The DEC VAX-11 instruction set had SVPCTX and LDPCTX for the kernel which made c...

The DEC VAX-11 instruction set had SVPCTX and LDPCTX for the kernel which made context switch simple. But they were reserved instructions; so they couldn't be used for setjmp() and longjmp(). VAX-11 also had queue instructions which made rescheduling simple. This was basically:

  SETIPL
  SVPCTX
  INSQUE
  FFS
  REMQUE
  LDPCTX
  SETIPL

Yeah, there was a little more. See page 188 of ... http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/vax/vms...