Sinclair ZX80 / ZX81 / Z88 Forums

for instance dizzy would b cool. i know the zx81 can do hi res, so y not?

Hi, welcome to the forums.

Generally ZX81 games happens because of what people want to develop, so if you wanted to learn Z80 to make a Dizzy-inspired game then this is the place to be.

Regards,

Shaun.

cant some of the code just b ported directly across?

I have no experience with ZX spectrum but have read in these forums that it's quite similar to ZX81 and that you only need to change the graphic routine to port a game (if the game is monochrome)

The ZX81 and ZX Spectrum have a lot of similarities, but also some significant differences.

The basic ZX81 with 16k byte RAM pack (or equivalent) has RAM between 16k and 32k, the same as a 16k ZX Spectrum. They both use a Z80 CPU, but the ZX Spectrum runs slightly faster at approx 3.5MHz compared to approx 3.25MHz for the ZX81. The keyboard electronics hardware is similar.

The ZX81 has no sound.

In terms of a high resolution graphics display, yes the ZX81 can display the same as the ZX Spectrum. However, unlike the ZX Spectrum where all the CPU has to do is put the screen data into screen memory (display file), the ZX81 CPU also has to help draw the picture on the screen as the ULA in this machine cannot produce a video picture on it's own. By this, I mean the CPU and the ULA work together to get each byte of the display file and then the ULA converts the byte that the CPU has just fetched to a serial bit stream to send to the display device (monitor/TV). The drawing of the video picture takes up a significant amount of the CPUs time. Because the CPU is used for the video picture generation, timing in the ZX81 is critical.

Mark

xxx777 wrote: ↑Wed Aug 30, 2017 2:06 am cant some of the code just b ported directly across?

The Spectrum is built upon a ZX81, which in turn is an upgraded ZX80 with a ULA as I understand it.

You'll have to look at what options for extra memory are available for real hardware, so consider that the Spectrum has ~40K free to the programmer for the 48K model (where as the 16K Spectrum has ~8k to the programmer). The ZX81 has ~15K for a typically expanded machine. It depends on how much more memory - if anything - will be required for the high resolution solutions available, something that I've not looked into.

Limitations are easily broken with emulation and modern upgrades, so this is where you'll have to find out more. There is a lot of knowledge here so feel free to ask more technical questions.

Regards,

Shaun.

If you want to do it, it is better when you have the source.

I have converted my "Shogun" from the ZX Spectrum to the ZX81.

http://minigamecompo.weebly.com/2k-page.html

Problems I met, but I knew in advance:
1) IX and AF' can't be used. I had to alter some routines.
2) the game uses the RST16 from the ZX Spectrum. This had to get its own printroutine : CALL RST16
3) Characterset is different. All printed text needed alterations.
4) colour had to be replaced by something else.
5) write your own hiresdisplayroutine.
6) speed can be an issue since the ZX81 has a slower Z80 and does a lot screentime.

To compare the games: both ingame screens

xxx777 wrote: ↑Wed Aug 30, 2017 12:13 am for instance dizzy would b cool. i know the zx81 can do hi res, so y not?

Yes, you'r true...
No Basic compatibilities : multi-commands line allowed on the spectrum!

spectrum:
10 print:print:print

ZX81:
10 print
20 print
30 print

You had to convert the Basic programme to a text file, add lines and delete all illegals commands/functions...
And build a P file from text file, and convert it in a wav file...
Or rewrite it on a ZX81!

->The ZX81 to Spectrum is ok...

Of course, one must remember that the best ZX81 titles are ZX81 exclusive titles. It's better to build from within the limitations than to down-scale from the Spectrum to the ZX81.

Regards,

Shaun