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Format of DOS 4.0-7.0 drive data table

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Offset Size Description

00h

DWORD

pointer to next table (offset FFFFh if last table)

04h

BYTE

physical unit number (for INT 13)

05h

BYTE

logical drive number
0 - A 13 - N
1 - B 14 - O
2 - C 15 - P
3 - D 16 - Q
4 - E 17 - R
5 - F 18 - S
6 - G 19 - T
7 - H 20 - U
8 - I 21 - V
9 - J 22 - W
10 - K 23 - X
11 - L 24 - Y
12 - M 25 - Z

06h

25 BYTEs

BIOS Parameter Block (see also INT 21/AH=53h)
Offset Size Description

00h

WORD

bytes per sector

02h

BYTE

sectors per cluster, FFh if unknown

03h

WORD

number of reserved sectors

05h

BYTE

number of FATs

06h

WORD

number of root dir entries

08h

WORD

total sectors (refer to offset 15h if zero)

0Ah

BYTE

media descriptor, 00h if unknown

0Bh

WORD

sectors per FAT

0Dh

WORD

sectors per track

0Fh

WORD

number of heads

11h

DWORD

number of hidden sectors

15h

DWORD

total sectors if WORD at 08h is zero

1Fh

BYTE

flags
bit 6 : 16-bit FAT instead of 12-bit
bit 7 : unsupportable disk (all accesses will return Not Ready)

20h

WORD

device-open count

22h

BYTE

device type (see #01561 at INT 21/AX=440Dh"DOS 3.2+")

23h

WORD

bit flags describing drive (see #02604)

25h

WORD

number of cylinders (for partition only, if hard disk)

27h

25 BYTEs

BIOS Parameter Block for default (highest) capacity supported

40h

6 BYTEs

reserved (part of BPB above)

46h

BYTE

last track accessed

---removable media---

47h

DWORD

time of last access in clock ticks (FFFFFFFFh if never)

---fixed media---

47h

WORD

partition
FFFFFFh = primary
0001h = extended
always 0001h for DOS 5+

49h

WORD

absolute cylinder number of partition's start on physical drive (FFFFh if primary partition in DOS 4.x)

------

4Bh

11 BYTEs

4Bh 11 BYTEs volume label or "NO NAME " if none (apparently taken from extended boot record rather than root directory)

56h

BYTE

terminating null for volume label

57h

DWORD

serial number

5Bh

8 BYTEs

filesystem type ("FAT12 " or "FAT16 ")

63h

BYTE

terminating null for filesystem type

See Also:

#02601,#02602