Mapping DOS FAT to MDFAT

  Any system which analyses a CVF▲ must trace FAT chains and examine the  correspondingMdFatEntryRecs in the MDFAT.  Observe:DirEntryRecstarting cluster number══╗Directory ╓───────────────────┬─┬───────────────────┬───┬───┬─▼─┬───────╖Entry ═══►║M Y F I L E   T X T│a│                   │tim│dat│003│ size  ║          ╙─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─┴─║─┴─┴─┴─┴─╜    ╔══════════════════════════════════════════╝FAT 000  001  002║03  004  005  006  007  008  009  00a  00b ...   ┌─┬─┐┌─┬─┐┌─┬─┐┌▼┬─┐┌─┬─┐╔═══╗┌─┬─┐┌─┬─┐┌─┬─┐┌─┬─┐┌─┬─┐┌─┬─┐   │mid││fff││000││004═►006═╝fff╚►007═►fff││000││000││000││000│...   └─┴─┘└─┴─┘└─┴─┘└─┴─┘└─┴─┘└─┴─┘└─┴─┘└─┴─┘└─┴─┘└─┴─┘└─┴─┘└─┴─┘╚═╦═╝╚═╦═╝╚═╦═╝╚══╩═════════════════════════╗║╚═══════╗╚══════════════════════╗║╚══════════╗╚══════╗║║MDFATn+000n+001n+002║n+003║n+004n+005║n+006║n+007   ┌─┬─┬─┬─┐┌─┬─┬─┬─┐┌─┬─┬─┬─┐┌▼┬─┬─┬─┐┌▼┬─┬─┬─┐┌─┬─┬─┬─┐┌▼┬─┬─┬─┐┌▼┬─┬─┬─┐   │       ││       ││       ││f 308e││f 1092││0 0 000││f 6094││1 009b│ ..└─┴─┴─┴─┘└─┴─┴─┴─┘└─┴─┴─┴─┘└─┴─┴─┴║┘└─┴─┴─┴║┘└─┴─┴─┴─┘└─┴─┴─┴║┘└─┴─┴─┴║┘(n=MdBpbRec.wFirstData)╔════════════╝║╔═══════════╝║║╔═════════════╝║║║║╔══════╝╔══════════════╝CVF Sector Heap╒═════╩════╕╒═╩══╕╒════════╩══════════╕╒╩╕   ┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐┌─┐   │ ││ ││ ││ ││ ││ ││ ││ ││ ││ ││ ││ ││ ││ ││ ││ ││ ││ ││ ││ ││ │...   └─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘└─┘ .. 89 8a 8b 8c 8d8e 8f 90 9192 9394 95 96 97 98 99 919b 9c 9d This diagram illustrates how FAT entries correspond to MDFAT entries. ■ MYFILE.TXT, starts at cluster 003H (as found in aDirectory Entry).   The FAT chain shows that it occupies clusters 003, 004, 006, and 007   (cluster 005 is apparently in use by another file). ■ To get theMdFatEntryRecs of interest, we addMdBpbRec.wFirstData to   each cluster number.  For instance, ifwFirstData is 0010H, then we   are interested in MDFAT entries 013H, 014H, 016H, and 017H. ■ To read a particularMdFatEntryRec,m, seek in the CVF to the start   of the MDFAT+(m*4) and read 4 bytes (the size of an MdFatEntryRec).   Each entry describes the physical location of the compressed data. ■ The low 21 bits indicate a CVF logical sector number and bits 22-25   contain one less than the count of the sectors occupied by the   cluster.  To convert a CVF logical sector number to a file offset,   add 1 to it and multiply the sum byMdBpbRec.wSectSize (usually 512). In the diagrammed example, we could access the first byte of the file as follows...   wClusterNo       = 0003H                 (obtained from directory entry)   mdClusterNo      = wClusterNo + MdBpbRec.wFirstData   lMdFatFileOffset = (MdBpbRec.wMdFatStart+1) * MdBpbRec.wSectSize   lItemOffset      = lMdFatFileOffset + (wMdClusterNo * 4) ...we seek to lItemOffset and read the 4-byteMdFatEntryRec.  We find that the low 21 bits contain 00008eH (thelocation) and that bits 22-25 contain 03H (sizCmpr) and bits 26-29 contain 0fH (sizRaw).  We calculate...   lClustStartSect =location+1   lCvfSectOffset  = lClustStartSect * MdBpbRec.wSectSize   wCmprsdBytes    = (sizCmpr + 1) * MdBpbRec.wSectSize   wUnCmprsdBytes  = (sizRaw + 1) * MdBpbRec.wSectSize ...and at last, we can seek to CVF offset lCvfSectOffset and read wCmprsdBytes of data into an internal buffer.  If we bother to examine the compressed data we'd see that the first four bytes are something like 'MD0' (4dH,44H,00H,02H). We could useMRCI Fn 0002H (settingMRCRequestRec.wDestLen=wUnCmprsdBytes) to decompress the data.   Notes: ■ COMMAND.COM accesses both the FAT and the MDFAT when you useDir /c.  In order to obtain the initial FAT entry, it uses the            long-obsolete fn11H (find first file via FCB).  The advantage            of this over fn4eH is that it returns a raw directory entry,            and therefore supplies a pointer into the FAT.          ■ As you trace a FAT chain, you may encounter a corresponding            MDFatEntryRec containing00000000H.  Such an entry indicates            thatno sectors are used by that cluster.            This can occur, for instance, if you open a file, seek at least            8K past the end of the last cluster in the file, then write at            least one byte.  To "decompress" such a cluster, just fill the            buffer with zeros.See Also:CVF Region: MDFATCVF LayoutDoubleSpace OverviewDoubleSpace APIDOS Functions-♦-