You are thinking of Emu42 which is written by the author of Emu48. The problem is Emu42 requires ROM images for the calculators it emulates. HP has not released any of the ROMs for those calculators so you have to download them yourself. This simulator does not need a ROM image.
This is the one you are thinking of
http://christophgiesselink.homepage....e.de/emu42.htm Just be prepared to spend quite a bit of time figuring out how to download and compile your ROM image.