Here's something that HP9825 owners may find useful.
The HP9825 is covered by US patent US4075679 Feb 21 1978.
The patent gives a full description of the HP9825 including schematics,
processor description and internal schematic, flow charts, memory
maps, data structures etc....and a source listing !!. (600 pages total)
Patent is available from
http://gb.espacenet.com as scanned images
free of charge.
I've typed up all of the source and assembled it with no errors.
Assembler is based on HP21XX assembler by Jeff Moffatt at
http://oscar.taurus.com/~jeff/2100. Email me for the HP9825
source and assembler C source.
Chris Leyson