BPB: BIOS Parameter Block

 This is a subset of the data found in the Boot Sector Layout used by
 device drivers. The 'Build BPB' device driver command (DvRq 02H) expects
 the driver to fill a buffer in this layout.
 Note the DOS 4.0 extensions (Extended BPB) at the end of the block.
BPBRec
 Offset Size Contents
 ▀▀▀▀▀▀ ▀▀▀▀ ▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
 +0 2 wSectSize bytes per sector (usually 512; 200H)
 +2 1 bClustSects sectors per allocation unit (cluster)
 +3 2 wResSects boot and reserved sectors
 +5 1 bFatCnt number of File Allocation Tables. See FAT.
 +6 2 wRootEntries max number of 32-byte DirEntryRecs in root
 +8 2 wTotSects total number of sectors in media
 0000H means>32 MB, so use lBigTotSects
 +0aH 1 bMedia media descriptor (same as 1st byte in FAT)
 +0bH 2 wFatSects number of sectors in one FAT
 ───────────────────────── Extended BPB additions
 +0dH 2 wSectsPerTrk sectors per track
 +0fH 2 wHeads number of read/write heads
 +11H 4 lHidSects hidden sectors (ignore hiword in pre-DOS 4.0)
 +15H 4 lBigTotSects 32-bit TotSects in volume (partitions> 32M)
 25 Length of a BPBRec (was 13 in pre-DOS 4.0)
 wResSects The number of "reserved sectors", starting with absolute
 sector 0. This counts how many sectors come before the first
 FAT in the partition. It is usually 0001H; that is only the
 boot sector comes before the first FAT sector.
 bFatCnt The number of File Allocation Tables. Usually 02H; The second
 FAT is just a copy of the first.
wRootEntries The size of the root directory, given in 32-byte units. The
 root directory has a fixed size, but subdirectory space is
 allocated as needed.
 wTotSects Total size of the media, in sectors (wSecSize bytes each).
 DOS 4.0+: This field will be 0000H to indicate a DOS partition
 that is larger than 32 MB. In that case, the total sector
 count is the 32-bit value lBigTotSects.
 bMedia Might identify the media. Media Descriptor.
 ───────────────────────────────────── Extended BPB (DOS 4.0+)
 These fields may help the driver and/or disk utility programs
 to understand the media better. Also, you may need to know
 this stuff when using BIOS Disk I/O (INT 13H).
wSectsPerTrk Sectors per track (one surface of one platter)
 wHeads Disk read/write heads (total surfaces all platters)
 lHidSects Sectors before the start of the 1st FAT in the partition.
lBigTotSects Dword total sectors in the partition. When a partition is
 larger than 32 MB, there are more than 65535 sectors--the
 actual count won't fit in wTotSects so it goes here.
See Also: Boot Sector Layout
 Partition Table
 File Allocation Table
 Device Drivers
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