INT 21 - DOS 2+ - "EXEC" - LOAD AND/OR EXECUTE PROGRAM

	AH = 4Bh
	AL = type of load
	    00h load and execute
	    01h load but do not execute
	    03h load overlay (see #01591)
	    04h load and execute in background (European MS-DOS 4.0 only)
		"Exec & Go" (see also AH=80h)
	DS:DX -> ASCIZ program name (must include extension)
	ES:BX -> parameter block (see #01590,#01591,#01592)
	CX = mode (subfunction 04h only)
		0000h child placed in zombie mode after termination
		0001h child's return code discarded on termination
Return: CF clear if successful
	    BX,DX destroyed
	    if subfunction 01h, process ID set to new program's PSP; get with
		INT 21/AH=62h
	CF set on error
	    AX = error code (01h,02h,05h,08h,0Ah,0Bh) (see #01680 at AH=59h)
Notes:	DOS 2.x destroys all registers, including SS:SP
	under ROM-based DOS, if no disk path characters (colons or slashes)
	  are included in the program name, the name is searched for in the
	  ROM module headers (see #01595) before searching on disk
	for functions 00h and 01h, the calling process must ensure that there
	  is enough unallocated memory available; if necessary, by releasing
	  memory with AH=49h or AH=4Ah
	for function 01h, the AX value to be passed to the child program is put
	  on top of the child's stack
	for function 03h, DOS assumes that the overlay is being loaded into
	  memory allocated by the caller
	function 01h was undocumented prior to the release of DOS 5.0
	some versions (such as DR DOS 6.0) check the parameters and parameter
	  block and return an error if an invalid value (such as an offset of
	  FFFFh) is found
	background programs under European MS-DOS 4.0 must use the new
	  executable format
	this function ignores the filename extension, instead checking the
	  first two bytes of the file to determine whether there is a valid
	  .EXE header (see #01594); if not, the file is assumed to be in .COM
	  format.  If present, the file may be in any of several formats which
	  are extensions of the original .EXE format (see #01593)
	.COM-format executables begin running with the following register
	  values:
		AL = 00h if first FCB has valid drive letter, FFh if not
		AH = 00h if second FCB has valid drive letter, FFh if not
		CS,DS,ES,SS = PSP segment
		SP = offset of last word available in first 64K segment
		(note: AX is always 0000h under DESQview)
	old-format executables begin running with the following register
	  values:
		AL = 00h if first FCB has valid drive letter, FFh if not
		AH = 00h if second FCB has valid drive letter, FFh if not
		DS,ES = PSP segment
		SS:SP as defined in .EXE header
		(note: AX is always 0000h under DESQview)
	new executables begin running with the following register values
		AX = environment segment
		BX = offset of command tail in environment segment
		CX = size of automatic data segment (0000h = 64K)
		ES,BP = 0000h
		DS = automatic data segment
		SS:SP = initial stack
	  the command tail corresponds to an old executable's PSP:0081h and
	  following, except that the 0Dh is turned into a NUL (00h); new
	  format executables have no PSP
	under the FlashTek X-32 DOS extender, only function 00h is supported
	  and the pointers are passed in DS:EDX and ES:EBX
	DR DOS 6 always loads .EXE-format programs with no fixups and
	  .COM-format programs starting with 9Ch 55h (PUSHF/PUSH BP) above the
	  64K mark to avoid the EXEPACK bug, by extending the memory block
	  containing the program's environment; this code is disabled if the
	  name of the parent program as stored in the MCB is 'WIN'.
	DR DOS 3.41 and 5.0 check for a valid filename before testing the
	  subfunction number, so the otherwise invalid subfunction 02h will
	  only return error code 01h if the given filename actually exists;
	  otherwise, errors 02h, 03h, or 05h are returned
BUGS:	DOS 2.00 assumes that DS points at the current program's PSP
	Load Overlay (subfunction 03h) loads up to 512 bytes too many if the
	  file contains additional data after the actual overlay
	Load but Do Not Execute (subfunction 01h) is reported to corrupt the
	  top word of the caller's stack if the loaded module terminates with
	  INT 21/AH=4Ch in some versions of MS-DOS, including v5.00.
SeeAlso: AX=4B05h,AH=4Ch,AH=4Dh,AH=64h/BX=0025h,AH=8Ah,INT 2E,INT 60/DI=0604h