Some consider the 6502 MPU a RISC processor avant la lettre, while others strictly oppose the idea. (And, in the strict sense, they are absolutely right.) – But, as can be learned from the following recount by Pete Foley, one of earliest chip design engineers at Apple, a RISC 6502, “potentially outperforming the 68000”, nearly became a thing!
- A RISC based implementation of the Apple II 6502 Processor: In mid ’85 I performed an analysis that showed a simple RISC style implementation of a 16‐bit binary compatible superset of the 8‐bit microprocessor used in the Apple II 6502, along with some judicious use of on‐chip caching, could substantially improve performance – to the point of potentially outperforming the 68000 used in the Mac, and given the simplicity of the 6502 the implementation was “doable” by a small team. This was a more direct approach than emulating 6502 compiled binaries by a different processor as was done some four years later in the Mobius project in the Advanced Technology Group (ATG). I set about completing a feasibility study that went through several revisions (Turbo‐I and Turbo‐II), which included a complete micro‐architecture design of the processor along with resource usage diagrams for every clock phase of every instruction. When the design seemed solid and I was ready to move on to an implementation, I sought the counsel and the support of my mentors in the IC Technology group (to whom I owe a huge debt of gratitude), Bob Bailey and Walt Peschke. As usual, when they felt it was time to impart some wisdom upon me, they said, “Pete, lets go for a walk”. As we walked around the local residential neighborhood in Cupertino they explained to me that marketing/sales/biz dev would have no idea what to do (how to position, etc) with such a thing and I would just end up with a black eye. Of course they were right and I stopped working on it. Their warnings were prescient, as four years later Jean-Louis Gassee was to shut down a similar project called Mobius in the Advanced Technology Group (ATG) where the ARM microprocessor was used to emulate another architecture.
(Found via HN, https://news.ycombinator.com/item?id=27843189)