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