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I struggle to see what is RISC-like about the 6502. 8-bit processors are usually too quirky to put into one of the categories. RISC hallmarks are: - Fixed-widt

I struggle to see what is RISC-like about the 6502. 8-bit processors are usually too quirky to put into one of the categories. RISC hallmarks are: - Fixed-width encoding: No. It varies from one to three bytes. - Large register set: No, it's an accumulator architecture. I don't count the zero page because every program that ran on the computer had to use the same zero page. IIRC, user programs on some late-era 6502 OSes would have only a handful of those 256 bytes to work with, rest being claimed by the system software. - Pipelining: There's some optimization with prefetching of instructions. It is not the multi-step pipeline of RISC, nor does it need to be. - Load/store architecture: Plenty of instructions work directly on memory.

HarHarVeryFunny 4y ago
RISC wasn't even a thing when the 6502 was designed, but in terms of complexity it was definitely at the reduced/simple instruction set end of the spectrum, as evidenced by it's transistor count. I never had the conversation with Wilson/Furber, but I'm assuming that CISC was essentially off the table in the first place. Both had zero CPU design experience, so to have any hope for success it had to be simple. The 6502 provided plenty of validation that simple could be fast, and no-one at Acorn had been impressed with next-gen 16/32-bit CISC designs anyway. No doubt other early RISC designs were an influence, but I don't believe there was any RISC vs CISC religion involved. It's amusing how today's worldwide domination of the processor market by ARM is really based on design decisions that were born of necessity for a chip that was really targeted for desktop use. It had to be simple, hence the die was small and cheap. It had to use low cost plastic packaging, so it had to be low power to avoid overheating. Low cost and low power is what led to ARM's mobile success.
- shadowofneptune 4y ago
  Just being simple in electronic design doesn't make it RISC, that makes the term useless. The PDP-8 shares a lot of similarities with the 6502, and in one implementation comes in at only 1409 transistors. If you look at what you need to do to write a program in it, it's anything but RISC in concept. A small set of operations is the least useful marker, the ones I gave above are common criteria.
  - HarHarVeryFunny 4y ago
    Sure, but RISC vs CISC is at heart just a design trade off. Once you've decided (or been forced by pragmatisim) to go with a "simple but fast" instruction set then design choices like large register set and load-store architecture (to keep instruction set small and orthogonal) are all obvious choices, and don't need to have come from some rulebook of RISC design principles. All I'm saying is that I heard at the time that the well-liked 6502 was a source of inspiration for the design (and someone else has noted Hermann also remembering something similar), notwithstanding that there was no doubt much borrowing from the contemporary (UC Berkeley, Stanford) RISC designs as well. As a 6502 programmer the utility of zero page variables is pretty obvious and there is a parallel there to a large register bank.. Maybe the 6502 wasn't "RISC like" in sense of incorporating what would become RISC design principles (it was too primitive for that), but it certainly validated a KISS design approach, just as contemporary next-gen CISC designs did nothing in Acorn's eyes to sell that approach. Back at that time we were all just seeing/discovering what was possible, and I'd have to guess that the ARM instruction set was more pragmatic than dogmatic - a combination of what seemed desireable, appeared reasonable to implement, and performed well in simulation.

HarHarVeryFunny 4y ago

RISC wasn't even a thing when the 6502 was designed, but in terms of complexity it was definitely at the reduced/simple instruction set end of the spectrum, as evidenced by it's transistor count. I never had the conversation with Wilson/Furber, but I'm assuming that CISC was essentially off the table in the first place. Both had zero CPU design experience, so to have any hope for success it had to be simple. The 6502 provided plenty of validation that simple could be fast, and no-one at Acorn had been impressed with next-gen 16/32-bit CISC designs anyway. No doubt other early RISC designs were an influence, but I don't believe there was any RISC vs CISC religion involved. It's amusing how today's worldwide domination of the processor market by ARM is really based on design decisions that were born of necessity for a chip that was really targeted for desktop use. It had to be simple, hence the die was small and cheap. It had to use low cost plastic packaging, so it had to be low power to avoid overheating. Low cost and low power is what led to ARM's mobile success.

shadowofneptune 4y ago
Just being simple in electronic design doesn't make it RISC, that makes the term useless. The PDP-8 shares a lot of similarities with the 6502, and in one implementation comes in at only 1409 transistors. If you look at what you need to do to write a program in it, it's anything but RISC in concept. A small set of operations is the least useful marker, the ones I gave above are common criteria.
- HarHarVeryFunny 4y ago
  Sure, but RISC vs CISC is at heart just a design trade off. Once you've decided (or been forced by pragmatisim) to go with a "simple but fast" instruction set then design choices like large register set and load-store architecture (to keep instruction set small and orthogonal) are all obvious choices, and don't need to have come from some rulebook of RISC design principles. All I'm saying is that I heard at the time that the well-liked 6502 was a source of inspiration for the design (and someone else has noted Hermann also remembering something similar), notwithstanding that there was no doubt much borrowing from the contemporary (UC Berkeley, Stanford) RISC designs as well. As a 6502 programmer the utility of zero page variables is pretty obvious and there is a parallel there to a large register bank.. Maybe the 6502 wasn't "RISC like" in sense of incorporating what would become RISC design principles (it was too primitive for that), but it certainly validated a KISS design approach, just as contemporary next-gen CISC designs did nothing in Acorn's eyes to sell that approach. Back at that time we were all just seeing/discovering what was possible, and I'd have to guess that the ARM instruction set was more pragmatic than dogmatic - a combination of what seemed desireable, appeared reasonable to implement, and performed well in simulation.