Acces to Original 1K with 16K RAM Pack Connected

[thewiz]

I had a thought the other day, and knowing nothing about hardware thought I would share it.

First, its possible to access the 1K of RAM during screen refresh. There is a program by Wilf that creates a very small true high-res display with it with no hardware modification.

Secondly, the 16Kb RAM Pack turns off the built in 1Kb.

Heres the challange. Could anything be done to make the builtin 1Kb available without too much hardware? If such a thing was possible, I think UDGs or possibly true high-res could be done in software.

Obviously there are other solutions for high-res, involving 8Kb of SRAM, however this is about minimal hardware for a contrived problem.

If its not possible, feel free to tell me to stay away from hardware and stick to software (politely .

thewiz

[siggi]

It it possible: the MEMOTECH centronics printer interface is connected between the ZX81 and the 16 K ram pack. It uses the internal 1K ram to remap it to the address range of the rom to "patch" the rom with new code to "install" the new printer driver.

To use the internal ram, you have to disconnect the /RAMCS line of the 16K ram pack, because the 16K ram pack pulls that signal HIGH and thus disables the internal 1K ram. Then you have to add an address decoder (LS138 or LS251), which delivers a new /RAMCS signal to the internal ram, which is fed to the egde connector pin of the ZX81. When the address decoder then enables the internal 1K ram at another address (e. g. at 32-33K), the address decoder has also to disable the mirrored rom at that address by pushing the /ROMCS (via a Diode) HIGH to avoid a collision between 1k RAM and ROM.

I think, the "normal" modification of a 16K ram pack for TRUE HRG (using 2 diodes and 1 resistor) is easier.

Siggi

[yerzmyey]

I think, the "normal" modification of a 16K ram pack for TRUE HRG (using 2 diodes and 1 resistor) is easier.
Siggi

Hi,

is there any scheme of this mod, on the Net?
To the moment I thought it's a modification of ZX81 itself, so I'm a bit suprised, as I'm not a hardware man.
Anyway, I could ask Factor6 if he can fix 16K RamPack into hi-res.

[siggi]

Just look in Matthias' manual of his HRG program for the modification of the ram pack:

http://www.swatosch.de/zx81/doku-en.pdf

HTH Siggi

[thewiz]

Thanks Siggi,

Hadn't realised it was so easy to get high-res. I just wonder why this wasn't standard in 1981. I guess we are all cleverer now.

Thewiz.