FATX - Technical Design - Boot Sector - Extended BIOS Parameter Block

Extended BIOS Parameter Block

Further structure used by FAT12 and FAT16 since OS/2 1.0 and DOS 4.0, also known as Extended BIOS Parameter Block (EBPB) (bytes below sector offset 0x024 are the same as for the DOS 3.31 BPB):

Sector Offset EBPB Offset Length (bytes) Description
0x00B 0x00 25 DOS 3.31 BPB
0x024 0x19 1 Physical drive number (0x00 for (first) removable media, 0x80 for (first) fixed disk as per INT 13h). Allowed values for possible physical drives depending on BIOS are 0x00-0x7E and 0x80-0xFE. Values 0x7F and 0xFF are reserved for internal purposes such as remote or ROM boot and should never occur on disk. Some boot loaders such as the MS-DOS/PC DOS boot loader use this value when loading the operating system, others ignore it altogether or use the drive number provided in the DL register by the underlying boot loader (f.e. with many BIOSes and MBRs). The entry is sometimes changed by SYS tools or it can be dynamically fixed up by the prior bootstrap loader in order to force the boot sector code to load the operating system from alternative physical disks than the default.

A similar entry existed (only) in DOS 3.2 to 3.31 boot sectors at sector offset 0x1FD.

If this belongs to a boot volume, the DR-DOS 7.07 enhanced MBR can be configured (see NEWLDR offset 0x014) to dynamically update this EBPB entry to the DL value provided at boot time or the value stored in the partition table. This enables booting off alternative drives, even when the VBR code ignores the DL value.
0x025 0x1A 1 Reserved;
  • In some MS-DOS/PC DOS boot code used as a scratchpad for the INT 13h current head high byte for the assumed 16-bit word at offset 0x024. Some DR-DOS FAT12/FAT16 boot sectors use this entry as a scratchpad as well, but for different purposes.
  • VGACOPY stores a CRC over the system's ROM-BIOS in this location.
  • Some boot managers use this entry to communicate the desired drive letter under which the volume should occur to operating systems such as OS/2 by setting bit 7 and specifying the drive number in bits 6-0 (C: = value 0, D: = value 1, ...). Since this normally affects the in-memory image of the boot sector only, this does not cause compatibility problems with other uses;
  • In Windows NT used for CHKDSK flags (bits 7-2 always cleared, bit 1: disk I/O errors encountered, possible bad sectors, run surface scan on next boot, bit 0: volume is "dirty" and was not properly unmounted before shutdown, run CHKDSK on next boot). Should be set to 0 by formating tools. See also: Bitflags in the second cluster entry in the FAT.
0x026 0x1B 1 Extended boot signature. (Should be 0x29 to indicate that an EBPB with the following 3 entries exists (since OS/2 1.2 and DOS 4.0). Can be 0x28 on some OS/2 1.0-1.1 and PC DOS 3.4 disks indicating an earlier form of the EBPB format with only the serial number following. MS-DOS/PC DOS 4.0 and higher, OS/2 1.2 and higher as well as the Windows NT family recognize both signatures accordingly.)
0x027 0x1C 4 Volume ID (serial number)

Typically the serial number "xxxx-xxxx" is created by a 16-bit addition of both DX values returned by INT 21h/AH=2Ah (get system date) and INT 21h/AH=2Ch (get system time) for the high word and another 16-bit addition of both CX values for the low word of the serial number. Alternatively, some DR-DOS disk utilities provide a /# option to generate a human-readable time stamp "mmdd-hhmm" build from BCD-encoded 8-bit values for the month, day, hour and minute instead of a serial number.
0x02B 0x20 11 Partition Volume Label, padded with blanks (0x20), e.g., "NO␠NAME␠␠␠␠" Software changing the directory volume label in the file system should also update this entry, but not all software does. The partition volume label is typically displayed in partitioning tools since it is accessible without mounting the volume. Supported since OS/2 1.2 and MS-DOS 4.0 and higher.

This area was used by boot sectors of DOS 3.2 to 3.3 to store a private copy of the Disk Parameter Table (DPT) instead of using the INT 1Eh pointer to retrieve the ROM table as in later issues of the boot sector. The re-usage of this location for the mostly cosmetical partition volume label minimized problems if some older system utilities would still attempt to patch the former DPT.
0x036 0x2B 8 File system type, padded with blanks (0x20), e.g., "FAT12␠␠␠", "FAT16␠␠␠", "FAT␠␠␠␠␠"

This entry is meant for display purposes only and must not be used by the operating system to identify the type of the file system. Nevertheless, it is sometimes used for identification purposes by third-party software and therefore the values should not differ from those officially used. Supported since OS/2 1.2 and MS-DOS 4.0 and higher.

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