Changes

Unlike many some DIY expansions from the 80s it's still possible to do it today - all required components can still be found easily. The logic ICs are standard ICs which most "makers" probably have lying around. If not you can find them in any electronics store or on Ebay for cheap. Even when if you have to source the 41256 ICs it should be possible to build this for less than 30€35€. To be fair, for that price you can already get an external 512KB expansion - but if you don't like this external add-ons, want to build it with your own hands and stay true to the 80s era, this expansion is might be for you.