It is very similar in mechanical and electrical design to the 9825 and nearly identical software-wise to the 9830. In fact, most of the internal parts are prefixed with 09825, meaning they were originally designed for the 9825 computer. For instance, the keyboard assembly is 09825-66537.

The 9831 (and 9825) use a three-chip central processing unit. The three chips are:

• Binary Processor Chip (BPC)
  • 59 byte-addressable instructions
  • similar to HP 2100 and HP 21MX instructions
  • Two general purpose 16-bit registers (A and B)
  • Program Counter P
  • Stack Pointer
  • Flags
• Input/Output Chip (IOC)
  • 12 instructions
• Extended Math Chip (EMC)
  • 15 instructions
  • Binary Coded Decimal operations

I am interested in finding any 9831 machines, ROMs, manuals, documents or other material, including information on the BPC processor. If you have any such equipment or documentation, please send me an e-mail!

I have more information and photographs here.

Plug-in ROM HP 98218A 9885 Flexible Disk ROM		09831-67905 BASIC Language plug-in ROM for use in the 9831A