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(not really about) problem with ux400

    Date: Fri, 29 Jun 90 10:42 EDT
    From: Scott Cyphers <Cyphers@JASPER.SCRC.Symbolics.COM>

	Date: Thu, 28 Jun 90 08:44 PDT
	From: Chucko@CHARON.arc.nasa.gov (Charles R. Fry)

	    Date: Tue, 26 Jun 90 18:02 EDT
	    From: barmar@Think.COM (Barry Margolin)

		[original problem report deleted]

	    I suspect some kind of hardware error.  That bogus argument,
	    #<DTP-PACKED-INSTRUCTION-77 37777777777> looks like something with all
	    its bits on (38 of the 40 bits in a word are evident from the numbers in
	    the printed rep, but I don't know the internal tag for
	    DTP-PACKED-INSTRUCTION offhand). [...]

	Yipes!!!  Any tagged computer architecture in which all-ones is a valid
	datum scares me.  (Likewise all-zeroes.)

	 -- Chucko

    All ones is only a valid datum for certain subprimitives that are used
    by things like the disassembler and the compiler.  As an instruction, it
    is invalid.

Then why didn't it cause a trap when it was referenced in this case?  It
wasn't noticed until the program tried to use it in a subtraction.  It
should trap at reference time, just like DTP-NULL does, unless one of
the special subprimitives is used.