Online HelpPC 2.10 Quick Reference Utility Copyright 1991 David Jurgens
FCB - Standard DOS File Control Block
Offset Size Description
-7 byte if FF this is an extended FCB ╪
-6 5bytes reserved ╪
-1 byte file attribute if extended FCB ╪
00 byte drive number (0 for default drive, 1=A:, 2=B:, ...)
01 8bytes filename, left justified with trailing blanks
09 3bytes filename extension, left justified w/blanks
0C word current block number relative to beginning of the
file, starting with zero
0E word logical record size in bytes
10 dword file size in bytes
14 word date the file was created or last updated
│F│E│D│C│B│A│9│8│7│6│5│4│3│2│1│0│ 15,14 (Intel reverse order)
│ │ │ │ │ │ │ │ │ │ │ └─┴─┴─┴─┴─ day 1-31
│ │ │ │ │ │ │ └─┴─┴─┴────────── month 1-12
└─┴─┴─┴─┴─┴─┴───────────────── year + 1980
16 word time of last write
│F│E│D│C│B│A│9│8│7│6│5│4│3│2│1│0│ 17,16 (Intel reverse order)
│ │ │ │ │ │ │ │ │ │ │ └─┴─┴─┴─┴── secs in 2 second increments
│ │ │ │ │ └─┴─┴─┴─┴─┴─────────── minutes (0-59)
└─┴─┴─┴─┴────────────────────── hours (0-23)
18 8bytes see below for version specific information ╪
1A dword address of device header if character device ╪
20 byte current relative record number within current BLOCK
21 dword relative record number relative to the beginning of
the file, starting with zero; high bit omitted if
record length is 64 bytes
DOS 2.x Values for reserved fields at offsets 18h-1Ah ╪
Offset Size Description
18 byte │7│6│5│4│3│2│1│0│
│ │ └─┴─┴─┴─┴─┴─── unknown
│ └────────────── 1 = open
└─────────────── 1 = logical device
19 word starting cluster number ╪
DOS 3.x Values for reserved fields at offsets 18h-19h ╪
Offset Size Description
18 byte System File Table (SFT) entry for file ╪
19 byte attributes ╪
│7│6│5│4│3│2│1│0│ attributes
│ │ └─┴─┴─┴─┴─┴──── unknown
└─┴─ share status
00 = SHARE not loaded block device
01 = SHARE not loaded characted device
10 = SHARE loaded, remote file
11 = SHARE loaded local file
DOS 3.x with SHARE, local file reserved offsets 1Ah-1Eh ╪
Offset Size Description
1A word starting cluster number
1C word offset within SHARE of sharing record
1E byte file attribute
DOS 3.x with SHARE, remote file reserved offsets 1Ah-1Eh ╪
Offset Size Description
1A word sector number containing directory entry
1C word last cluster accessed relative to beginning of file
1E byte absolute cluster number of last cluster accessed
DOS 3.x without SHARE reserved offsets 1Ah-1Fh ╪
Offset Size Description
1A byte ((device attribute word low byte) & 0Ch) || (open mode)
1B word starting cluster number
1D word sector number containing directory entry
1F byte number of directory entry within sector
The following are FCB related DOS functions:
INT 21,F Open file using FCB
INT 21,10 Close file using FCB
INT 21,11 Search for first entry using FCB
INT 21,12 Search for next entry using FCB
INT 21,13 Delete file using FCB
INT 21,14 Sequential read using FCB
INT 21,15 Sequential write using FCB
INT 21,16 Create a file using FCB
INT 21,17 Rename file using FCB
INT 21,21 Random read using FCB
INT 21,22 Random write using FCB
INT 21,23 Get file size using FCB
INT 21,24 Set relative record field for FCB
INT 21,27 Random block read using FCB
INT 21,28 Random block write using FCB
INT 21,29 Parse filename for FCB
╪ see Bibliography reference to "Undocumented DOS"
- see XFCB INT 21,52
[helppc.bosbyte.nl] File Control Block