INT 21 - DOS 1+ - NULL FUNCTION FOR CP/M(Control Program for Microcomputers) An early operating system for micros based on the 8-bit Intel 8080 CPU (and later the compatible 8085 and Zilog Z80 CPUs).  MSDOS version 1.0 was essentially a clone of CP/M for the Intel 8086. COMPATIBILITY
	AH = 20h
Return: AL = 00h
Note:	corresponds to the CP/M(Control Program for Microcomputers) An early operating system for micros based on the 8-bit Intel 8080 CPU (and later the compatible 8085 and Zilog Z80 CPUs).  MSDOS version 1.0 was essentially a clone of CP/M for the Intel 8086. BDOS function "get/set default user
	  (sublibrary) number", which is meaningless under MS-DOS
SeeAlso: AH=18h,AH=1Dh,AH=1Eh,AX=4459h