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."