INT 21,4B - EXEC/Load and Execute Program

	AH = 4B
	AL = 00  to load and execute program
	   = 01  (Undocumented)  create program segment prefix and load
		 program, but don't execute.  The CS:IP and SS:SP of the
		 program is placed in parameter block.	Used by debuggers
	   = 03  load program only
	   = 04  called by MSC spawn() when P_NOWAIT is specified
	DS:DX = pointer to an ASCIIZ filename
	ES:BX = pointer to a parameter block

	on return:
	AX = error code if CF set  (see DOS ERROR CODES)
	ES:BX = when AL=1, pointer to parameter block similar to:

	Offset Size	       Description

	  00   word   when AL=1, segment of env. or zero if using parents env.
	       word   when AL=3, segment of load point for overlay
	  02   dword  when AL=1, pointer to cmd line at PSP 80h
	       word   when AL=3, relocation factor for EXE overlay
	  06   dword  pointer to default FCB passed at PSP 5Ch
	  0A   dword  pointer to default FCB passes at PSP 6Ch
	  0E   dword  value of program SS:SP
	  12   dword  value of program CS:IP

	- allows execution of an external program as well as overlay
	  management from within an application
	- all registers except CS and IP are destroyed
	- SS and SP should be preserved in code segment before call
	  since a bug in DOS version 2.x destroys these
	- return code can be retrieved if child process exits via INT 21,4C
	- calling process must assure presence of enough unallocated memory
	- subfunction 4 returns with an error under DOS 4.x+
	- calls INT 21,55

	- see also  INT 21,26