Format of DOS 4.x data segment subsegment control blocks:

Offset  Size    Description
 00h    BYTE    subsegment type (blocks typically appear in this order)
                "D"  device driver
                "E"  device driver appendage
                "I"  IFS (Installable File System) driver
                "F"  FILES=  control block storage area (for FILES>5)
                "X"  FCBS=   control block storage area, if present
                "C"  BUFFERS EMS workspace area (if BUFFERS /X option used)
                "B"  BUFFERS=  storage area
                "L"  LASTDRIVE=  drive info table storage area
                "S"  STACKS=  code and data area, if present (see below)
 01h    WORD    paragraph of subsegment start (usually the next paragraph)
 03h    WORD    size of subsegment in paragraphs
 05h  3 BYTEs   unused
 08h  8 BYTEs   for types "D" and "I", base name of file from which the driver
                  was loaded (unused for other types)