Int 21H, AH=4BH

2 and higher

“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

 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)

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.

AX=4B05h,AH=4Ch,AH=4Dh,AH=64h/BX=0025h,INT 2E,INT 60/DI=0604h

Text based on Ralf Brown Interrupt List Release 61