Sinclair ZX80 / ZX81 Forums

[spodula]

Hi, Having recently got a zx81 (And a replacement keyboard, thanks Mr Rwap ) , I am interested in building a Ram pack for it. Now i have a couple of 128K sram chips around (being unable to buy only one of anything , ), and was wondering what the difficulties of creating 48k.

Now I know by default the zx81 will only use 16K of ram, but i have vaigue memories from years ago of the upper 32k of the memory map being used for other things,

However, i see that the old Memopacks managed it, so there must be a way.

So my questions are:

1: Is the upper ram are actually used? or is it just incompletely decoded.
2: If it is, can i get away with doing something like disabling my new upper 32k when an interrupt is occuring?

Thanks..

[Mike3]

Look in the Thread "32k 62256 mem mod help" in this same index "hardware" maybe there is something there you could use...

Regards.. Mike

[spodula]

Alas, that mod doesnt use the $8000-$bfff area, i assume because it uses the internal RamCS signal.
It looks a lot like the Rom appears at that location, so i may have to disable that.

I think i shall just try it and see.

[sirmorris]

Have you seen this thread?

64k-ram-best-configuration-t178s20.html

Charlie

[spodula]

Ta. I shall take a look. I'm currently on a concerted effort to fix all my old computers while i have a few free days, so I will take a look at it in a bit..

[ksng36]

Hello All,

It seems lately alot of folks are interested in adding more memory to the ZX81/TS1000's. It's a healthy sign that this computer still has value!! And alot of questions have been asked along the way. To keep disappointment down to a minimum, I have attached a zip file containing a study of the memory circuit designed by Wilf in his article on his website entitled "Thanks for all the Memories". First, go to his website (http://www.user.dccnet.com/wrigter/) and review his article. Next, download and unzip my study and review it along with his article. Doing both should help clear up the questions. Lastly, if you keep in mind that the address lines can be used as chip select lines, then most anything is possible to control, whether it's a memory module, printer interface, keyboard interface, etc.

Several other sources that are helpful are the following books: The INS and OUTS of the Timex TS1000 & ZX81 by Don Thomasson and The Explorer's Guide to the ZX81 and Timex Sinclair 1000 by Mike Lord. These are obviously both out of print, which means you'll have to try Ebay or something to find them, but I found myself referring back to them often when working out a hardware design.

Feedback concerning the zipped files would be grateful as it helps to know the audience.

Cheers!!

ksng36

[XavSnap]

Hi,
Something's wrong???

[siggi]

Hi,
Something's wrong???

Yes: the "ZX81-16K RAMPACK" contains 64K ram

Siggi

[XavSnap]

...
Oooops !

Yes ! It's a "Mega-16k rampack".

[sirmorris]

Unless you're a serious developer anything more than 32k is pointless. For a casual user 32k is useful as it enables hires programs of all types as long as it sits 8..40k. And you can play Xav's 32k graphic adventures in all their glory!

The upcoming SD-card/RAM board will be configured this way.

C