A decent Speccy 48k/Zeddy emulator for plain OCS/68k?

[dreadnought]

I don't want to be too pedantic, but "perfect" for me means full speed.

That's not to say this emu isn't a very impressive achievement, code-wise.

[jotd]

if it works perfectly for most games on a 68000 then it's just ... awesome. Not being able to do the same thing with the 6502 oric emulator on my A500 made me buy a A1200 back in the old days.

Accurate emulation is something else. Even WinUAE has a lot of settings to make emulation more or less accurate. And guess what? less accurate often means faster

[Bruce Abbott]

Originally Posted by commie1974

Hi there, mates!
During my "stay at home" period, I had this weird idea:
Would nowadays the creation from scratch of a good, usable and accurate ZX Spectrum 48k and/or ZX81 emulator be possible on a vanilla 512k OCS miggy, like the A500?

No, not if 'accurate' means at full speed.

But who cares? This type of thing is what accelerator cards were made for. A 20MHz 68000 (eg. Wicher 500i) should be able to do it, and any 020 or 030 board is more than enough.

We already have Speccy emulators like the very interesting KGB 1.3 from the nineties, but they seem poor for today's standards...

I run Speccylator on my A1200 with 50MHz 030 and its perfect (better than a real Spectrum!).

And of course, the 48k Spectrum or especially the 1/16k ZX81 must not be THAT complex systems to emulate on a way more powerful platform...

A stock A500 isn't that much more powerful when it comes to CPU emulation. The 68000 is only clocked twice as fast as the Z80, and being 16 bit doesn't help much when you have to emulate 8 bit instructions. All the peripheral hardware has to emulated too, so it's not surprising that you need about 20MHz to get accurate real time emulation.

Wouldn't it be great to be able to play e.g. Bomb Jack under speccy emulation or Mazogs under zeddy emulation at a good speed on a plain rev.5/6 500, or even a 1000?

Not really. I would at least want a hard drive to store games on, more RAM for multitasking etc. Otherwise I would rather use a real Spectrum. But a suitably expanded A500 or A1000 - sure!

[ReadOnlyCat]

I actually am relatively confident that accurate full speed emulation of <some> 8 bit machines is possible on the A500.

This may require more than 512KB though since a large amount of memory would be essential to avoid the trap of interpreting Z80/C6502 rather than converting it once and for all.

JIT have limitations but these limitations are not intrinsic to the method, just to the design choices of the JIT writer.
Also, a JIT is still essentially an interpreter with an executation cache, there are other approaches like static recompilation.

[ReadOnlyCat]

Originally Posted by Bruce Abbott

No, not if 'accurate' means at full speed.

Accuracy and speed are quasi orthogonal concerns.
Classic emulator designs have struggled with speed whenever they tried to be address accuracy because they were not design to take this constraint into account in the first place. Accuracy is a design requirement, not a feature one can simply plug in long after the design is solidified.

The latest BSNES version is a good example that starting from a slow, accurate emulator (Higan) it is possible to rewrite a fast, equally accurate one by making the proper design choices from the start rather than growing the emulator organically.

Originally Posted by Bruce Abbott

A stock A500 isn't that much more powerful when it comes to CPU emulation. The 68000 is only clocked twice as fast as the Z80, and being 16 bit doesn't help much when you have to emulate 8 bit instructions. All the peripheral hardware has to emulated too, so it's not surprising that you need about 20MHz to get accurate real time emulation.

This is not how problems are solved.

Interpretation of Z80 opcodes is slow? Yup, definitely.
How do we make it faster?
-> we don't.

We choose another method than interpretation, static recompilation for example.
Or redesigning JIT compilers to take accuracy into account instead of trying to shoehorn old JIT designs into the emulator.

Originally Posted by Bruce Abbott

Not really. I would at least want a hard drive to store games on, more RAM for multitasking etc. Otherwise I would rather use a real Spectrum. But a suitably expanded A500 or A1000 - sure!

Totally agree with you, a hard drive seems a necessity for proper comfort, if only to store the converted data the emulator would have to produce before it can actually emulate at full speed.

[Bruce Abbott]

Originally Posted by ReadOnlyCat

This is not how problems are solved.

Yes it is.

We choose another method than interpretation, static recompilation for example.
Or redesigning JIT compilers to take accuracy into account instead of trying to shoehorn old JIT designs into the emulator.

That's just silly. You want a faster Amiga to run native software that needs it and just for general enjoyment anyway, and emulation is another good justification for having it. Why spend so much time and effort trying (and failing) to get around it?

Even if you did manage to JIT the Spectrum on an unaccelerated A500, it wouldn't be stock. You would still need more RAM and a hard drive, and existing emulators already run well on a moderately accelerated machine so it's pointless. It might have been worthwhile 'back in the day' when accelerator cards were rare and expensive, but now there is no excuse not to upgrade. Do we really want to perpetuate the worst attitude of Amiga owners back then?