The Free On-line Dictionary of Computing (30 December 2018):

Motorola 6809
6809
MC6809

    (MC6809) An eight-bit microprocessor from Motorola,
   Inc..

   The 6809 was a major advance over both its predecessor, the
   Motorola 6800 and the 6502.  The 6809 had two 8-bit
   accumulators, rather than one in the 6502, and could combine
   them into a single 16-bit register.  It also featured two index
   registers and two stack pointers, which allowed for some very
   advanced addressing modes.  The 6809 was source compatible
   with the 6800, even though the 6800 had 78 instructions and the
   6809 only had around 59 (including a SEX instruction).  Some
   instructions were replaced by more general ones which the
   assembler would translate and some were replaced by addressing
   modes.

   The 6809 had one of the first multiplication instructions of the
   time, 16-bit arithmetic and a special fast interrupt.  But it
   was also highly optimised, gaining up to five times the speed of
   the 6800 series CPU.  Like the 6800, it included the undocumented
   HCF (Halt and Catch Fire) bus test instruction.

   The Hitachi 6309 was a version with extra registers.  The
   6809 was used in the UK "Dragon 32" personal computer and
   was followed by the Motorola 68000.

   Usenet newsgroup: news:comp.sys.m6809.

   Lennart Benschop  posted a
   emulator (originally called "usim") and a cross-assembler to
   Usenet newsgroup alt.sources on 1993-11-03.  Ray P. Bellis
    released a version 0.11.

   Benschop emulator (http://lennartb.home.xs4all.nl/m6809.html).

   (2014-06-24)