6502 illegal opcodes questions
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From: heuser.marcus on 17 May 2006 11:31 > Huh? They did move forward. 6510 -> 8500 -> 8502. And what exactly were the differences between these chips? > And also, I don't think it's any kind of problem for Commodore to use > customized CPUs since they owned MOS, the owners and producers > of 6502 tech back then :) And, yes, MOS made all the funky custom chips - but the 6502-core chips were AFAIK the most advanced CPUs designed/modified/produced by them. bye Marcus
From: Michael J. Mahon on 17 May 2006 17:45 John Selck wrote: > Michael J. Mahon wrote: > >> The fact that both companies used "custom" 6502 processors for their >> machines is no doubt part of the reason that they never moved forward. > > > Huh? They did move forward. 6510 -> 8500 -> 8502. And also, I don't > think it's any kind of problem for Commodore to use customized CPUs > since they owned MOS, the owners and producers of 6502 tech back then :) And these new processors preserved the undefined opcode behavior? If so, they are almost certainly *not* new processor designs. If not, then they are a good illustration of why it's a bad idea to depend on undefined behavior. >> Non-portable code is written for two quite different reasons: >> >> 1) Because the coder doesn't even think about portability or >> doesn't understand it, non-portablility happens. >> 2) Because the coder understands perfectly, and chooses to >> write non-portable code on purpose. (One-time use, static >> platform, compelling need,...?) > > > 3) Slow-as-hell 8 Bit platforms don't have a proper abstraction layer > for any of their hardware, no matter if sound, graphics, timers, ports > or CPU. Nonsense. The "abstraction" we are discussing is the published documentation for the processor/system. What it documents is the abstraction that future implementations will preserve. What it doesn't document will generally not be preserved. The only exception to this is when rampant exploitation of undocumented features makes their preservation a marketing necessity. In the latter case, many extensions to the system functionality will be hampered by the need to provide the undocumented functionality, making evolution less attractive. Most early micro systems suffered from this problem. Many didn't live long enough or attract enough user base to face the issue. But those who did face it had to decide what undocumented behaviors of the early platform to leave behind, along with any software that exploited them. -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: aiiadict on 17 May 2006 17:53 >It would actually be fun to see the real logic diagram of the >6502. ;-) I'd love to see it. Rich
From: John Selck on 18 May 2006 02:09 Michael J. Mahon wrote: > John Selck wrote: >> Huh? They did move forward. 6510 -> 8500 -> 8502. And also, I don't >> think it's any kind of problem for Commodore to use customized CPUs >> since they owned MOS, the owners and producers of 6502 tech back then :) > > And these new processors preserved the undefined opcode behavior? > > If so, they are almost certainly *not* new processor designs. Same opcodes but faster clockspeed. Also, CBM switched from NMOS to HMOS. >> 3) Slow-as-hell 8 Bit platforms don't have a proper abstraction layer >> for any of their hardware, no matter if sound, graphics, timers, ports >> or CPU. > > Nonsense. The "abstraction" we are discussing is the published > documentation for the processor/system. What it documents is the > abstraction that future implementations will preserve. What it > doesn't document will generally not be preserved. What about the bugs in the decimal mode? They were fixed on later processor designs, this also renders the CPUs incompatible for some programs.
From: John Selck on 18 May 2006 02:15
Michael J. Mahon wrote: > True, but the decoding in the 6502 is handled by a kind of PLA, so > it would likely not be very expensive (in real estate) to trap or > NOP the invalid combinations. The 6502 is a design to have as few transistors as possible. The whole CPU only has a few thousand of them. Handling the illegals would have increased this small amount a lot. > Admittedly, though, the microprocessor design culture at that time > was not oriented toward "protecting" undefined space, as they all > have been in the years since. It's not about culture, it's about being able to do more with just a few transistors. What CPU would you buy? The one which can do less has NOPs instead of illegals, or the one which can do more? Removing illegals is ok if you only waste 1000 transistors of 1000000, but if it's 1000 transistors of 6000, then it's a whole different matter. |