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The MS-DOS Encyclopedia
Appendix G: File Control Block (FCB) Structure
Figures G-1 and G-2 (memory block diagrams) and Tables G-1 and G-2
describe the structure of normal and extended file control blocks
(FCBs).
Offset
00H ┌──────────────────────────┐
│ Drive identifier │
01H ├──────────────────────────┤
│ │
│ │
│ │
│ │
│ │
│ Filename │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
09H ├──────────────────────────┤
│ │
│ File extension │
│ │
0CH ├──────────────────────────┤
│ │
│ Current block number │
│ │
0EH ├──────────────────────────┤
│ │
│ Record size (bytes) │
10H ├──────────────────────────┤
│ │
│ │
│ File size (bytes) │
│ │
│ │
14H ├──────────────────────────┤
│ │
│ Date stamp │
│ │
16H ├──────────────────────────┤
│ │
│ Time stamp │
│ │
18H ├──────────────────────────┤
│ │
│ │
│ │
│ Reserved │
│ │
│ │
20H ├──────────────────────────┤
│ Current record number │
21H ├──────────────────────────┤
│ │
│ │
│ Random record number │
│ │
│ │
└──────────────────────────┘
Figure G-1. Structure of a normal file control block.
Table G-1. Elements of a Normal File Control Block.
╓┌──────────────────────┌──────────┌─────────────────────────────────────────╖
Maintained
Element by Comments
──────────────────────────────────────────────────────────────────
Drive identifier Program Designates the drive on which the
file to be opened or created resides
(0 = default drive, 1 = drive A, 2 =
drive B, and so on). If the
application supplies a zero in this
byte, MS-DOS alters the byte during
the open or create operation to
reflect the actual drive used.
Filename Program Standard eight-character filename;
must be left justified and must be
padded with blanks if fewer than
eight characters. A device name (for
example, PRN) can be used; there is
no colon after a device name.
File extension Program Three-character file extension; must
be left justified and must be padded
with blanks if fewer than three
characters.
Current block Program Zero when the file is opened; the
number current block number and the current
record number combined make up the
record pointer during sequential
file access.
Record size Program Set to 128 when the file is opened or
created; the program can modify the
field afterward to any desired
record size.
File size MS-DOS The size of the file in bytes; the
first 2 bytes of this 4-byte field
are the least significant bytes of
the file size.
Date stamp MS-DOS The date of the last write operation
on the file; follows the same format
used by Interrupt 21H file handle
Function 57H (Get/Set Time and Date):
Bits Contents
9-15 Year (relative to 1980)
5-8 Month (1-12)
0-4 Day of month (1-31)
Time stamp MS-DOS The time of the last write operation
on the file; follows the same format
used by Interrupt 21H file handle
Function 57H (Get/Set Time and Date):
Bits Contents
11-15 Hours (0-23)
5-10 Minutes (0-59)
0-4 Number of 2-second
increments (0-29)
Current record Program Limited to the range 0 through 127; there
number are 128 records per block. The beginning
of a file is record 0 of block 0.
Together with the current block number,
this field constitutes the record pointer
used during sequential read and write
operations. MS-DOS does not automatically
initialize this field when a file is
opened.
Random record Program Identifies the record to be transferred
pointer by the Interrupt 21H random record
functions 21H, 22H, 27H, and 28H; if the
record size is 64 bytes or larger, only
the first 3 bytes of this field are used.
MS-DOS updates this field after random
block reads and writes (Functions 27H and
28H) but not after random record reads
and writes (Functions 21H and 22H).
Table G-2. Additional Elements of an Extended File Control Block.
╓┌──────────────────────┌──────────┌─────────────────────────────────────────╖
Maintained
Element by Comments
──────────────────────────────────────────────────────────────────
Extended FCB flag Program 0FFH tells MS-DOS this is an extended
(44-byte) FCB.
File attribute Program Must be initialized by the application
byte when an extended FCB is used to open or
create a file. The bits of this field
have the following significance:
Bit Meaning
0 Read-only
1 Hidden
2 System
3 Volume label
4 Directory
5 Archive
6 Reserved
7 Reserved
Offset
00H ┌──────────────────────────┐
│ Extended FCB flag (0FFH) │
01H ├──────────────────────────┤
│ │
│ │
│ │
│ │
│ │
│ Reserved │
│ │
│ │
│ │
06H ├──────────────────────────┤
│ File attribute byte │
07H ├──────────────────────────┤
│ Drive identifier │
08H ├──────────────────────────┤
│ │
│ Filename │
│ │
10H ├──────────────────────────┤
│ │
│ File extension │
│ │
13H ├──────────────────────────┤
│ │
│ Current block number │
15H ├──────────────────────────┤
│ │
│ Record size (bytes) │
17H ├──────────────────────────┤
│ │
│ File size (bytes) │
│ │
1BH ├──────────────────────────┤
│ │
│ Date stamp │
1DH ├──────────────────────────┤
│ │
│ Time stamp │
1FH ├──────────────────────────┤
│ │
│ │
│ │
│ │
│ Reserved │
│ │
│ │
27H ├──────────────────────────┤
│ Current record number │
28H ├──────────────────────────┤
│ │
│ │
│ Random record number │
│ │
│ │
└──────────────────────────┘
Figure G-2. Structure of an extended file control block.
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