Analysis of the HP-35 with extracted ROMs
Here are corrected HP-45 and HP-55 assembly code listings, plus a casmsim-compatible xio.c to display the HP-55 keyboard.
Two patents containing source code for the HP-45 and HP-55, along with some architectural details of their internals:
US Patent 4001569 (HP-45)
US Patent 4009379 (HP-55)
Eric Smith's assembler and simulator, CASMSIM
Dave Hicks' Java port with debugger console