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

Halt and Catch Fire

    (HCF) Any of several undocumented and
   semi-mythical machine instructions with destructive
   side-effects, supposedly included for test purposes on several
   well-known architectures going as far back as the IBM 360.

   The Motorola 6800 microprocessor was the first for which an
   HCF opcode became widely known.  This instruction caused the
   processor to read every memory location sequentially until reset.

   [Gerry Wheeler, Byte, December 1977, p46, "Undocumented M6800
   Instructions"
   (https://archive.org/details/byte-magazine-1977-12)].

   (2014-09-20)