🚀 go-pugleaf

In article <1992Dec14.203631.16388@CSD-NewsHost.Stanford.EDU>,
andy@SAIL.Stanford.EDU (Andy Freeman) writes:
| In article <id.08RV.WYB@ferranti.com> peter@ferranti.com (peter da silva)
| writes:
|>Close. As I recall, DEC is very fond of using even/odd for true/false
|>values, and the VMS interface definition has a lot of even/odd status
|>returns, so it's likely these instructions are to make it run VMS real good.
| 
| Does anyone know why they preferred even/odd to >=0/<0?  Sometimes one
| has to test the sign bit, so why not use it whenever one can use any
| bit to split things into two classes?

It's tough to reconstruct that kind of decision years after the fact.  I can
suggest two ways in which using the bottom bit works better:

	- Both TRUE=1 and TRUE=-1 are reasonably common representations in
		existing languages.  Using the bottom bit works for both;
		using the top doesn't.

	- The actual representation of a VMS condition value is more complex
		than has been described so far.  Reading from the least signi-
		ficant bit, we have:

			Severity:  3 bits.  Bottom bit set indicates some kind
				of successful completion (but it could be an
				"alternate success" status); bottom bit clear
				is some kind of failure, in which case the
				other two bits indicate how severe a failure.
			Message number:  13 bits.
			Facility number:  12 bits.  Globally assigned value
				that identifies the particular subsystem;
				most significant of these is 0 for DEC
				facilities, 1 for customer-defined facilities.
			Control:  4 bits.

		The "SYS" facility is facility 0; status values returned by
		this facility (which includes all values returned as a direct
		result of system calls, in particular) always have the top
		word of the condition value longword clear.  In some contexts,
		the condition value is stored as a (16-bit) word, with an
		implied zero-extension to a full longword condition value.

		This last is a hack that seems to have some connection to
		VMS's RSX heritage; it also saved space in a day when VMS
		ran on 128K systems.  But it means that you can't use the
		top bit of a condition longword to encode success information,
		since that bit may not be representable in some contexts.

							-- Jerry