INT 21 U - DR-DOS 5.0+ internal - GET POINTER TO TABLE OF ???
        AX = 4458h
Return: ES:BX -> internal table (see below)
        AX = ??? (0B50h for DR-DOS 5.0, 0A56h for DR-DOS 6.0)

Format of internal table:
Offset  Size    Description
 00h    DWORD   pointer to ???
 04h  7 BYTEs   ???
 0Bh    WORD    K of extended memory at startup
 0Dh    BYTE    number of far jump entry points
 0Eh    WORD    segment containing far jumps to DR-DOS entry points (see below)
 10h    WORD    ???
 12h    WORD    pointer to segment of environment variables set in CONFIG,
                or 0000h if already used (DR-DOS 6.0 only)
 14h    ???     ???
Note:   the segment used for the DR-DOS 6.0 CONFIG environment variables
          (excluding COMSPEC, VER and OS) is only useful for programs/drivers
          called from CONFIG.SYS. The word is set to zero later and the area
          lost.

Format of jump table for DR-DOS 5.0-6.0:
Offset  Size    Description
 00h  5 BYTEs   far jump to entry point corresponding to CP/M CALL 5
 05h  5 BYTEs   far jump to entry point corresponding to INT 20
 0Ah  5 BYTEs   far jump to entry point corresponding to INT 21
 0Fh  5 BYTEs   far jump to entry point corresponding to ???
 14h  5 BYTEs   far jump to entry point corresponding to ???
 19h  5 BYTEs   far jump to entry point corresponding to ???
 1Eh  5 BYTEs   far jump to entry point corresponding to ???
 23h  5 BYTEs   far jump to entry point corresponding to ???
 28h  5 BYTEs   far jump to entry point corresponding to ???
 2Dh  5 BYTEs   far jump to entry point corresponding to ???
 32h  5 BYTEs   far jump to entry point corresponding to ??? (IRET)
 37h  5 BYTEs   far jump to entry point corresponding to ??? (IRET)
 3Ch  5 BYTEs   far jump to entry point corresponding to ??? (IRET)
 41h  5 BYTEs   far jump to entry point corresponding to ??? (IRET)
 46h  5 BYTEs   far jump to entry point corresponding to ??? (IRET)
 4Bh  5 BYTEs   far jump to entry point corresponding to ???