Changes

* ROM is paged into the range &0000-&3fff when the red button is pressed and the NMI vector is executed.

* When the black button is pressed, the ROM is paged into the range &0000-&3fff and executes it at &0000. This checks the BASIC ROM version, and clears the extra ram if a BASIC ROM from a CPC6128 is found.

0000 f3 di ;; disable maskable interrupts