Has anyone tried a CPU replacement for CPC?

[zhulien]

like...


https://www.ebay.com.au/itm/164985208999?hash=item2669e280a7:g:HFkAAOSw5J1fMkZ~


Aside from whether the onboard memory mapping is compatible with CPC i/o ports or not, and the fact that I am aware that people have fitted a 6Mhz and 8Mhz CPU to their CPCs...


What other similar solutions could there be for Plus machines?

[TotO]

No CPU are 100% compatible for the CPC usage than the original NMOS Z80.
(and looking the price, it is a nice joke.... You can rebuild 10 of that board)

[genesis8]

See https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/ize80-in-circuit-simulation-of-a-z80/

[GUNHED]

The HD64180 suxx, because it lacks lots of undocumented instructions regarding IX and IY. (And yes, I'm talking about undocumented instructions, not unwanted instructions. That's a difference! Demos and advanced OS / Apps use both groups). Better use a Z180 or Z280 instead - but they bring their problems too.


The perfect thing would be a Z80 'clone' doing things more quick at the same MHz. 

[JupiterJones]

The HD64180 suxx, because it lacks lots of undocumented instructions regarding IX and IY. (And yes, I'm talking about undocumented instructions, not unwanted instructions. That's a difference! Demos and advanced OS / Apps use both groups). Better use a Z180 or Z280 instead - but they bring their problems too.


The perfect thing would be a Z80 'clone' doing things more quick at the same MHz. 

Hi!


In fact, they already exist, made by Zilog (or at least were built by Zilog):


There are Z80 capable of running at up to 20 MHz. The problem would be to set the clock (I guess minor problem) and avoid unwanted behaviours at such great frequencies of other components, maybe PIO, AY, or any other component (I don't have schematics at hand, so this is my guess...)

By the way, using a higher-rated Z80 as a replacement in any CPC (in fact in any Z80-built computer) is not a problem, as frequency seems not to rate the needed frequnecy but maximum frequency. I have one CPC and one ZX Spectrum + with CPU replaced with higher reted freq. and everything works like a charm.

[Bryce]

This subject seems to get re-discussed every 6 months. No, you guessed wrong, it's not a minor issue to change the clock frequency, it's a major one, because everything is connected to the same clock and many things only work at the frequency Amstrad designed for. ie: changing the clock speed puts the RAM out of sync, changes the dimensions of the screen, messes up the sound and makes disks incompatible.

There are many threads here discussing these issues, so I'm not going to repeat all the reasons here. Search out some of the other threads for details.

Bryce.

[JupiterJones]

This subject seems to get re-discussed every 6 months. No, you guessed wrong, it's not a minor issue to change the clock frequency, it's a major one, because everything is connected to the same clock and many things only work at the frequency Amstrad designed for. ie: changing the clock speed puts the RAM out of sync, changes the dimensions of the screen, messes up the sound and makes disks incompatible.

There are many threads here discussing these issues, so I'm not going to repeat all the reasons here. Search out some of the other threads for details.

Bryce.

Yes, that was my intended point. Upscaling frequency for processor *ONLY* maybe would be easy. For any other devices (I dismissed RAM, that's true) would be a major issue. My point was that there are processors capable to run at higher frequencies out of the box, nothing else. Within my limited knowledge, I don't need to search to understand that 


As a personal note, I wouldn't modify any system, specially the old ones, I prefer to keep them at original condition.

[Bryce]

Unfortunately (but necessarily) the CPU is synchronised with most of the rest of the CPC, so it's not possible to change the CPU frequency without changing the frequency of all the other parts. You can boost the frequency for the small window of time where the CPU is only calculating internally, but that's only a fraction of the entire time.

Bryce.

[GUNHED]

My idea was not to change the clock. It was to have a Z80(or similar really compatible) with less machine cycles for an opcode. Let's say ADD HL,DE will then take 2 us instead of 3 us at the same 4 MHz frequency.

[TotO]

The Z280 allows to be connected with a 4MHz bus but run 2x or 4x faster inside using PLL for a fastest computing.
But the main problem is that no CPU are 100% compatible with the NMOS Z80 CPU. (bug fix or removed features)

[zhulien]

Imagine a Z80 replacement that ran at 8Mhz, couldn't that 8Mhz Z80 be on a daughter board with externally halved clock cycles?  Couldn't that same Z80 have local fast RAM local to itself that runs at the full 8Mhz?


In theory I don't understand why the above is not relatively easily possible especially given the more complex hardware we are now seeing on the CPC - and if 8Mhz like above with local fast RAM could be possible, why wouldn't faster clock speeds?


At the end of the day, a CPU accelerator could be in the form of a PC connected to the CPC via a serial interface, however we could only use specially written software for it... why just plug that PC directly into the CPC's socket?


Amiga does it well, although the concept was there from the start so the OS was able to cater for it to varying degrees from day one.  In a CPC it could be more akin to a PowerUP board - still a fun thing to have in a CPC if someone made one.

[Bryce]

You would still need to "feed" the original RAM used for screen content. I a game, this is probably the most frequently accessed RAM.

Bryce.

[TotO]

In fact, Amstrad missed to have a faster PLUS by allowing to run at twice the speed for cartridge games.
The ASIC is clocked at 40MHz, so it was probably possible.

Booting the computer with:
- 32MHz "GA"
- 2MHz "CRTC"
- 8MHz Z80
- 2MHz AY

In theory, it allow 640x200 with 4 colours, 320x200 with 16 colours and 160x200 with 256 colours.
Nice to use the PLUS sprites with 4bit or 8bit colour depth for the 1:1 or 1:2 ratio.

[Bryce]

In fact, Amstrad missed to have a faster PLUS by allowing to run at twice the speed for cartridge games.
The ASIC is clocked at 40MHz, so it was probably possible.

Booting the computer with:
- 32MHz "GA"
- 2MHz "CRTC"
- 8MHz Z80
- 2MHz AY

In theory, it allow 640x200 with 4 colours, 320x200 with 16 colours and 160x200 with 256 colours.
Nice to use the PLUS sprites with 4bit or 8bit colour depth for the 1:1 or 1:2 ratio.

Yes, they focused a bit too much on backwards compatibility.

Bryce.

[TotO]

Yes, they focused a bit too much on backwards compatibility.

Yes. Probably both was possible, depending of "how the computer boot".
Finally you reach something close to the Atari ST... With a 8bit CPU.

[Bryce]

Yes. Probably both was possible, depending of "how the computer boot".
Finally you reach something close to the Atari ST... With a 8bit CPU.

Close to an ST?? So great sound but crap games?

Bryce.

[TotO]

Here is! No regret.

[Bryce]

I owned an ST for several years, but it always felt like owning the loser of the Amiga / ST battle. At least for gaming.

Bryce.

[eto]

I owned an ST for several years, but it always felt like owning the loser of the Amiga / ST battle. At least for gaming.

Bryce.

Wasn't that the same with the Amstrad vs. C64? Gaming was not everything. And that's where the CPC and the ST had strengths. Honestly, with a C64 or an Amiga I doubt I would have done so much "serious" stuff.

[GUNHED]

This can be used:
https://easyeda.com/hlide.dev.mobile/kc80-z80-adapter

Also check this:
https://www.msx.org/forum/msx-talk/hardware/little-kawasaki-z80-experiment?page=0

DRAM refresh in the CPC is NOT done by the Z80.
So this would be a working solution.