INT 0x21 - 0x4b - "EXEC" - Load and/or Execute Program (DOS 2+)

Request/Call:

reg	value	descr
AH	0x4b
AL		type of load
DS:DX		ASCIIz program name (must include extentsion)
ES:BX		parameter block (see below)

Offset	Size	Description
0x00	WORD	segment of environment to copy for child process (copy caller's environment if 0000h)
0x02	DWORD	pointer to command tail to be copied into child's PSP
0x06	DWORD	pointer to first FCB to be copied into child's PSP
0x0A	DWORD	pointer to second FCB to be copied into child's PSP
0x0E	DWORD	(AL=01h) will hold subprogram's initial SS:SP on return
0x12	DWORD	(AL=01h) will hold entry point (CS:IP) on return

Offset	Size	Description
0x00	WORD	segment at which to load overlay
0x02	WORD	relocation factor to apply to overlay if in .EXE format

CL cleared if successfull

BX, DX are destroyed
if sub function 0x01 is called, process ID set to new program's PSP can be requested via Int 0x21, 0x62

CL is set on error

AX = error code (0x01, 0x02, 0x05, 0x08, 0x0a, 0x0b) see Int 0x21, 0x59, DOS error Code

DOS 2.x destroys all registers, including SS:SP
for function 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 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 has been documented for DOS 5+, but was undocumented in prior versions

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

BUG: DOS 2.00 assumes that DS points at the current program's PSP