Now this:

I think these two sections of the AVR Instruction Set Manual illustrate the concept of HAL I'm talking about.

The paragraph titled AVR Instruction Set Summary explains it all. It's the same machine code for all CPUs, but in several models certain features removed lead to certain instructions removed. The size of the included memory in the rest of the MCU does also makes a difference, and AFAIU the evolution on how internally the cores are designed and built resulted in the same instruction taking more or less clock cycles to complete in different models -- sort of what happens with the several 6502 implementations.

The AVR opcodes are all 16 bit, the CPUs being 8 bit. Seems not to be a problem even for the smallest MCU implementations.

Side note: from the eyes of a total ignorant, it seems not that hard to port 65xx code to AVR code.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

About having several accumulators or more registers.

As GARTHWILSON said, having Zero and First page as registers would lead to not using the system bus at all for things happening there.

And I add that if all the zero page registers were accumulators, the software coded math operations would be much faster because the results of multiple additions could be reused directly instead of SToring them in memory and LoaDing them from memory for everyone of them, providing that there would be an instruction that would store the result of an math operation directly to a ZP location instead of the accumulator like it happens right now.

If the definition of HAL as I've used it is wrong, what would be the right term to be used in this case?

I've already mentioned upthread the virtual machine. Except you seem to be looking at something more like a configurable computing.

It's all a bit unsatisfactory because you haven't given a coherent top-to-bottom description. It's as if you have a sketch and you'd like others to fill in the details for you.

I think the thing about the AVR example is that it's easier to design a complex ISA and microarchitecture and then subset it, than to make ad-hoc additions to something existing.

That said, point accelerations like multiply are fairly easy to add and to make use of.

Is Configurable computing the right term for defining a CPU from the opcodes it could use?



This thread is a discussion for exposition and interchange of ideas, comments and knowledge. It something arises from it, that someone with the appropiate knowledge could implement, it would be great.

Possibly Reconfigurable Computing is a relevant term (instead of Configurable!)
https://en.wikipedia.org/wiki/Reconfigurable_computing

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

> At a given clock speed (and the same percentage of the maximum speed they were ever spec'ed for), the 68000 took longer to do a register-to-register byte move than the 65c02 took to do a memory-to-register byte move.

You need to scale this by the memory access being multiple cycles on a 68k. "At a given clock speed" makes no sense as a comparison of such different architectures.

> the same percentage of the maximum speed they were ever spec'ed for
Although that is an interesting qualification, it brings in questions of implementation technology choices and market forces. It is possible that the internal complexity of the 68k limited the maximum clock speed enough for this to be a limiting factor. It's another reason to keep the complexity of CPUs under control. A massive microcode ROM (and nanocode ROM) is in a sense a simple thing, not a complex one, but by being large it might be slow, on chip.

Adventures on anycpu.org show that minor changes to a machine spec or to implementation tactics can reduce the clock speed on FPGA from 60MHz to 50MHz. That's a big step-down in performance, and might negate the advantage of extra accumulators.

Parenthetically, I think this thread shows
- the merit in a good choice of title, which doesn't distract from the point in question
- a clear first post which sets up the actual conditions and the question being asked
- the difficulty of speculating about improved CPUs when not informed by considerations of the tooling, the microarchitecture, the instruction encoding, the influences on clock speed
- the improbability of bringing a napkin sketch to a forum and ending up with someone skilled having been inspired into building the thing

I don't wish to put anyone off, and I have nothing against wild ideas and people enjoying speculation, but making a working system is a bit more difficult.

If you really want to make an improved 6502 I might suggest this path
- become pretty good at programming the 6502
- gain a passing familiarity with two or three other CPUs, at machine code level
- understand what it is that an assembler does, and how instruction encodings work
- design and simulate a very simple accumulator machine
- add indexing to it
- design and simulate other simple machines
- implement at least one machine and study what resources are needed - busses, flops, logic, registers, adders, incrementers
- understand the reports you get from timing analysis
- make changes and investigate the consequences

There are many books to guide you on this journey. Recent books include Petzold's Code, and Nand to Tetris.

There are a few YouTubers who build CPUs from scratch and explain what they are doing. In fact I think Ben Eater started with a CPU build, before making a minimal 6502 system.

Tokafonda, in the light of what you've learned from this thread, it might be worth starting a new one. A good title and a strong first post should set it off in the right direction, although it's an impossible hope for a thread to stay on-topic for long.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?