Source : Free On-Line Dictionary of Computing

Motorola 6809
     
        (MC6809) An eight-bit {microprocessor} from {Motorola, Inc.}
     
        The 6809 was a major advance over both its predecessor, the
        {Motorola 6800} and also over the {6502}.  The 6809 had two 8-bit
     {accumulator}s, rather than one in the 6502, and could
        combine them into a single 16-bit register.  It also featured
        two {index register}s and two {stack pointer}s, which allowed
        for some very advanced {addressing mode}s.  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 even replaced by {addressing mode}s.
     
        Other features were 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 {register}s.  The
        6809 was used in the UK "{Dragon 32}" {personal computer} and
        was followed by the {Motorola 68000}.
     
        See also {SEX}.
     
        {Usenet} newsgroup: {news:comp.sys.m6809}.
     
        There is a simulator called {usim} and an {assembler} by
        Lennart Benschop  was posted
        to {Usenet} newsgroup {alt.sources} on 1993-11-03.
     
        (1995-02-01)