ZX80 on ZX81???

[kokkiklhs]

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!

[Moggy]

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.

[kokkiklhs]

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...

[1024MAK]

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

[Moggy]

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.

[kokkiklhs]

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!

[dr beep]

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.

[1024MAK]

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

[kokkiklhs]

@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!

[dr beep]

@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.