tjentzsch on 25 Apr 2006 06:38
Actually there are some nice and reliable(!) opcodes which save space and CPU time. Especially LAX, DCP, ISC and the various NOPs are used by many homebrew programmers for the Atari 2600 console. Just stay away from those which are marked as unreliable (e.g. see http://oxyron.de/html/opcodes02.html) |