[Coco] 9809 Halt and Catch Fire (HCF)

[Melanie and John Mark Mobley]

All,

I did not know the 6809 had undocumented commands.  I thought the
undocumented commands were all NOP, BRN, LBRN.

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Source:
https://en.wikipedia.org/wiki/Motorola_6809

Like the 6800, the 6809 includes an undocumented address bus test
instruction which came to be nicknamed Halt and Catch Fire (HCF).[1]

References
[1] "Archived copy". Archived from the original on 2013-07-01. Retrieved
2013-07-01.

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Archived copy:
https://archive.is/20130701061311/https://student.brighton.ac.uk/burks/pcinf
o/hardware/cpu.htm#selection-1145.427-1149.177

Part VI: The 6809, extending the 680x (1977) . . . . . . . .
...
Like the 6800, it included the undocumented HCF (Halt Catch Fire)
instruction to incrementally strobe the address lines for bus testing ("jump
to accumulator (A or B)" in the 6800, implemented and documented as $00 in
the 68HC11 which is described below).

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John Mark Mobley