Low-Level Disk Access Information

This Page contens stuff compiled by Günther Brunthaler

1. DOS Disk Generic Layout

• 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 (per default 32 Sectors on hard disks)
• Data area (first cluster has cluster number 2)

1. The CMOS Parameter Info

Purpose

total # of cylinders of a physical disk drive as auto-detected or specified in the drive documentation. A cylinder is comprised of all the tracks on all discs within a hard disk drive that have the same track number but are read from and written to the discs by different reading/writing heads.

# of heads of a physical drive. Each cylinder is made up of that number of tracks. Head indexes are zero-based, i. e. from 0 to the # of heads minus 1.

# of sectors of a physical drive. Sectors are numbered from 1 to this value on each track. There is the same number of sectors on every track as far as accessing software is concerned, although the actual physical drive layout may be completely different - but this is completely transparent even for the disk controller.

cylinder index for write precompensation or 0FFFFh if unused. Cylinders and tracks are specified as zero-based indexes, i. e. from 0 to the total # of cylinders minus 1.

cylinder index used as landing zone for parking the drive’s heads. Usually the last cylinder of the drive - not used for anything else (thus the highest used cylinder index is the total number of cylinders minus 2). If you set the landing zone to the total number of cylinders, no cylinder is reserved as landing zone.

2. The Boot Sector

Offset	Size	Purpose
000h	3 Bytes	For DOS 2.x, 3 byte near jump (0E9h). For DOS 3.x, 2 byte near jump (0EBh) followed by a NOP (90h). Will be executed at 0000:7C00.
003h	8 Bytes	OEM name and version, e. g. "MSWIN4.0", "MSDBL6.0", "MSDOS5.0", "<5BytesTrash>IHC", "PC TOOLS", "Olivetti", "PSA 1.04"
00Bh	Word	bytes per sector (should be set to 200h)
00Dh	Byte	# of sectors per cluster (must be a power of 2)
00Eh	Word	# of reserved sectors (starting at logical sector 0)
010h	Byte	# of FATs. Usually 2.
011h	Word	max # of root directory entries (32 bytes each). Usually 512 (or 32 sectors) for hard disks.
013h	Word	# of sectors in DOS partition or whole diskette (total # of sectors in media, including boot sector directories, etc.) or 0h if unused (and entry at offset 20h is used instead).
015h	Byte	media descriptor (see section 1.4)
016h	Word	# of sectors occupied by a single FAT. Usually ceiling(HighestUsedClusterIndex / NumberOfFATEntriesPerFATSector). The latter is floor(BytesPerSector / 2) for 16 bit FATs or floor(BytesPerSector / 1.5) for 12 bit FATs.
018h	Word	# of sectors per track
01Ah	Word	# of heads - this equals the number of tracks on each cylinder.
01Ch	DWord	# of hidden sectors (relative sector offset from the containing partition table). Same as relative sectors from partition table entry.
020h	DWord	total # of sectors in logical image (only used if entry at offset 13h is set to 0h)
024h	Byte	physical drive number (00h for first diskette dive:, 01h for next diskette drive, 80h for first hard disk, 81h for next hard disk, etc.)
025h	Byte	? - pad byte? - set to 0h
026h	Byte	extended boot sector signature, set to 29h in order to enable the special meanings for offsets between 26h and 40h. Only this way DOS uses the serial number field and only then the LABEL command writes a copy of the volume name into the boot sector. However, the smallest offset ever observed to be used for the boot program has been 030h so far.
027h	DWord	Serial Number, will be displayed as "<highWord>-<lowWord>"
02Bh	0Bh Bytes	Disk Label, padded with spaces (20h). This entry is set in addition to a corresponding "Volume"-entry in the root directory.
036h	8 Bytes	File System ID "FAT12" or "FAT16" - padded with spaces (20h). Regardless of what is specified here, the actual FAT size will be determined run time by performing the following check: If the highest used cluster index is less than 4087, a 12 bit FAT is assumed, otherwise it is assumed to be a 16 bit FAT.
03Eh	1C0h Bytes	remaining boot code and data, actual entry point will be jumped at from code at offset 000h
1FEh	2 Bytes	Boot Sector Valid signature: bytes 55h, Aah

3. Media Descriptor Byte

Bit	Purpose
7-3	always set to "1"
2	"0" for fixed (i. e. non-removable) devices
1	"0" if there are other than exactly 8 sectors per track
0	"0" if medium is not double-sided

4. DOS Disk Standard Formats

Inch	Total Capacity	DOS Version	Heads	Sectors/ Track	Tracks	Sectors/ FAT	RootDir Sectors	RootDir Entries	Sectors/ Cluster	Total Sectors
5.25	160K	1.0	1	8	40	1	4	64	1	320
5.25	320K	1.1	2	8	40	1	7	112	2	360
5.25	180K	2.0	1	9	40	2	4	64	1	640
5.25	360K	2.0	2	9	40	2	7	112	2	720
5.25	1.2M	3.0	2	15	80	7	14	224	1	2400
3.5	720K	3.2	2	9	80	3	7	112	2	1440
3.5	1.44M	3.3	2	18	80	9	14	224	1	2880

5. Master Boot Record Layout

Offset	Bytes	Purpose
000h	1BEh	Master Boot Code; will be run at 0000:7C00
1Beh	010h	Partition Table Entry for partition #1
1CEh	010h	Partition Table Entry for partition #2
1DEh	010h	Partition Table Entry for partition #3
1EEh	010h	Partition Table Entry for partition #4
1FEh	002h	MBR Valid signature: bytes 055h, 0Aah

6. Partition Table Entry Layout

Offset	Bytes	Purpose
0h	1	boot indicator: 00h if non-bootable, 80h if bootable partition. (This is also the drive number of the boot drive.)
1h	1	partition starting head
2h	1	bits 7-6: bits 9-8 of starting cylinder; bits 5-0: starting sector. Partitions normally start at sector 1 of a cylinder for a given head.
3h	1	bits 7-0 of starting cylinder
4h	1	Operating System Indicator Byte (see section 1.8)
5h	1	partition ending head. Partitions normally end with the last sector of a cylinder for a given head.
6h	1	bits 7-6: bits 9-8 of ending cylinder; bits 5-0: ending sector
7h	1	bits 7-0 of ending cylinder
8h	4	relative sector offset of the first actual partition sector
Ch	4	total # of sectors in partition

7. Operating System Indicator Byte

Value	(Symbol and) Meaning
00h	"Free" - Empty or unspecified non-DOS
01h	"DOS-12" - DOS 12-bit FAT (£ 20740 sectors)
02h	"XENIX" - XENIX root file system
03h	XENIX /usr file system (obsolete)
04h	"DOS-16" - DOS 16-bit FAT (³ 20740 sectors)
05h	"EXTEND" - DOS 3.3+ extended partition, contains nested logical drives (FDISK). An extended partition contains the following components: a partition table in the first sector, and up to four partitions nested within the containing extended partition. But in practice, in each partition table, only the first partition table entry refers to an actual partition, while the second partition is an extended partition and the remaining two partition table entries are unused. Logical drives in the extended partition are actually the first partitions of deeper and deeper nested extended partitions.
06h	"BIGDOS" - DOS 3.31+ Large File System, required for MS-DOS partitions ³ 33 MB
07h	"HPFS" - QNX, OS/2 HPFS, NTFS or Advanced Unix
08h	"Split" - AIX bootable partition or SplitDrive
09h	AIX data partition or Coherent
0Ah	OPUS
10h	PUS
40h	ENIX 80286
50h	(OnTrack?) Disk Manager, read-only partition
51h	Novell??? or (OnTrack?) Disk Manager, read/write partition
52h	CP/M or Microport System V/386
56h	GoldenBow Vfeature
61h	"Speed" - SpeedStor
63h	"386/ix" - Unix SysV/386, 386/ix; or Mach, MtXinu BSD 4.3 on Mach; or GNU HURD
64h	"NET286" - Novell NetWare
65h	"NET386" - Novell NetWare
75h	"PCIX" - PC/IX
80h	Minix v1.1 - 1.4a
81h	Minix v1.4b+ or Linux
82h	Linux Swap partition (planned)
93h	Amoeba file system
94h	Amoeba bad block table
B7h	BSDI file system (secondarily swap)
B8h	BSDI swap partition (secondarily file system)
C6h	DR-DOS 6.0 LOGIN.EXE-secured partition
DBh	"CP/M" - CP/M, Concurrent CP/M, Concurrent DOS; or CTOS (Convergent Technologies OS)
E1h	SpeedStor 12-bit FAT extended partition
E4h	SpeedStor 16-bit FAT extended partition
F2h	DOS 3.3+ secondary
FEh	LANstep
FFh	"BBT" - Xenix bad block table

8. The FAT