AH = 53h
        DS:SI points to BPB (Bios Parameter Block)
        ES:BP points to area for DOS Disk Block
Translates BPB (Bios Parameter Block, see below)
into a DOS Disk Block (see function call 32h).

BPB
Bytes   Value
0-1     Bytes/sector. Get from DDB bytes 2-3.
2       Sectors/cluster. Get from: (DDB byte 4) + 1
3-4     Reserved sectors. Get from: DDB bytes 6-7
5       Number of FATs. Get from: DDB byte 8
6-7     Number of root dir entries. Get from: DDB bytes 9-A
8-9     Total # of sectors. Get from:
        ((DDB bytes D-E) - 1) * (sectors per cluster (BPB byte 2))
          + (DDB Bytes B-C)
A       Media descriptor byte. Get from: DDB byte 16
B-C     Number of sectors/FAT. Get from: DDB byte F
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