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The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.

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                       ┌───┬───────────────────────┬───┐
           Extension → │ -1│        rsv            │att│
                       └───┴───────────────────────┴───┘
 PSP                   ↓                               ↓
 Offset
 ──────                                                ┌───┬───────────┐
 50h                                          FCB #1 → │drv│  name∙∙∙  │
       ┌───────────────────┬───────────┬───────┬───────┼───┴───────────┤
 60h   │∙∙∙                │    ext    │ block │ recsz │    filesz     │
       ├───────┬───────┬───┴───────────┴───────┴───────┼───┬───────────┤
 70h   │  date │  time │              rsv              │rec│  rand∙∙∙  │
       ├───┬───┴───────┴───────────────────────────────┴───┴───────────┘
 80h   │∙∙∙│
       └───┘
       └───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┴───┘
         0   1   2   3   4   5   6   7   8   9   A   B   C   D   E   F
 
 
  Field     Extension
 
  -1         First byte = 0FFh.
  rsv        Reserved.
  att        File attribute.
 
 
  Field      FCB
 
  drv        Drive code (0=default, 1=A, 2=B, etc).
 
  name       8-character filename, left-justified with trailing blanks.
 
  ext        3-character extension, left-justified with trailing blanks.
 
  block      Current block number relative to beginning of file,
             starting with 0. Each block comprises 128 records of the
             size specified by the logical record size (below). Used
             with current record for sequential file operations.
 
  recsz      Logical record size in bytes (set to 128 when file is
             opened).
 
  filesz     File size in bytes (set by DOS).
 
  date       Date file was created or last updated (set by DOS).
 
  time       Time file was created or last updated (set by DOS). Date
             and time fields are in standard file date/time formats
             (see: DOS File Date/Time Formats).
 
  rsv        Reserved.
 
  rec        Current record number (beginning with 0) relative to
             current block. Must be initialized before doing sequential
             file operations.
 
  rand       Record number (beginning with 0) relative to beginning of
             file. Must be initialized before doing random file
             operations.
 
  NOTES:
 
     ■ The FCB is used by certain interrupt 21h functions called "FCB
       functions," between 0Fh (Open File) and 29h (Parse Filename).
       These functions provide a method of file access compatible with
       CP/M. They are not recommended in DOS Versions 2.0 and higher,
       which offer superior file handling.
 
     ■ The FCB extension is a 7-byte header used to create or access a
       file with a read-only, hidden, system, subdirectory, and/or
       volume-label attribute. To use the extension with any of the FCB
       functions, set the pointer (usually DS:DX) to the first byte of
       the extension header rather than to the FCB itself.
 
     ■ The program segment prefix (PSP) holds two FCBs, identified as
       #1 (beginning at offset 5Ch) and #2 (beginning at offset 6Ch).
       Each FCB uses the format shown in the Summary screen. Both FCBs
       cannot be open at the same time, since each overwrites the
       other.
 
     ■ The application program is responsible for initializing the
       fields at FCB offsets 0-15 and 32-36. Offsets 16-31 (filesz,
       date, time, and rsv) are set by DOS from information in the
       file's directory entry and must not be altered.
 
     See also: Program Segment Prefix (PSP), Interrupt 21h FCB Functions,
               DOS File Date/Time Formats
                                    -♦-