Format of EXEC parameter block for AL=00h,01h,04h:
Offset	Size	Description	)
 00h	WORD	segment of environment to copy for child process (copy caller's
		  environment if 0000h)
 02h	DWORD	pointer to command tail to be copied into child's PSP
 06h	DWORD	pointer to first FCB to be copied into child's PSP
 0Ah	DWORD	pointer to second FCB to be copied into child's PSP
 0Eh	DWORD	(AL=01h) will hold subprogram's initial SS:SP on return
 12h	DWORD	(AL=01h) will hold entry point (CS:IP) on return
SeeAlso: #01591,#01592