vax compatibility mode mark

[cottrell at nbs-vms.ARPA]

/* Henry Spencer writes...
> Actually, a friend who got one of the first 780's outside DEC, and had
> DEC people working with them for a while getting it running, mentioned
> one other thing that compatibility mode doesn't support:  the MARK
> instruction.  This omission is (I believe) undocumented, but it's not
> surprising that nobody has noticed the absence of the most useless
> instruction ever added to the 11...
> -- 

It's WORSE than useless. I suspect the reason DEC shyed away from
supporting separate I/D in space in RSX etc. is that it won't work with
mark. Mark executes off the stack! Somebody goofed royally. 

	jim
*/