Sinclair ZX80 / ZX81 / Z88 Forums

I was looking at the superb dr beep's Sp-2-ZX80 emulator for the Spectrum and I just had a silly idea:
Could it somehow be ported to the Zeddy? Would this be possible?


It would be an out-of-this-world experience to see an emulator running on Zeddy!

Better still, solder 4 resistors to the right places on the CPU, program an EPROM with the ZX80's ROM code, fit a reset switch across the reset capacitor and have the real thing on a ZX81.

Moggy wrote: ↑Sun May 28, 2023 10:11 pm Better still, solder 4 resistors to the right places on the CPU, program an EPROM with the ZX80's ROM code, fit a reset switch across the reset capacitor and have the real thing on a ZX81.

Yes, I am aware of that (haven't tried it yet, though) but a 100% software emulator (IF such a thing is possible, I am not a programmer myself and was just wondering) would be a big programming achievement!! There could also be (in theory) e.g. a Jupiter Ace emulator for the Zeddy, AFAIK hardware-wise these machines are almost the same. Just checking the possibilities.

Another question: is there any other old, Z80 based monochrome and soundless system that could theoretically be emulated by a Zeddy? I know of the Tandy TRS-80 model I, but this has 32 and 64 character screenmodes, so this would be a major problem...

Although both the Jupiter Ace and the ZX81 both use the same microprocessor (Z80A or equivalent) and hence have ROM in the lower part of the memory map, and use a similar matrix keyboard, otherwise, the hardware is very different.

The ZX81 uses a mix of software and limited hardware to create a video picture. The Jupiter Ace video system is based on hardware, it will continue to produce a video image even if the processor is stopped (HALTed) or busy (unlike the Zeddy). Hence 2K bytes of the 3K bytes of RAM in a Jupiter Ace is for primary for the video system (only a small amount of video system RAM can be used for general purpose use).

That does not mean that an emulator can’t be written to run on a ZX81. But it does mean that there will be limitations, such as reduced speed.

Mark

1024MAK wrote: ↑Mon May 29, 2023 4:03 pm

The ZX81 uses a mix of software and limited hardware to create a video picture. The Jupiter Ace video system is based on hardware, it will continue to produce a video image even if the processor is stopped (HALTed) or busy (unlike the Zeddy). Hence 2K bytes of the 3K bytes of RAM in a Jupiter Ace is for primary for the video system (only a small amount of video system RAM can be used for general purpose use).



Mark

Now that's something I didn't know! I always assumed Altwasser and Vickers just copied the Zeddy.
You wouldn't have a link to something that explains it in depth would you Mark or just a pointer so I can learn a bit more about it?

Thanks in advance.

Moggy wrote: ↑Mon May 29, 2023 4:58 pm

1024MAK wrote: ↑Mon May 29, 2023 4:03 pm

The ZX81 uses a mix of software and limited hardware to create a video picture. The Jupiter Ace video system is based on hardware, it will continue to produce a video image even if the processor is stopped (HALTed) or busy (unlike the Zeddy). Hence 2K bytes of the 3K bytes of RAM in a Jupiter Ace is for primary for the video system (only a small amount of video system RAM can be used for general purpose use).



Mark

Now that's something I didn't know!

...me neither!!!
Mark is an infinite source of useful information! Thanks for letting us know!

kokkiklhs wrote: ↑Mon May 29, 2023 1:27 pm Yes, I am aware of that (haven't tried it yet, though) but a 100% software emulator (IF such a thing is possible, I am not a programmer myself and was just wondering) would be a big programming achievement!! There could also be (in theory) e.g. a Jupiter Ace emulator for the Zeddy, AFAIK hardware-wise these machines are almost the same. Just checking the possibilities.

A software emulator would need a ZX8 ROM loaded in RAM, like SP-2-ZX80 / SP-2-ZX81.
This would also mean that a full emulator is needed.
Since the ZX81 is using most CPU time for the screen the emulator would be pooly slow.
Just to compare: SP-2-ZX81 is running on app 10% when each intrupt the screen is updated and the ZX Spectrum is updating the screen itself.

Sites that have information about the Jupiter Ace computer:

• Grant Searle's Jupiter Ace page (his home page is here)
• Jupiter ACE Archive site (everything you ever wanted to know about it…)
• old-computers.com site (it does not have a “user port” despite what this site says)
• Tynemouth Software Blog entry
• Simply Eighties (excuse the errors and inaccuracies about the Sinclair computers).

I hope that helps

Mark

@Johan: Thanks for the detailed answer, I was just wondering (and hoping)... Now I understand that even though it IS possible, it is useless...
Well, our beloved Zeddy is capable of many other impressive and unexpected things, so I won't consider that as a big disadvantage...

@Mark: Wow, thank you, time for some good reading!

kokkiklhs wrote: ↑Mon May 29, 2023 9:22 pm @Johan: Thanks for the detailed answer, I was just wondering (and hoping)... Now I understand that even though it IS possible, it is useless...

And you would need enough RAM 16K is not much, morelike 32K.