File Control Block (FCB) Structure

     Offset  Length  Field           Initialized to       Initialized by

00h     1     Drive code      Drive specified           User
01h     8     Filename        Filename                  User
09h     3     Extension       Filename extension        User
0Ch     2     Current Block   00h                       DOS
0Eh     2     Record Size     80h                       DOS; see below
10h     4     File Size       Value in directory        DOS
14h     2     Date            Value in directory        DOS
16h     2     Time            Value in directory        DOS
18h     8     Reserved
20h     1     Current Record  See notes below           User
21h     4     Random Record   See notes below           User
Number

An extended File Control Block is used to access files with special
attributes. An extended File Control Block has three additional
fields, starting at offset -07h (minus 7), as follows:

Offset  Length  Field             Initialized to     Initialized by

-07h     1     Extended FCB ID   FFh                     User
-06h     5     Reserved          Must be zeros           User
-01h     1     Attribute Byte    File attribute          User

                                                                          

Notes:         The record-size field is set by DOS to 80h (128)
bytes. If you want to use a different value, change
the field after the file has been opened and before
you perform any disk operation. If the record size
is less than 64 bytes, all four bytes of this field
are used; otherwise only the lower three bytes are
used.

The current record field is used for sequential
read/write operations, and must be set before
performing any sequential read/writes. Set the
current record field to the record number that you
wish to access within the current block. The current
record field can take on any value between 0 and
127.

The random record number field must be set before
any random read/write operations can be performed.
Set this field to the record number relative to the
beginning of the file that you wish to access.

File Control Blocks are a holdover from DOS 1. It is
preferable to manipulate files using the newer DOS
services that deal with files via handles. (All file
functions above 2Eh use handles rather than FCBs.)

See also: File attributes

DOS - FCB Fields