Note: This API call is for DOS and Win16 personality only. Use Family API for portability.

2018/09/07 05:04 · prokushev · 0 Comments

Int 21H, AH=4BH


2 and higher



Family API


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
	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
	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.

See also

AX=4B05h,AH=4Ch,AH=4Dh,AH=64h/BX=0025h,AH=8Ah,INT 2E,INT 60/DI=0604h


2018/09/04 17:23 · prokushev · 0 Comments