The bizarre Casio FX-840P

[Jeff_Birt]

I picked up a Casio FX-840P recently and noticed it has the same PCB as the FX-850P and FX-880P. The FX-840P and FX-850P come with 8K internal RAM and the FX-880p 32K. For the FX-850P you can swap the 4464 SRAM chip for a 62256 and change one solder jumper. Oddly this does not work on the FX-840P! The FX-840P will see and RP-8 and RP-33 option RAM packs though.

I can POKE and PEEK into the full 64K address range with a 32K RAM installed internally and an RP-33 Redux installed. It would seem then that the initialization function in the ROM does not even look for the extra internal RAM. The size of the ROM in the FX-840P is smaller than the FX-850P/FX-880P so perhaps they left that out to save a few bytes?

I suspect if someone understood where the RAM pointers lived in the Casio memory map it would be possible to POKE those in by hand and let BASIC see the entire amount of RAM available.