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1994-01-19
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** Programmer's Technical Reference for MSDOS and the IBM PC **
USA copyright TXG 392-616 ALL RIGHTS RESERVED
──────────────────────────┤ DOSREF (tm) ├───────────────────────────
ISBN 1-878830-02-3 (disk-based text)
Copyright (c) 1987, 1994 Dave Williams
┌─────────────────────────────┐
│ Shareware Version, 01/20/94 │
│ Please Register Your Copy │
└─────────────────────────────┘
C H A P T E R E I G H T
DOS DISK INFORMATION
C O N T E N T S
The DOS Area .......................................................... 8**1
The Boot Record ....................................................... 8**2
DOS File Allocation Table (FAT) ....................................... 8**3
Media Descriptor Byte ......................................... 8**4
12 Bit FATs ................................................... 8**5
16 Bit FATs ................................................... 8**6
DOS Disk Directory .................................................... 8**8
The Data Area ......................................................... 8**9
Floppy Disk Types ..................................................... 8**10
Hard Disk Layout ...................................................... 8**11
System Initialization ................................................. 8**12
Boot Record/Partition Table ........................................... 8**13
Hard Disk Technical Information ....................................... 8**14
Determining Hard Disk File Allocation ................................. 8**15
BIOS Disk Functions ................................................... 8**16
00h Reset
01h Get Status
02h Read Sectors
03h Write Sectors
04h Verify
05h Format Track (floppy disk)
06h Hard Disk - format track
07h Hard Disk - format drive
08h Read Drive Parameters
09h Initialize Two Fixed Disk Base Tables
0Ah Read Long (Hard disk)
0Bh Write Long (Hard disk)
0Ch Seek To Cylinder
0Dh Alternate Hard Disk Reset
0Eh Read Sector Buffer
0Fh Write sector buffer
10h Test For Drive Ready
11h Recalibrate Drive
12h Controller RAM Diagnostic
13h Controller Drive Diagnostic
14h Controller Internal Diagnostic
15h Get Disk Type
16h Get Disk Change Status (diskette)
17h Set Disk Type for Format (diskette)
18h Set Media Type For Format (diskette)
19h Park Hard Disk Heads
1Ah ESDI Hard Disk - Low Level Format
1Bh ESDI Hard Disk - Get Manufacturing Header
1Ch ESDI Hard Disk - Get Configuration
THE DOS AREA├────────────────────────────────────────────────────────── 8**1
All disks and diskettes formatted by DOS are created with a sector size of 512
bytes. The DOS area (entire area for a diskette, DOS partition for hard disks)
is formatted as follows:
┌────────────────────────────────────────────────────────────┐
│ D O S A R E A │
├────────────────────────────────────────────────────────────┤
│ partition table - variable size (hard disk only) │
│ boot record - 1 sector │
│ first copy of the FAT - variable size │
│ second copy of the FAT - same size as first copy │
│ root directory - variable size │
│ data area - variable depending on disk size │
└────────────────────────────────────────────────────────────┘
The following sections describe each of the allocated areas:
THE BOOT RECORD├─────────────────────────────────────────────────────── 8**2
The boot record resides on track 0, sector 1, side 0 of every diskette
formatted by the DOS FORMAT program. For hard disks the boot record resides on
the first sector of the DOS partition. It is put on all disks to provide an
error message if you try to start up with a nonsystem disk in drive A:. If the
disk is a system disk, the boot record contains a JMP instruction pointing to
the first byte of the operating system.
If the device is IBM compatible the first sector of the first FAT must be
located at the same sector for all disk types. This is because the FAT sector
is read before the disk type is actually determined.
The information relating to the BPB for a particular media is kept in the
disk's boot sector. The format of the boot sector is:
┌──────────────────────────────────────────────────────────────────────────────┐
│ D O S B O O T R E C O R D │
├───┬───────┬──────────────────────────────────────────────────────────────────┤
│00h│3 bytes│ JMP to executable code. For DOS 2.x, 3 byte near jump (0E9h). │
│ │ │ For DOS 3.x, 2 byte near jump (0EBh) followed by a NOP (90h) │
├───┼───────┼──────────────────────────────────────────────────────────────────┤
│03h│8 bytes│ optional OEM name and version (such as IBM 2.1) │
├───┼───────┼──────────────────────────────────────────────────────────────────┤
│0Bh│2 bytes│ bytes per sector │
├───┼───────┼─────┬────────────────────────────────────────────────────────────┤
│0Dh│ byte │ │ sectors per allocation unit (must be a power of 2) │
├───┼───────┤ ├────────────────────────────────────────────────────────────┤
│0Eh│2 bytes│ B │ reserved sectors (starting at logical sector 0) │
│ │ │ │ 01 for 1.x-3.31, 02 for 4.0+ │
├───┼───────┤ ├────────────────────────────────────────────────────────────┤
│10h│ byte │ │ number of FATs │
├───┼───────┤ ├────────────────────────────────────────────────────────────┤
│11h│2 bytes│ │ maximum number of root directory entries │
├───┼───────┤ P ├────────────────────────────────────────────────────────────┤
│13h│2 bytes│ │ number of sectors in logical image (total number of │
│ │ │ │ sectors in media, including boot sector directories, etc.)│
│ │ │ │ If logical disk size is geater than 32Mb, this value is 0 │
│ │ │ │ and the actual size is reported at offset 26h (DOS 4.0+) │
├───┼───────┤ ├────────────────────────────────────────────────────────────┤
│15h│ byte │ B │ media descriptor byte │
├───┼───────┤ ├────────────────────────────────────────────────────────────┤
│16h│2 bytes│ │ number of sectors occupied by a single FAT │
├───┼───────┼─────┴────────────────────────────────────────────────────────────┤
│18h│2 bytes│ sectors per track │
├───┼───────┼──────────────────────────────────────────────────────────────────┤
│1Ah│2 bytes│ number of heads │
├───┼───────┼──────────────────────────────────────────────────────────────────┤
│1Ch│2 bytes│ # of hidden sectors (sectors before this volume) (1st part) │
└───┴───────┼──────────────────────────────────────────────────────────────────┤
│ EXTENDED BOOT RECORD (DOS 4.0+) │
┌───┬───────┼──────────────────────────────────────────────────────────────────┤
│1Eh│2 bytes│ # of hidden sectors (sectors before this volume) (2nd part) │
├───┼───────┼──────────────────────────────────────────────────────────────────┤
│20h│4 bytes│ # sectors in this disk (see offset 13h, if 0) │
├───┼───────┼──────────────────────────────────────────────────────────────────┤
│24h│2 bytes│ physical drive number (max 2 for DOS 4, max 8 for DOS 5) │
├───┼───────┼──────────────────────────────────────────────────────────────────┤
│26h│ byte │ extended boot record signature (29h) │
├───┼───────┼──────────────────────────────────────────────────────────────────┤
│27h│4 bytes│ volume serial number (assigned with a random function) │
├───┼───────┼──────────────────────────────────────────────────────────────────┤
│2Bh│11 byte│ volume label │
├───┼───────┼──────────────────────────────────────────────────────────────────┤
│36h│7 bytes│ file system ID (FAT12 ), (FAT16 ) etc. ("reserved") │
└───┴───────┴──────────────────────────────────────────────────────────────────┘
The three words at the end return information about the media. The number of
heads is useful for supporting different multihead drives that have the same
storage capacity but a different number of surfaces. The number of hidden
sectors is useful for drive partitioning schemes.
DOS 3.2 uses a table called the BIOS Parameter Block (BPB) to determine if a
disk has a valid File Allocation Table. The BPB is located in the first sector
of a floppy disk. Although the BPB is supposed to be on every formatted floppy
disk, some earlier versions of DOS did not create a BPB and instead assumed that
the FAT begins at the second sector of the disk and that the first FAT byte
(Media Descriptor Byte) describes the disk format.
DOS 3.2 reads in the whole of the BPB and tries to use it - although strangely
enough, it seems as if DOS is prepared to cope with a BPB that is more or less
totally blank (it seems to ignore the descriptor byte and treat it as a DSDD
9-sector disk).
DOS 3.2 determines if a disk has a valid boot sector by examining the first
byte of logical sector 0. If that byte it a jump instruction 0E9h, DOS 3.2
assumes the rest of the sector is a valid boot sector with a BPB. If the first
byte is not 0E9h DOS 3.2 behaves like previous versions, assumes the boot sector
is invalid and uses the first byte of the FAT to determine the media type.
If the first byte on the disk happens to be 0E9h, but the disk does not have a
BPB, DOS 3.2 will return a disk error message.
The real problems occur if some of the BPB data is valid and some isn't.
Apparently some OEMs have assumed that DOS would continue to ignore the
formatting data on the disk, and have failed to write much there during FORMAT
except the media descriptor byte (or, worse, have allowed random junk to be
written there). While this error is understandable, and perhaps even
forgiveable, it remains their problem, not IBM's, since the BPB area has always
been documented as containing the format information that IBM DOS 3.2 now
requires to be there.
When the BPB problems first became evident with DOS 3.2 a number of reports
circulated claiming DOS looked for the letters "IBM" in the OEM ID field. This
was incorrect. IBM DOS 4.0 *did* check for the letters "IBM" and would refuse
to recognize hard drives formatted under MSDOS 4.0. IBM corrected this with
their 4.01 revision.
THE DOS FILE ALLOCATION TABLE (FAT)├─────────────────────────────────── 8**3
The File Allocation Table, or FAT, has three main purposes:
1) to mark bad sectors on the media
2) to determine which sectors are free for use
3) to determine the physical location(s) of a file on the media.
DOS uses one of two schemes for defining the File Allocation Table:
1) a 12-bit FAT, for DOS 1.x, 2.x, all floppies, and small hard disks
2) a 16-bit FAT, for DOS 3.x+ hard disks from 16.8 to 32Mb
This section explains how DOS uses the FAT to convert the clusters of a file
into logical sector numbers. It is recommended that system utilities use the
DOS handle calls rather than interpreting the FAT, particularly since
aftermarket disk partitioning or formatting software may have been used.
The FAT is used by DOS to allocate disk space for files, one cluster at a time.
In DOS 4.0, clusters are referred to as "allocation units." It means the same
things; the smallest logical portion of a drive.
The FAT consists of a 12 bit entry (1.5 bytes) for each cluster on the disk or
a 16 bit (2 bytes) entry when a hard disk has more than 20740 sectors as is the
case with fixed disks larger than 10Mb.
The first two FAT entries map a portion of the directory; these FAT entries
contain indicators of the size and format of the disk. The FAT can be in a 12
or 16 bit format. DOS determines whether a disk has a 12 or 16 bit FAT by
looking at the total number of allocation units on a disk. For all diskettes
and hard disks with DOS partitions less than 20,740 sectors, the FAT uses a 12
bit value to map a cluster. For larger partitions, DOS uses a 16 bit value.
The second, third, and fourth bit applicable for 16 bit FAT bytes always
contains 0FFFFh. The first byte is used as follows:
Media Descriptor Byte ................................................. 8**4
┌──────────────────────────────────────────────────────────────────────────────┐
│ M E D I A D E S C R I P T O R B Y T E │
├──────────┬──────────────────────────────────┬────────────────────────────────┤
│hex value │ meaning │ normally used │
├──────────┼──────────────────────────────────┼────────────────────────────────┤
│ 00 │ hard disk │ 3.3+ extended DOS partition │
├──────────┼──────────────────────────────────┼────────────────────────────────┤
│ ED │ double sided 9 sector 80 track │ Tandy 2000 720k (5¼) │
├──────────┼──────────────────────────────────┼────────────────────────────────┤
│ F0 │ double sided 18 sector diskette │ PS/2 1.44 meg DSHD │
├──────────┼──────────────────────────────────┼────────────────────────────────┤
│ F8 │ hard disk │ bootable hard disk at C:800 │
├──────────┼──────────────────────────────────┼────────────────────────────────┤
│ F8 │ 720k floppy, 9 sector 80 track │ Sanyo 55x, DS-DOS 2.11 (5¼) │
├──────────┼──────────────────────────────────┼────────────────────────────────┤
│ F9 │ double sided 15 sector diskette │ AT 1.2 meg DSHD │
│ │ double sided 9 sector diskette │ Convertible 720k DSQD │
├──────────┼──────────────────────────────────┼────────────────────────────────┤
│ FA │ IBM Displaywriter System disk │ 287k │
│ │ Kodak "4 meg" (Pelican) │ 4.4 meg (5¼) │
├──────────┼──────────────────────────────────┼────────────────────────────────┤
│ FB │ IBM Displaywriter System disk │ 1 meg (5¼) │
│ │ Kodak "6 meg" (Pelican) │ 5.5 meg (5¼) │
├──────────┼──────────────────────────────────┼────────────────────────────────┤
│ FC │ single sided 9 sector diskette │ DOS 2.0, 180k SSDD (5¼) │
├──────────┼──────────────────────────────────┼────────────────────────────────┤
│ FD │ double sided 9 sector diskette │ DOS 2.0, 360k DSDD (5¼) │
├──────────┼──────────────────────────────────┼────────────────────────────────┤
│ FF │ double sided 36 sector diskette │ Practidisk 2.88mb DSED (3½) │
│ │ single sided 8 sector diskette │ DOS 1.0, 160k SSDD (5¼) │
│ │ double sided 8 sector diskette │ DOS 1.1, 320k SSDD (5¼) │
│ │ hard disk │ Sanyo 55x with DS-DOS 2.11 │
├──────────┴───────────┬──────────────────────┴────────────────────────────────┘
│for 8 inch diskettes: │
├──────────┬───────────┴──────────────────────┬────────────────────────────────┐
│ FD │ double sided 26 sector diskette │ IBM 3740 format DSSD │
├──────────┼──────────────────────────────────┼────────────────────────────────┤
│ FE │ single sided 26 sector diskette │ IBM 3740 format SSSD │
│ ├──────────────────────────────────┼────────────────────────────────┤
│ │ double sided 8 sector diskette │ IBM 3740 format DSDD │
└──────────┴──────────────────────────────────┴────────────────────────────────┘
The third FAT entry begins mapping the data area (cluster 002).
NOTE: These values are provided as a reference. Therefore, programs should not
make use of these values.
Each entry contains three hexadecimal characters for 12-bit FATs or four for
16-bit FATs.
The possible entries are:
12-bit | 16-bit
|
000h | 0000h if the cluster is unused and available
0FF7h | 0FFF7h bad cluster (if not part of the allocation chain)
|
0FF0h-0FF7h | 0FFF0h-0FFF7h to indicate reserved clusters
|
0FF8h-0FFFh | 0FFF8h-0FFFFh to indicate the last cluster of a file (EOF)
|
xxxH | xxxxH any other hexadecimal numbers are the cluster
| number of the next cluster in the file. The
| cluster number is the first cluster in the file
| that is kept in the file's directory entry.
The file allocation table always occupies the sector or sectors immediately
following the boot record. If the FAT is larger than 1 sector, the sectors
occupy consecutive sector numbers. Two copies of the FAT are written, one
following the other, for integrity. The FAT is read into one of the DOS buffers
whenever needed (open, allocate more space, etc).
12 Bit File Allocation Table .......................................... 8**5
Obtain the starting cluster of the file from the directory entry.
Now, to locate each subsequent sector of the file:
1. Multiply the cluster number just used by 1.5 (each FAT entry is 1.5
bytes long).
2. The whole part of the product is offset into the FAT, pointing to the entry
that maps the cluster just used. That entry contains the cluster number of
the next cluster in the file.
3. Use a MOV instruction to move the word at the calculated FAT into a register.
4. If the last cluster used was an even number, keep the low order 12 bits of
the register, otherwise, keep the high order 12 bits.
5. If the resultant 12 bits are (0FF8h-0FFFh) no more clusters are in the file.
Otherwise, the next 12 bits contain the cluster number of the next cluster in
the file.
To convert the cluster to a logical sector number (relative sector, such as
that used by int 25h and 26h and DEBUG):
1. Subtract 2 from the cluster number
2. Multiply the result by the number of sectors per cluster.
3. Add the logical sector number of the beginning of the data area.
12-bit FAT if DOS partition is smaller than 32,680 sectors (16.340 MB).
16 Bit File Allocation Table .......................................... 8**6
Obtain the starting cluster of the file from the directory entry. Now to
locate each subsequent cluster of the file:
1. Multiply the cluster number used by 2 (each FAT entry is 2 bytes long).
2. Use the MOV word instruction to move the word at the calculated FAT offset
into a register.
3. If the resultant 16 bits are (0FF8h-0FFFFh) no more clusters are in the
file. Otherwise, the 16 bits contain the cluster number of the next cluster
in the file.
DOS Disk Directory .................................................... 8**8
The FORMAT command initially builds the root directory for all disks. Its
location (logical sector number) and the maximum number of entries are
available through the device driver interfaces.
Since directories other than the root directory are actually files, there is
no limit to the number of entries that they may contain.
All directory entries are 32 bytes long, and are in the following format:
┌───────┬─────────┬────────────────────────────────────────────────────────────
│offset │ size │ DISK DIRECTORY ENTRY
├───────┼─────────┼────────────────────────────────────────────────────────────
│ 00h │ 8 bytes │ Filename
│ ├─────────┴────────────────────────────────────────────────────────────
│ │ The first byte of the filename indicates the file status.
│ │ The file status byte may contain the following values:
│ ├──────┬───────────────────────────────────────────────────────────────
│ │ 00h │ Directory entry has never been used. This is used to limit
│ │ │ the length of directory searches, for performance reasons.
│ │ 05h │ Indicates that the first character of the filename actually
│ │ │ has an 0EDh character.
│ │ 0E5h │ Filename has been used but the file has been erased.
│ │ 2Eh │ This entry is for a directory. If the second byte is also
│ │ │ 2Eh, the cluster field contains the cluster number of this
│ │ │ directory's parent directory. (0000h if the parent directory
│ │ │ is the root directory). Otherwise, bytes 00h-0Ah are all
│ │ │ spaces and the cluster field contains the cluster number of
│ │ │ the directory.
│ ├──────┴───────────────────────────────────────────────────────────────
│ │ Any other character is the first character of a filename. Filenames
│ │ are left-aligned and if necessary padded with blanks.
├───────┼─────────┬────────────────────────────────────────────────────────────
│ 08h │ 3 bytes │ Filename extension if any
│ ├─────────┴────────────────────────────────────────────────────────────
│ │ Three characters, left-aligned and padded with blanks if necessary.
│ │ If there is no file extension, this field contains all blanks
├───────┼─────────┬────────────────────────────────────────────────────────────
│ 0Bh │ 1 byte │ File attributes
│ ├─────────┴────────────────────────────────────────────────────────────
│ │ The attribute byte is mapped as follows:
│ ├─────┬───┬────────────────────────────────────────────────────────────
│ │ hex │bit│ meaning
│ ├─────┼───┼────────────────────────────────────────────────────────────
│ │ 00h │ │ (no bits set) normal; can be read or written without
│ │ │ │ restriction
│ │ 01h │ 0 │ file is marked read-only. An attempt to open the file for
│ │ │ │ output using int 21h/fn 3Dh will fail and an error code
│ │ │ │ will be returned. This value can be used with other values
│ │ │ │ below.
│ │ 02h │ 1 │ indicates a hidden file. The file is excluded from normal
│ │ │ │ directory searches.
│ │ 04h │ 2 │ indicates a system file. The file is excluded from normal
│ │ │ │ directory searches.
│ │ 08h │ 3 │ indicates that the entry contains the volume label in the
│ │ │ │ first 11 bytes. The entry has no other usable information
│ │ │ │ and may exist only in the root directory.
│ │ 10h │ 4 │ indicates that the file is a subdirectory
│ │ 20h │ 5 │ indicates an archive bit. This bit is set to on whenever
│ │ │ │ the file is written to and closed. Used by BACKUP and
│ │ │ │ RESTORE.
│ │ │ 6 │ reserved, set to 0
│ │ │ 7 │ reserved, set to 0
│ ├─────┴───┴────────────────────────────────────────────────────────────
│ │ note 1) Bits 6 and 7 may be used in OS/2.
│ │ note 2) Attributes 08h and 10h cannot be changed using int21/43h.
│ │ note 3) The system files IBMBIO.COM and IBMDOS.COM (or customized
│ │ equivalent) are marked as read-only, hidden, and system
│ │ files. Files can be marked hidden when they are created.
│ │ note 4) Read-only, hidden, system and archive attributes may be
│ │ changed with int21h/fn43h.
├───────┼─────────┬────────────────────────────────────────────────────────────
│ 0Ch │ 10 bytes│ Reserved by DOS; value unknown
├───────┼─────────┼────────────────────────────────────────────────────────────
│ 16h │ 2 bytes │ File timestamp
│ ├─────────┴────────────────────────────────────────────────────────────
│ │ These bytes contain the time when the file was created or last
│ │ updated. The time is mapped in the bits as follows:
│ ├───────────────────────────────┬───────────────────────────────┐
│ │ B Y T E 16h │ B Y T E 17h │
│ ├───────────────────────────────┼───────────────────────────────┤
│ │ F E D C B A 9 8 │ 7 6 5 4 3 2 1 0 │
│ ├───────────────────┬───────────┴───────────┬───────────────────┤
│ │ H H H H H │ M M M M M M │ D D D D D │
│ ├───────────────────┼───────────────────────┼───────────────────┤
│ │ binary # hrs 0-23 │ binary # minutes 0-59 │ bin. # 2-sec incr │
│ ├───────────────────┴───────────────────────┴───────────────────┘
│ │ note: The time is stored with the least significant byte first.
├───────┼─────────┬────────────────────────────────────────────────────────────
│ 18h │ 2 bytes │ File datestamp
│ ├─────────┴────────────────────────────────────────────────────────────
│ │ This area contains the date when the file was created or last
│ │ updated. The mm/dd/yy are mapped in the bits as follows:
│ ├───────────────────────────────┬───────────────────────────────┐
│ │ B Y T E 18h │ B Y T E 19h │
│ ├───────────────────────────────┼───────────────────────────────┤
│ │ F E D C B A 9 8 │ 7 6 5 4 3 2 1 0 │
│ ├───────────────────────────┬───┴───────────┬───────────────────┤
│ │ Y Y Y Y Y Y Y │ M M M M │ D D D D D │
│ ├───────────────────────────┼───────────────┼───────────────────┤
│ │ 0-119 (1980-2099) │ 1-12 │ 1-31 │
│ ├───────────────────────────┴───────────────┴───────────────────┘
│ │ note: The date is stored with the least significant byte first.
├───────┼─────────┬────────────────────────────────────────────────────────────
│ 1Ah │ 2 bytes │ First file cluster number
│ ├─────────┴────────────────────────────────────────────────────────────
│ │ * (reserved in DOS 2, documented in DOS 3+)
│ │ This area contains the starting cluster number of the first cluster
│ │ in the file. The first cluster for data space on all fixed disks and
│ │ floppy disks is always cluster 002. The cluster number is stored
│ │ with the least significant byte first.
├───────┼─────────┬────────────────────────────────────────────────────────────
│ 1Ch │ 4 bytes │ File size
│ ├─────────┴────────────────────────────────────────────────────────────
│ │ This area contains the file size in bytes. The first word contains
│ │ the low order part of the size. Both words are stored with the least
│ │ significant byte first.
└───────┴──────────────────────────────────────────────────────────────────────
The Data Area ......................................................... 8**9
Allocation of space for a file (in the data area) is done only when needed
(it is not preallocated). The space is allocated one cluser (unit allocation)
at a time. A cluster is always one or more consecutive sector numbers, and all
of the clusters in a file are "chained" together in the FAT.
The clusters are arranged on disk to minimize head movement for multisided
media. All of the space on a track (or cylinder) is allocated before moving
on to the next track. This is accomplished by using the sequential sector
numbers on the lowest-numbered head, then all the sector numbers on the next
head, and so on until all sectors of all heads of the track are used. Then the
next sector used will be sector 1 of head 0 on the next track.
An interesting innovation that was introduced in MS-DOS 3.0: disk space that
is freed by erasing a file is not re-used immediately, unlike earlier versions
of DOS. Instead, free space is obtained from the area not yet used during the
current session, until all of it is used up. Only then will space that is freed
during the current session be re-used.
This feature minimizes fragmentation of files, since never-before-used space
is always contiguous. However, once any space has been freed by deleting a file,
that advantage vanishes at the next system boot. The feature also greatly
simplifies un-erasing files, provided that the need to do an un-erase is found
during the same session and also provided that the file occupies contiguous
clusters.
However, when one is using programs which make extensive use of temporary
files, each of which may be created and erased many times during a session,
the feature becomes a nuisance; it forces the permanent files to move farther
and farther into the inner tracks of the disk, thus increasing rather than
decreasing the amount of fragmentation which occurs.
The feature is implemented in DOS by means of a single 16-bit "last cluster
used" (LCU) pointer for each physical disk drive; this pointer is a part of
the physical drive table maintained by DOS. At boot time, the LCU pointer is
zeroed. Each time another cluster is obtained from the free-space pool (the
FAT), its number is written into the LCU pointer. Each time a fresh cluster
is required, the FAT is searched to locate a free one; in older versions of
DOS this search always began at Cluster 0000, but in 3.x it begins at the
cluster pointed to by the LCU pointer.
For hard disks, the size of the file allocation table and directory are
determined when FORMAT initializes it and are based on the size of the DOS
partition.
Floppy Disk Types ..................................................... 8**10
The following tables give the specifications for floppy disk formats:
IBM PC-DOS disk formats:
# of FAT size DIR total
sides (sectors)(entries) sectors
│ sectors │ DIR │ sectors│
│ /track │sectors│/cluster│
│ │ │ │ │ │ │
┌─────┬──┬───────┬─┴─┬───┴───┬─┴─┬─┴─┬─┴─┬─┴─┬──┴─┬────────────────────────────
│ 160k│5¼│DOS 1.0│ 1 │ 8 (40)│ 1 │ 4 │ 64│ 1 │ 320│Original PC-0, 16k mbd
│ 320k│5¼│DOS 1.1│ 2 │ 8 (40)│ 1 │ 7 │112│ 2 │ 360│PC-1, 64k mbd
│ 180k│5¼│DOS 2.0│ 1 │ 9 (40)│ 2 │ 4 │ 64│ 1 │ 640│PC-2, 256k mbd
│ 360k│5¼│DOS 2.0│ 2 │ 9 (40)│ 2 │ 7 │112│ 2 │ 720│PC/XT
│ 1.2M│5¼│DOS 3.0│ 2 │15 (80)│ 7 │14 │224│ 1 │2400│PC/AT, PC/RT, XT/286
│ 720k│3½│DOS 3.2│ 2 │ 9 (80)│ 3 │ 7 │112│ 2 │1440│Convertible, PS/2 25+
│1.44M│3½│DOS 3.3│ 2 │18 (80)│ 9 │14 │224│ 1 │2880│PS/2 50+
└─────┴──┴───────┴───┴───────┴───┴───┴───┴───┴────┴────────────────────────────
various MS-DOS disk formats:
┌─────┬──┬───────┬─┴─┬───┴───┬─┴─┬─┴─┬─┴─┬─┴─┬──┴─┬────────────────────────────
│ 200k│5¼│ * │ 1 │10 (40)│ │ │ │ │ │
│ 400k│5¼│ * ** │ 2 │10 (40)│ │ │ │ │ │
│ 800k│5¼│ * │ 2 │10 (80)│ │ │ │ │ │
│ 720k│2 │ │ │ │ │ │ │ │ │Zenith SuperSport 2-inch
│ 720k│5¼│DOS2.11│ 2 │ 9 (80)│ 3 │ 7 │112│ 2 │1440│Tandy 2000 (discontinued)
│2.88M│3½│ │ 2 │36 (80)│ │ │ │ │5760│Practidisk 2.88mb floppy
│2720k│5¼│ *** │ 2 │17(192)│ 8 │ │272│ 4 │5440│Pelican (Kodak 3.3Mb)(disc.)
│5570k│5¼│ *** │ 2 │17(384)│ 8 │ │272│ 4 │10880Pelican (Kodak 6.6Mb)(disc.)
└─────┴──┴───────┴───┴───────┴───┴───┴───┴───┴────┴────────────────────────────
* Michtron DS-DOS 2.11 Plus and one version of MS-DOS 3.11 (vendor unknown)
** TallTree JFormat program
*** Pelican driver source calls for 2 sectors/cluster, Norton Utils reports 4.
┌─────┬──┬───────┬─┴─┬───┴───┬─┴─┬─┴─┬─┴─┬─┴─┬──┴─┬────────────────────────────
│ 400k│5¼│DOS2.11│ 1 │10 (80)│ │ │ │ │ 800│DEC Rainbow SS/HD (disc.)
│ 720k│5¼│DOS2.11│ 2 │variable number of sectors │Victor 9000 PC (discont'd)
└─────┴──┴───────┴───┤per track, more sectors on ├────────────────────────────
│outer tracks than inner │
│tracks. Special DSDD drive. │
└────────────────────────────┘
Some oddball DOS versions specify "zero" heads in their data area. HP's
single-sided disk format (16 256-byte sectors/track, model unknown) uses a
zero-based parameter for the number of heads. Without special device driver
software to "fix" this, these disks are basically unusable by normal DOS.
A couple of people have reported that the IBM "Gearbox" industrial PC uses
an 800k 3.5 inch floppy with 10 sectors and 80 tracks. I've been unable to
confirm this.
Files in the data area are not necessarily written sequentially. The data area
space is allocated one cluster at a time, skipping over clusters already
allocated. The first free cluster found is the next cluster allocated,
regardless of its physical location on the disk. This permits the most efficient
utilization of disk space because clusters freed by erasing files can be
allocated for new files. Refer back to the description of the DOS FAT in this
chapter for more information.
SSDD single sided, double density (160-180k) 5¼
DSDD double sided, double density (320-360k) 5¼
DSQD double sided, quad density (720k) 5¼, 3½, 2
DSHD double sided, high density (1.2-1.44M) 5¼, 3½
DSED double sided, extra high density (2.88M) 3½
Much of the trouble with AT 1.2 meg drives has been through the inadverdent
use of quad density disks in the high density drives. The high density disks
use a higher-coercivity media than the quads, and quads are not completely
reliable as 1.2Mb. Make sure you have the correct disk for your application.
ROTATION SPEEDS:
720k, 3½" (unknown) note: Zenith has discontinued 2" floppies
720k, 3½" 300 RPM
1.44Mb, 3½" 300 RPM
360k, 5¼" 300 RPM
720k, 5¼" 300 RPM
1.2mb, 5¼" 360 RPM (even when reading 360k diskettes)
all 8" 360 RPM
The Victor 9000's 5¼" floppies could vary their rotational speed as required.
This allowed them to put 720k on a standard 360k floppy, using a constant
density throughout.
The Central Point CopyIIPC Option Board emulates the Macintosh GCR recording
format by varying the data rate instead of the rotational speed.
HARD DISK LAYOUT ...................................................... 8**11
The DOS hard disk routines perform the following services:
1) Allow multiple operating systems to be installed on the hard disk at the
same time.
2) Allow a user-selected operating system to be started from the hard disk.
I) In order to share the hard disk among operating systems, the disk may be
logically divided into 1 to 4 partitions. The space within a given
partition is contiguous, and can be dedicated to a specific operating
system. Each operating system may "own" only one partition in DOS versions
2.0 through 3.2. DOS 3.3 introduced the "Extended DOS Partition" which
allows multiple DOS partitions on the same hard disk. FDISK (or a
similar program from other DOS vendors) utility allows the user to select
the number, type, and size of each partition. The partition information is
kept in a partition table that is embedded in the master hard disk boot
record on the first sector of the disk. The format of this table varies
from version to version of DOS.
II) An operating system must consider its partition to be the entire disk,
and must ensure that its functions and utilities do not access other
partitions on the disk.
III) Each partition may contain a boot record on its first sector, and any
other programs or data that you choose, including a different operating
system. For example, the DOS FORMAT command may be used to format and
place a copy of DOS in the DOS partition in the same manner that a
diskette is formatted. You can use FDISK to designate a partition as
"active" (bootable). The master hard disk boot record causes that
partition's boot record to receive control when the system is
initialized. Additional disk partitions could be FORTH, UNIX, Pick,
CP/M-86, OS/2 HPFS, Concurrent DOS, Xenix, or the UCSD p-System.
SYSTEM INITIALIZATION ................................................. 8**12
The boot sequence is as follows:
1. System initialization first attempts to load an operating system from
diskette drive A. If the drive is not ready or a read error occurs, it then
attempts to read a master hard disk boot record on the first sector of the
first hard disk in the system. If unsuccessful, or if no hard disk is
present, it invokes ROM BASIC in an IBM PC or displays a disk error
message on most compatibles.
2. If initialization is successful, the master hard disk boot record is given
control and it examines the partition table embedded within it. If one of
the entries indicates an active (bootable) partition, its boot record is
read from the partition's first sector and given control.
3. If none of the partitions is bootable, ROM BASIC is invoked on an IBM PC or
a disk error on most compatibles.
4. If any of the boot indicators are invalid, or if more than one indicator is
marked as bootable, the message "INVALID PARTITION TABLE "is displayed and
the system stops.
5. If the partition's boot record cannot be successfully read within five
retries due to read errors, the message "ERROR LOADING OPERATING SYSTEM"
appears and the system stops.
6. If the partition's boot record does not contain a valid "signature", the
message "MISSING OPERATING SYSTEM" appears, and the system stops.
NOTE: When changing the size or location of any partition, you must ensure that
all existing data on the disk has been backed up. The partitioning program
will destroy the data on the disk.
System programmers designing a utility to initialize/manage a hard disk must
provide the following functions at a minimum:
1. Write the master disk boot record/partition table to the disk's first
sector to initialize it.
2. Perform partitioning of the disk - that is, create or update the partition
table information (all fields for the partition) when the user wishes
to create a partition. This may be limited to creating a partition for only
one type of operating system, but must allow repartitoning the entire disk,
or adding a partition without interfering with existing partitions (user's
choice).
3. Provide a means for marking a user-specified partition as bootable and
resetting the bootable indicator bytes for all other partitions at the same
time.
4. Such utilities should not change or move any partition information that
belongs to another operating system.
BOOT RECORD/PARTITION TABLE ........................................... 8**13
A boot record must be written on the first sector of all hard disks, and
must contain the following:
1. Code to load and give control to the boot record for one of four possible
operating systems.
2. A partition table at the end of the boot record. Each table entry is 16
bytes long, and contains the starting and ending cylinder, sector, and head
for each of four possible partitions, as well as the number of sectors
preceding the partition and the number of sectors occupied by the partition.
The "boot indicator" byte is used by the boot record to determine if one of
the partitions contains a loadable operating system. FDISK initialization
utilities mark a user-selected partition as "bootable" by placing a value
of 80h in the corresponding partition's boot indicator (setting all other
partitions' indicators to 0 at the same time). The presence of the 80h tells
the standard boot routine to load the sector whose location is contained in
the following three bytes. That sector is the actual boot record for the
selected operating system, and it is responsible for the remainder of the
system's loading process (as it is from the diskette). All boot records are
loaded at absolute address 0:7C00.
The partition table with its offsets into the boot record is:
┌──────────┬──────────┬──────────┬────────┬────────────────────────────────────
│ Offset │ Offset │ Offset │ │
│from Start│from Start│from Start│ Size │ Description
│ of Disk │ of Entry │ of Disk │ │
├──────────┼──────────┼──────────┼────────┼────────────────────────────────────
│ │ 00h │ 0BEh │ 1 byte │ boot indicator
│ │ 01h │ 0BFh │ 1 byte │ beginning head
│ 1BEh │ 02h │ 0C0h │ 1 byte │ beginning sector
│ (part 1) │ 03h │ 0C1h │ 1 byte │ beginning cylinder
│ 16 bytes │ 04h │ 0C2h │ 1 byte │ system indicator
│ │ 05h │ 0C3h │ 1 byte │ ending head
│ │ 06h │ 0C4h │ 1 byte │ ending sector
│ │ 07h │ 0C5h │ 1 byte │ ending cylinder
│ │ 08h │ 0C6h │ 4 bytes│ relative (starting) sector
│ │ 0Ch │ 0DAh │ 4 bytes│ number of sectors
├──────────┼──────────┼──────────┼────────┼────────────────────────────────────
│ │ 00h │ 0DEh │ 1 byte │ boot indicator
│ │ 01h │ 0DFh │ 1 byte │ beginning head
│ 1CEh │ 02h │ 0E0h │ 1 byte │ beginning sector
│ (part 2) │ 03h │ 0E1h │ 1 byte │ beginning cylinder
│ 16 bytes │ 04h │ 0E2h │ 1 byte │ system indicator
│ │ 05h │ 0E3h │ 1 byte │ ending head
│ │ 06h │ 0E4h │ 1 byte │ ending sector
│ │ 07h │ 0E5h │ 1 byte │ ending cylinder
│ │ 08h │ 0E6h │ 4 bytes│ relative (starting) sector
│ │ 0Ch │ 0EAh │ 4 bytes│ number of sectors
├──────────┼──────────┼──────────┼────────┼────────────────────────────────────
│ │ 00h │ 0FEh │ 1 byte │ boot indicator
│ │ 01h │ 0FFh │ 1 byte │ beginning head
│ 1DEh │ 02h │ 0100h │ 1 byte │ beginning sector
│ (part 3) │ 03h │ 0101h │ 1 byte │ beginning cylinder
│ 16 bytes │ 04h │ 0102h │ 1 byte │ system indicator
│ │ 05h │ 0103h │ 1 byte │ ending head
│ │ 06h │ 0104h │ 1 byte │ ending sector
│ │ 07h │ 0105h │ 1 byte │ ending cylinder
│ │ 08h │ 0106h │ 4 bytes│ relative (starting) sector
│ │ 0Ch │ 010Ah │ 4 bytes│ number of sectors
├──────────┼──────────┼──────────┼────────┼────────────────────────────────────
│ │ 00h │ 010Eh │ 1 byte │ boot indicator
│ │ 01h │ 011Fh │ 1 byte │ beginning head
│ 1EEh │ 02h │ 0110h │ 1 byte │ beginning sector
│ (part 4) │ 03h │ 0111h │ 1 byte │ beginning cylinder
│ 16 bytes │ 04h │ 0112h │ 1 byte │ system indicator
│ │ 05h │ 0113h │ 1 byte │ ending head
│ │ 06h │ 0114h │ 1 byte │ ending sector
│ │ 07h │ 0115h │ 1 byte │ ending cylinder
│ │ 08h │ 0116h │ 4 bytes│ relative (starting) sector
│ │ 0Ch │ 011Ah │ 4 bytes│ number of sectors
├──────────┼──────────┴──────────┼────────┼────────────────────────────────────
│ 1FEh │ │ 2 bytes│ 055AAh signature
└──────────┴─────────────────────┴────────┴────────────────────────────────────
Boot indicator (boot ind): The boot indicator byte must contain 0 for a non-
bootable partition or 80h for a bootable partition. Only one partition can be
marked as bootable at a time.
System Indicator (sys ind): The sys ind field contains an indicator of the
operating system that "owns" the partition. IBM PC-DOS can only "own" one
partition, though some versions of MSDOS allow all four partitions to be used
by DOS.
The system indicators are:
┌─────────────────────────────────────────────────────────────┐
│ System Indicator (sys ind) │
├───────┬─────────────────────────────────────────────────────┤
│ 00h │ unknown or no partition defined │
├───────┼─────────────────────────────────────────────────────┤
│ 01h │ DOS 12 bit FAT (DOS 2.x all and 3.x+ under 16 Mb) │
│ │ less than 4086 clusters │
├───────┼─────────────────────────────────────────────────────┤
│ 02h │ Xenix │
├───────┼─────────────────────────────────────────────────────┤
│ 03h │ Xenix │
├───────┼─────────────────────────────────────────────────────┤
│ 04h │ DOS 16 bit FAT (DOS 3.0+. Not recognized by 2.x) │
│ │ less than 65,536 sectors │
├───────┼─────────────────────────────────────────────────────┤
│ 05h │ extended DOS partition, some 3.2 and all 3.3+ │
│ │ (pointer to further partition table) │
├───────┼─────────────────────────────────────────────────────┤
│ 06h │ Compaq DOS 3.31, DOS 4.0+ partitions over 32 megs │
│ │ Digital Research DRDOS 3.4, 3.41 over 32 megs │
├───────┼─────────────────────────────────────────────────────┤
│ 06h │ PC-MOS/386 partitions over 32 megs (NOT compatible │
│ │ with the DR, Compaq, and MSDOS big partitions! │
├───────┼─────────────────────────────────────────────────────┤
│ 07h │ OS/2 High Performance File System │
├───────┼─────────────────────────────────────────────────────┤
│ 051h │ Ontrack Disk Manager "read/write" partitions │
├───────┼─────────────────────────────────────────────────────┤
│ 0DBh │ DRI Concurrent DOS (>32mb partitions?)│
│ │ DRI Concurrent CP/M? │
├───────┼─────────────────────────────────────────────────────┤
│ 0E4h │ Speedstor, small partitions (?) (under 1024cyl?) │
├───────┼─────────────────────────────────────────────────────┤
│ 0F2h │ 2nd DOS partition, some OEM customized DOS 3.2 │
├───────┼─────────────────────────────────────────────────────┤
│ 0F4h │ Speedstor, large partitions (?) │
├───────┼─────────────────────────────────────────────────────┤
│ 0FEh │ Speedstor, partitions >1024 cylinders │
└───────┴─────────────────────────────────────────────────────┘
There are ID bytes for proprietary formatting schemes. Some manufacturers
(such as Zenith, Wyse, and Tandon) diddle with these system bytes to implement
more than one DOS partition per disk.
note 1) Xenix doesn't like extended DOS partitions a'la DOS 3.3, limiting you
to a DOS partition of 32Mb. Xenix doesn't recognize DOS 4.0x at all,
so to use it you need to boot from a floppy. Early versions of OS/2
also have this problem.
2) I have found one source listing Minix partitions as "40" and some
Unix partitions as "63". I don't know if these are decimal or
hexadecimal figures.
Cylinder (CYL) and Sector (S): The 1 byte fields labelled CYL contain the low
order 8 bits of the cylinder number - the high order 2 bits are in the high
order 2 bits of the sector (S) field. This corresponds with the ROM BIOS
interrupt 13h (disk I/O) requirements, to allow for a 10 bit cylinder number.
The fields are ordered in such a manner that only two MOV instructions are
required to properly set up the DX and CX registers for a ROM BIOS call to
load the appropriate boot record (hard disk booting is only possible from the
first hard disk in the system, where a BIOS drive number of 80h corresponds
to the boot indicator byte).
All partitions are allocated in cylinder multiples and begin on sector 1,
head 0, with the exception that the partition that is allocated at the beginning
of the disk starts at sector 2, to account for the hard disk's master boot
record.
Relative (starting) Sector: The number of sectors preceding each partition
on the disk is kept in this 4 byte field. This value is determined by counting
the sectors beginning with cylinder 0, sector 1, head 0 of the disk, and
incrementing the sector, head, and then track values up to the beginning of
the partition. This, if the disk has 17 sectors per track and 4 heads, and the
second partition begins at cylinder 1, sector 1, head 0, then the partition's
starting relative sector is 68 (decimal) - there were 17 sectors on each of 4
heads on 1 track allocated ahead of it. The field is stored with the least
significant word first.
Number of sectors (#sects): The number of sectors allocated to the partition
is kept in the "# of sects" field. This is a 4 byte field stored least
significant word first.
Signature: The last 2 bytes of the boot record (55AAh) are used as a signature
to identify a valid boot record. Both this record and the partition boot record
are required to contain the signature at offset 1FEh.
HARD DISK TECHNICAL INFORMATION ....................................... 8**14
Western Digital's hard disk installation manuals make the claim that MSDOS
can support only 2 hard drives. This is entirely false, and their purpose for
making the claim is unclear. DOS merely performs a function call pointed at
the hard disk driver, which is normally in one of three locations; a ROM at
absolute address C:800, the main BIOS ROM if the machine is an AT, or a device
driver installed through the CONFIG.SYS file. Two hard disk controller cards
can normally not reside in the same machine due to lack of interrupt
arbitration. Perstor's ARLL controller and some cards marketed by Novell can
coexist with other controllers. Perstor's technical department has had four
controllers and eight hard disks in the same IBM XT functioning concurrently.
A valid hard disk has a boot record arranged in the following manner:
db drive ; 0 or 80h (80h marks a bootable, active
partition)
db head1 ; starting head
dw trksec1 ; starting track/sector (CX value for INT 13)
db system ; SYS IND ID from table above
db head2 ; ending head
dw trksec2 ; ending track/sector
dd sector1 ; absolute # of starting sector
dd sector2 ; absolute # of last sector
The master disk boot record invokes ROM BASIC if no indicator byte reflects a
bootable system.
When a partition's boot record is given control, it is passed its partition
table entry address in the DS:SI registers.
DETERMINING HARD DISK ALLOCATION ...................................... 8**15
DOS determines disk allocation using the following formula:
D * BPD
TS - RS - ───────────
BPS
SPF = ──────────────────────────────
BPS * SPC
CF + ──────────────
BPC
where:
TS Total number of sectors on the disk
RS The number of sectors at the beginning of the disk that are
reserved for the boot record. DOS normally reserves 1 sector.
D The number of directory entries in the root directory.
BPD The number of bytes per directory entry. This is always 32.
BPS The number of bytes per logical sector. Typically 512, but you can
specify a different number with VDISK.
CF The number of FATS per disk. Usually 2. VDISK is 1.
SPF The number of sectors per FAT. Maximum 64.
SPC The number of sectors per allocation unit (cluster).
BPC The number of bytes per FAT entry. BPC is 1.5 for 12 bit FATs.
2 for 16 bit FATS.
To calculate the minimum partition size that will force a 16-bit FAT:
CYL = (max clusters * 8)/(HEADS * SPT)
where:
CYL number of cylinders on the disk
max clusters 4092 (maximum number of clusters for a 12 bit FAT)
HEADS number of heads on the hard disk
SPT sectors per track (normally 17 on MFM)
DOS 2.0 through 3.3 limit partition sizes to 32 megabytes. The limit arises
from the fact that DOS does things by sector number, and each sector is stored
as a word. So the largest sector number DOS can count to is 64k. As each
sector is 512 bytes long, 64k * .5k = 32Mb. The easiest way for an aftermarket
disk handler to break the 32Mb barrier is probably to increase the sector size
- with 2k sectors, maximum partiton size increases to 128Mb. However, the BIOS
boot routines and IBMBIO.COM are hardwired for 512 byte sectors, so you won't
be able to boot from a drive with oversize sectors. That's why Disk Manager
formats a small boot partition by default.
DOS 2.x uses a "first fit" algorithm when allocating file space on the hard
disk. Each time an application requests disk space, it will scan from the
beginning of the FAT until it finds a contiguous peice of storage large enough
for the file.
DOS 3.x+ keeps a pointer into the disk space, and begins its search from the
point it last left off. This pointer is lost when the system is rebooted.
This is called the "next fit" algorithm. It is faster than the first fit and
helps minimize fragmentation.
In either case, if the FCB function calls are used instead of the handle
function calls, the file will be broken into pieces starting with the first
available space on the disk.
BIOS Disk Routines .................................................... 8**16
┌─────────────────────────────────────────────────────────────────────────────┐
│Interrupt 13h Disk I/O - access the disk drives (floppy and hard disk) │
└─────────────────────────────────────────────────────────────────────────────┘
(0:004Ch) 1) These calls do not try rereading disk if an error is returned.
2) In the IBM OS/2 Tech Ref Volume 1, page 7-33, under "DOS
Environment Software Interrupt Support", it lists:
13h disk/diskette for non-removable media only, these
functions are supported:
01h read status
02h read sectors
0Ah read long
15h read DASD (disk) type
3) On hard disk systems these calls may be vectored through the
int 40h hard disk BIOS.
Function 00h Reset - reset the disk controller chip
entry AH 00h
DL drive (if bit 7 is set both hard disks and floppy disks reset)
00h-7Fh floppy disk
80h-0FFh hard disk
return AH status (see 01h below)
note 1) Forces controller chip to recalibrate read/write heads.
2) Some systems (Sanyo 55x, Columbia MPC) this resets all drives.
3) This function should be called after a failed floppy disk Read, Write,
Verify, or Format request before retrying the operation.
4) If called with DL >= 80h (i.e., selecting a hard drive), the floppy
controller and then the hard disk controller are reset.
5) Function 0Dh allows the hard disk controller to be reset without
affecting the floppy controller.
Function 01h Get Status of Disk System
entry AH 01h
DL drive (hard disk if bit 7 set)
00h-7Fh floppy disk
80h-0FFh hard disk
return AH 00h
AL status of most recent disk operation
00h successful completion, no errors
01h bad command
02h address mark not found
03h tried to write on write-protected disk (floppy only)
04h sector not found
05h reset failed (hard disk)
06h diskette removed or changed (floppy only)
07h bad parameter table (hard disk)
08h DMA overrun (floppy only)
09h attempt to DMA across 64K boundary
0Ah bad sector detected (hard disk)
0Bh bad track detected (hard disk)
0Ch unsupported track or media type not found (floppy disk)
0Dh invalid number of sectors on format (hard disk)
0Eh control data address mark detected (hard disk)
0Fh DMA arbitration level out of range (hard disk)
10h uncorrectable CRC/EEC on read
11h ECC corrected data error (hard disk)
20h controller failure
40h seek failed
80h timeout
0AAh drive not ready (hard disk)
0BBh undefined error (hard disk)
0CCh write fault (hard disk)
0E0h status error (hard disk)
0FFh sense operation failed (hard disk)
note 1) For hard disks, error code 11h (ECC data error) indicates that a
recoverable error was detected during a preceding int 13h fn 02h
(Read Sector) call.
Function 02h Read Sectors - read one or more sectors from diskette
entry AH 02h
AL number of sectors to read
BX address of buffer (ES=segment)
CH track (cylinder) number (0-39 or 0-79 for floppies)
(for hard disk, bits 8,9 in high bits of CL)
CL sector number (1 to 18, not value checked)
DH head number (0 or 1)
DL drive (0=A, 1=B, etc.) (bit 7=0) (drive 0-7)
00h-7Fh floppy disk
80h-FF0h hard disk
ES:BX address to store/fetch data (buffer to fill)
[0000:0078] dword pointer to diskette parameters
return CF clear successful
AL number of sectors transferred
set error
AH status (00h, 02h, 03h, 04h, 08h, 09h, 10h,
0Ah, 20h, 40h, 80h)
note 1) Number of sectors begins with 1, not 0.
2) Trying to read zero sectors is considered a programming error; results
are not defined.
3) For hard disks, the upper 2 bits of the 10-bit cylinder number are
placed in the upper 2 bits of register CL.
4) For hard disks, error code 11h indicates that a read error occurred
that was corrected by the ECC algorithm; in this case, AL contains the
burst length. The data read is good within the limits of the ECC code.
If a multisector transfer was requested, the operation was terminated
after the sector containing the read error.
5) For floppy drives, an error may result from the drive motor being off
at the time of the request. The BIOS does not automatically wait for
the drive to come up to speed before attempting the read operation. The
calling program should reset the floppy disk system with function 00h
and retry the operation three times before assuming that the error
results from some other cause.
Function 03h Write Sectors - write from memory to disk
entry AH 03h
AL number of sectors to write (1-8)
CH track number (for hard disk, bits 8,9 in high bits of CL)
CL beginning sector number
(if hard disk, high two bits are high bits of track #)
DH head number (head 0=0)
DL drive number (0-7)
00h-7Fh floppy disk
80h-FF0h hard disk
ES:BX address of buffer for data
return CF clear success
AL number of sectors written
set error
AH status (see 01h above)
note 1) Number of sectors begins with 1, not 0.
2) Trying to write zero sectors is considered a programming error; results
are not defined.
3) For hard disks, the upper 2 bits of the 10-bit cylinder number are
placed in the upper 2 bits of register CL.
4) For floppy drives, an error may result from the drive motor being off
at the time of the request. The BIOS does not automatically wait for
the drive to come up to speed before attempting the read operation. The
calling program should reset the floppy disk system with function 00h
and retry the operation three times before assuming that the error
results from some other cause.
Function 04h Verify - verify that a write operation was successful
entry AH 04h
AL number of sectors to verify (1-8)
CH track number (for hard disk, bits 8,9 in high bits of CL)
CL beginning sector number
DH head number
DL drive number (0-7)
DL drive number (0-7)
00h-7Fh floppy disk
80h-FF0h hard disk
ES:BX address of buffer for data
return CF set on error
AH status (see 01h above)
AL number of sectors verified
note 1) With IBM PC, XT, and AT with ROM BIOS earlier than 11/15/85, ES:BX
should point to a valid buffer.
2) For hard disks, the upper 2 bits of the 10-bit cylinder number are
placed in the upper 2 bits of register CL.
3) This function can be used to test whether a readable media is in a
floppy drive. An error may result from the drive motor being off at the
time of the request since the BIOS does not automatically wait for the
drive to come up to speed before attempting the verify operation. The
requesting program should reset the floppy disk system with function
00h and retry the operation three times before assuming that a readable
disk is not present.
Function 05h Format Track - write sector ID bytes for 1 track (floppy disk)
entry AH 05h
AL number of sectors to create on this track
interleave (for XT hard disk only)
CH track (or cylinder) number (bits 8,9 in high bits of CL)
CL sector number
DH head number (0, 1)
DL drive number (0-3)
00h-7Fh floppy disk
80h-0FFh hard disk
ES:BX pointer to 4-byte address field (C-H-R-N) (except XT hard disk)
byte 1 = (C) cylinder or track
byte 2 = (H) head
byte 3 = (R) sector
byte 4 = (N) bytes/sector (0 = 128, 1 = 256, 2 = 512, 3 = 1024)
return CF set if error occurred
AH status code (see 01h above)
note 1) Not valid for ESDI hard disks on PS/2.
2) For floppy disks, the number of sectors per track is taken from the
BIOS floppy disk parameter table whose address is stored in the vector
for int 1Eh.
3) When this function is used for floppies on ATs or the PS/2, it should
be preceded by a call to int 13h/fn 17h to select the type of media to
format.
4) For hard disks, the upper 2 bits of the 10-bit cylinder number are
placed in the upper 2 bits of CL.
5) On the XT/286, AT, and PS/2 hard disks, ES:BX points to a 512-byte
buffer containing byte pairs for each physical disk sector as follows:
Byte Contents
0 00h good sector
80h bad sector
1 sector number
For example, to format a track with 17 sectors and an interleave of
two, ES:BX would point to the following 34-byte array at the beginning
of a 512-byte buffer:
db 00h, 01h, 00h, 0Ah, 00h, 02h, 00h, 0Bh, 00h, 03h, 00h, 0Ch
db 00h, 04h, 00h, 0Dh, 00h, 05h, 00h, 0Eh, 00h, 06h, 00h, 0Fh
db 00h, 07h, 00h, 10h, 00h, 08h, 00h, 11h, 00h, 09h
Function 06h Hard Disk - format track and set bad sector flags
(PC2, PC-XT, and Portable)
entry AH 06h
AL interleave value (XT only)
CH cylinder number (bits 8,9 in high bits of CL)
CL sector number
DH head
DL drive (80h-0FFh for hard disk)
ES:BX 512 byte format buffer
the first 2*(sectors/track) bytes contain f,n for each sector
f 00h good sector
80h bad sector
n sector number
return CF error
AH status code (see 01h above)
Function 07h Hard Disk - format the drive starting at the desired track
(PC2, PC-XT and Portable)
entry AH 07h
AL interleave value (XT only) (01h-10h)
CH cylinder number (bits 8,9 in high bits of CL) (00h-03FFh)
CL sector number
DH head number (0-7)
DL drive number (80h-0FFh, 80h=C, 81h=D,...)
ES:BX format buffer, size = 512 bytes
the first 2*(sectors/track) bytes contain f,n for each sector
f 00h good sector
80h bad sector
n sector number
return CF set on error
AH status code (see 01h above)
note Award AT BIOS routines are extended to handle more than 1024 cylinders.
AL number of sectors
CH cylinder numberm low 8 bits
CL sector number bits 0-5, bits 6-7 are high 2 cylinder bits
DH head number (bits 0-5) bits 6-7 are extended high cyls (>1024)
DL drive number (0-1 for diskette, 80h-81h for hard disk)
ES:BX transfer address
Function 08h Read Drive Parameters (except PC, Jr)
entry AH 08h
DL drive number
00h-7Fh floppy disk
80h-0FFh hard disk
return CF set on error
AH status code (see above)
BL drive type (AT/PS2 floppies only)
01h if 360 Kb, 40 track, 5¼"
02h if 1.2 Mb, 80 track, 5¼"
03h if 720 Kb, 80 track, 3½"
04h if 1.44 Mb, 80 track, 3½"
CH low 8 bits of maximum useable value for cylinder number
CL bits 6-7 high-order 2 bits of maximum cylinder number
0-5 maximum sector number
DH maximum usable value for head number
DL number of consecutive acknowledging drives (0-2)
ES:DI pointer to drive parameter table
note 1) On the PC and PC/XT, this function is supported on hard disks only.
2) The Columbia MPC supports functions 6-14 for its hard disk. It returns
drive information, same as int 13 function 8, except that the BL and
ES:DI values are omitted and AL <- burst length.
Function 09h Initialize Two Fixed Disk Base Tables (XT, AT, XT/286, PS/2)
(install nonstandard drive)
entry AH 09h
DL 80h-0FFh hard disk number
return CF set on error
AH status code (see 01h above)
For PC, XT hard disks, the disk parameter block format is:
00h-01h maximum number of cylinders
02h maximum number of heads
03h-04h starting reduced write current cylinder
05h-06h starting write precompensation cylinder
07h maximum ECC burst length
08h drive options
bits 7 1 disable disk access retries
6 1 disable ECC retries
3-5 set to 0
0-2 drive option
09h standard timeout value
0Ah timeout value for format drive
0Bh timeout value for check drive
0Ch-0Fh reserved
For AT and PS/2 hard disks:
00h-01h maximum number of cylinders
02h maximum number of heads
03h-04h reserved
05h-06h starting write precompensation cylinder
07h maximum ECC burst length
08h drive options byte
bits 6-7 nonzero (10, 01, or 11) if retries disabled
5 1 if manufacturer's defect map present at
maximum cylinder + 1
4 not used
3 1 if more than 8 heads
0-2 not used
09h-0Bh reserved
0Ch-0Dh landing zone cylinder
0Eh sectors per track
0Fh reserved
note 1) For the XT, int 41h must point to the Disk Parameter Block.
2) For the AT and PS/2, int 41h points to table for drive 0 and int 46h
points to table for drive 1.
3) Initializes the hard disk controller for subsequent I/O operations
using the values found in the BIOS disk parameter block(s).
4) This function is supported on hard disks only.
Function 0Ah Read Long (Hard disk) (XT, AT, XT/286, PS/2)
entry AH 0Ah
CH cylinder number (bits 8,9 in high bits of CL)
CL sector number (upper 2 bits of cyl # in upper 2 bits of CL)
DH head number
DL drive ID (80h-0FFh hard disk)
ES:BX pointer to buffer to fill
return CF set on error
AH status code (see 01h above)
AL number of sectors actually transferred
note 1) A "long" sector includes a 4 byte EEC (Extended Error Correction) code.
2) Used for diagnostics only on PS/2 systems.
3) This function is supported on fixed disks only.
4) Unlike the normal Read Sector (02h) function, ECC errors are not
automatically corrected. Multisector transfers are terminated after any
sector with a read error.
Function 0Bh Write Long (XT, AT, XT/286, PS/2)
entry AH 0Bh
AL number of sectors
CH cylinder (bits 8,9 in high bits of CL)
CL sector number
DH head number
DL drive ID (80h-0FFh hard disk)
ES:BX pointer to buffer containing data
return CF set on error
AH status code (see 01h above)
AL number of sectors actually transferred
note 1) A "long" sector includes a 4 byte EEC (Extended Error Correction) code.
2) Used for diagnostics only on PS/2 systems.
3) Valid for hard disks only.
Function 0Ch Seek To Cylinder (except PC, PCjr)
entry AH 0Ch
CH lower 8 bits of cylinder
CL upper 2 bits of cylinder in bits 6-7
DH head number
DL drive number (0 or 1) (80h-0FFh for hard disk)
return CF set on error
AH status code (see 01h above)
note 1) Positions heads over a particular cylinder, but does not move any data.
2) This function is supported on hard disks only.
3) The upper 2 bits of the 10-bit cylinder number are placed in the upper
2 bits of CL.
4) The Read Sector, Read Sector Long, Write Sector, and Write Sector Long
functions include an implied seek operation and need not be preceded by
an explicit call to this function.
Function 0Dh Alternate Hard Disk Reset (except PC, PCjr)
entry AH 0Dh
DL hard drive number (80h-0FFh hard disk)
return CF set on error
AH status code (see 01h above)
note 1) Not for PS/2 ESDI hard disks.
2) Resets the hard disk controller, recalibrates attached drives (moves
the read/write arm to cylinder 0), and prepares for subsequent disk I/O.
3) This function is for hard disks only. It differs from fn 00h by not
resetting the floppy disk controller.
Function 0Eh Read Sector Buffer (XT, Portable, PS/2)
entry AH 0Eh
ES:BX pointer to buffer
return CF set on error
AH status code (see 01h above)
AL number of sectors actually transferred
note 1) Transfers controller's sector buffer. No data is read from the drive.
2) Used for diagnostics only on PS/2 systems.
3) This fn is supported by the XT's hard disk adapter only. It is "not
defined" for hard disk adapters on the AT or PS/2.
Function 0Fh Write sector buffer (XT, Portable)
entry AH 0Fh
ES:BX pointer to buffer
return CF set if error
AH status code (see 01h above)
AL number of sectors actually transferred
note 1) Should be called before formatting to initialize the controller's
sector buffer.
2) Used for diagnostics only on PS/2 systems.
3) Transfers data from system RAM to the hard disk adapter's internal
sector buffer.
4) No data is written to the physical disk drive.
5) This fn is for the XT hard disk controller only. It is "not defined"
for AT or PS/2 controllers.
Function 10h Test For Drive Ready (XT, AT, XT/286, PS/2)
entry AH 10h
DL hard drive number 0 or 1 (80h-0FFh)
return CF set on error
AH status code (see 01h above)
note 1) Tests whether the specified hard disk drive is operational and returns
the drive's status.
2) This function is supported on hard disks only.
3) Perstor and Novell controllers allow more than one controller. Does
not work for multiple Perstor controllers. (reports first two drives
only).
4) Does not work with network drives.
Function 11h Recalibrate Drive (XT, AT, XT/286, PS/2)
entry AH 11h
DL hard drive number (80h-0FFh hard disk)
return CF set on error
AH status code (see 01h above)
note 1) Causes the HD controller to recalibrate itself for the specified drive,
positioning the read/arm to cylinder 0, and returns the drive's status.
2) This function is for hard disks only.
Function 12h Controller RAM Diagnostics (XT, Portable, PS/2)
entry AH 12h
return CF set on error
AH status code (see fn 01h above)
note 1) Used for diagnostics only on PS/2 systems.
2) Makes the hard disk controller carry out a built-in diagnostic test on
its internal sector buffer.
Function 13h Controller Drive Diagnostic (XT, Portable, PS/2)
entry AH 13h
return CF set on error
AH status code (see 01h above)
note 1) Used for diagnostics only on PS/2 systems.
2) Causes HD controller to run internal diagnostic tests of the attached
drive, indicating whether the test was passed by the returned status.
3) This function is supported on XT HDs only.
Function 14h Controller Internal Diagnostic (AT, XT/286)
entry AH 14h
return CF set on error
AH status code (see 01h above)
note 1) OEM is Western Digital 1003-WA2 hard/floppy combination controller
in AT and XT/286.
2) Used for diagnostics only in PS/2 systems.
3) Causes HD controller to do a built-in diagnostic self-test, indicating
whether the test was passed by the returned status.
4) This function is supported on hard disks only.
Function 15h Get Disk Type (except PC and XT)
entry AH 15h
DL drive ID
00h-7Fh floppy disk
80h-0FFh fixed disk
return CF set on error
AH error code (see 01h above)
AH disk type
00h no drive is present
01h diskette, no change detection present
02h diskette, change detection present
03h hard disk
CX:DX number of 512-byte sectors
note 1) Returns a code indicating the type of disk referenced by the specified
drive code.
2) This function is not supported on the PC or XT.
Function 16h Get Disk Change Status (diskette) (except PC, XT, & Jr)
entry AH 16h
DL drive to check
return CF set on error
AH disk change status
00h no disk change
01h disk changed
DL drive that had disk change (00h-07Fh floppy disk)
note Returns the status of the change line, indicating whether the disk in
the drive may have been replaced since the last disk access. If this
function returns with CF set, the disk has not necessarily been
changed; the change line can be activated by simply unlocking and
relocking the disk drive door without removing the floppy disk.
Function 17h Set Disk Type for Format (diskette) (except PC and XT)
entry AH 17h
AL 00h not used
01h 160, 180, 320, or 360Kb diskette in 360kb drive
02h 360Kb diskette in 1.2Mb drive
03h 1.2Mb diskette in 1.2Mb drive
04h 720Kb diskette in 720Kb drive
DL drive number (0-7)
return CF set on error
AH status of operation (see 01h above)
note 1) This function is probably enhanced for the PS/2 series to detect
1.44 in 1.44 and 720k in 1.44.
2) This function is not supported for floppy disks on the PC or XT.
3) If the change line is active for the specified drive, it is reset.
4) The BIOS sets the data rate for the specified drive and media type.
The rate is 250k/sec for double-density media and 500k/sec for high
density media. The proper hardware is required.
Function 18h Set Media Type For Format (diskette) (AT, XT2, XT/286, PS/2)
entry AH 18h
CH lower 8 bits of number of tracks
CL high 2 bits of number of tracks (6,7) sectors per track
(bits 0-5)
DL drive number (0-7)
return CF clear no errors
AH 00h if requested combination supported
01h if function not available
0Ch if not suppported or drive type unknown
80h if there is no media in the drive
ES:DI pointer to 11-byte disk parameter table for media type
CF set error code (see 01h above)
note 1) A floppy disk must be present in the drive.
2) This function should be called prior to formatting a disk with Int 13h
Fn 05h so the BIOS can set the correct data rate for the media.
3) If the change line is active for the specified drive, it is reset.
Function 19h Park Hard Disk Heads (PS/2)
entry AH 19h
DL drive number (80h-0FFh)
return CF set on error
AH error code (see fn 01h)
note This function is defined for PS/2 fixed disks only.
Function 1Ah ESDI Hard Disk - Low Level Format (PS/2)
entry AH 1Ah
AL Relative Block Address (RBA) defect table count
00h no errors on disk
01h+ number of disk errors
CL format modifiers byte
bits 0 ignore primary defect map
1 ignore secondary defect map
2 update secondary defect map
3 perform extended surface analysis
4 generate periodic interrupt after each cylinder format
5 reserved - must be 0
6 reserved - must be 0
7 reserved - must be 0
DL drive (80h-0FFh)
ES:BX pointer to RBA defect table
return CF set on error
AH error code (see fn 01h above)
note 1) Initializes disk sector and track address fields on a drive attached
to the IBM "ESDI Fixed Disk Drive Adapter/A."
2) If periodic interrupt selected, int 15h/fn 0Fh is called after each
cylinder is formatted
3) If bit 4 of CL is set, Int 15h, AH=0Fh, AL=phase code after each
cylinder is formatted or analyzed. The phase code is defined as:
0 reserved
1 surface analysis
2 formatting
4) If bit 2 of CL is set, the drive's secondary defect map is updated to
reflect errors found during surface analysis. If both bit 2 and bit 1
are set, the secondary defect map is replaced.
5) For an extended surface analysis, the disk should first be formatted by
calling this function with bit 3 cleared and then analyzed by calling
this function with bit 3 set.
Function 1Bh ESDI Hard Disk - Get Manufacturing Header (PS/2)
entry AH 1Bh
AL number of record
DL drive
ES:BX pointer to buffer for manufacturing header (defect list)
return CF set on error
AH status
note Manufacturing header format (Defect Map Record format) can be found
in the "IBM 70Mb, 115Mb Fixed Disk Drives Technical Reference."
Function 1Ch ESDI Hard Disk - Get Configuration (PS/2)
entry AH 1Ch
AL 0Ah Get Device Configuration
DL drive
ES:BX pointer to buffer for device configuration
(drive physical parameter)
0Bh Get Adapter Configuration
ES:BX pointer to buffer for adapter configuration
0Ch Get POS Information
ES:BX pointer to POS information
0Dh unknown
0Eh Translate RBA to ABA
CH low 8 bits of cylinder number
CL sector number, high two bits of cylinder number
in bits 6 and 7
DH head number
DL drive number
ES:BX pointer to ABA number
return CF set on error
AH status (see 01h)
note 1) Device configuration format can be found in IBM ESDI Fixed Disk Drive
Adapter/A Technical Reference.
2) ABA (absolute block address) format can be found in IBM ESDI Adapter
Technical Reference by using its Device Configuration Status Block.
Function 1Dh IBMCACHE.SYS (PS/2 50+)
entry AH 1Dh
other parameters unknown
note IBMCACHE.SYS comes on the setup disk for MCA-bus PS/2 machines.
Function 20h Western Digital HD SuperBIOS
entry AH 20h
other parameters unknown
note SuperBIOS may be purchased separately from Western Digital and added
to standard HD controllers. SuperBIOS contains additional setup
tables and parameters.
