Note: This API call is for DOS personality only. Use Family API for portability.
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,AH=8Ah,INT 2E,INT 60/DI=0604h
Text based on Ralf Brown Interrupt List Release 61