Category: DOS kernel
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