REP - Repeat String Operation

	Usage:	REP
	Modifies flags: None

	Repeats execution of string instructions while CX != 0.  After
	each string operation, CX is decremented and the Zero Flag is
	tested.  The combination of a repeat prefix and a segment override
	on CPU's before the 386 may result in errors if an interrupt occurs
	before CX=0.  The following code shows code that is susceptible to
	this and how to avoid it:

	 again:  rep movs  byte ptr ES:[DI],ES:[SI]   ; vulnerable instr.
		     jcxz  next 	     ; continue if REP successful
		     loop  again	     ; interrupt goofed count

				 Clocks			Size
	Operands	 808x  286   386   486		Bytes

	none		  2	2     2			  1

	- see Instruction Timing   REPE   REPNE