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Updated FAT overhead notes

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Jeff Parsons 2017-06-01 21:34:06 -07:00 • committed by Jeff Parsons
commit 81591ad8d5
1 changed files with 28 additions and 10 deletions
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38
modules/diskdump/lib/diskdump.js
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@ -531,17 +531,35 @@ DiskDump.aDefaultBPBs = [
* Here's some useful background information on a 10Mb PC XT fixed disk, partitioned with a single DOS partition.
*
* The BPB for a 10Mb "type 3" PC XT drive specifies 0x5103 or 20739 for TOTAL_SECS, which is the partition
* size in sectors (10,618,368 bytes), whereas the total disk size is 20808 sectors (10,653,696 bytes). The partition
* is 69 sectors smaller than the disk because the first sector is reserved for the MBR and 68 sectors (the entire last
* cylinder) are reserved for diagnostics, head parking, etc. This cylinder usage is confirmed by FDISK, which reports
* that 305 cylinders (not 306) are assigned to the DOS partition.
* size in sectors (10,618,368 bytes), whereas total disk size is 20808 sectors (10,653,696 bytes). The partition
* is 69 sectors smaller than the disk because the first sector is reserved for the MBR and 68 sectors (the entire
* last cylinder) are reserved for diagnostics, head parking, etc. This cylinder usage is confirmed by FDISK,
* which reports that 305 cylinders (not 306) are assigned to the DOS partition.
*
* That 69-sector overhead is NOT the overhead incurred by the FAT file system, which is the boot sector (1), FAT
* sectors (16), and root directory sectors (32), for a total of 49 sectors, leaving 20739 - 49 or 20690 sectors.
* However, free space is measured in clusters, not sectors, and the partition uses 8 sectors/cluster, leaving room
* for 2586.25 clusters. Since a fractional cluster is not allowed, another 2 sectors are lost to FAT overhead,
* for a total of 51 sectors. So actual free space is (20739 - 51) * 512, or 10,592,256 bytes -- which is exactly
* what is reported as the available space on a freshly formatted PC XT 10Mb fixed disk.
* That 69-sector overhead is NOT the overhead incurred by the FAT file system. That overhead is the boot sector
* (1), FAT sectors (2 * 8), and root directory sectors (32), for a total of 49 sectors, leaving 20739 - 49 or
* 20690 sectors. However, free space is measured in clusters, not sectors, and the partition uses 8 sectors/cluster,
* leaving room for 2586.25 clusters. Since a fractional cluster is not allowed, another 2 sectors are lost, for
* a total of 51 sectors of FAT overhead. So actual free space is (20739 - 51) * 512, or 10,592,256 bytes -- which
* is exactly what is reported as the available space on a freshly formatted 10Mb PC XT fixed disk.
*
* Some sources on the internet (eg, http://www.wikiwand.com/en/Timeline_of_DOS_operating_systems) claim that the
* file system overhead for the XT's 10Mb disk is "50 sectors". As they explain:
*
* "The fixed disk has 10,618,880 bytes of raw space: 305 cylinders (the equivalent of tracks) × 2 platters
* × 2 sides or heads per platter × 17 sectors per track = 20,740 sectors × 512 bytes per sector = 10,618,880
* bytes...."
*
* and:
*
* "With DOS the only partition, the combined overhead is 50 sectors leaving 10,592,256 bytes for user data:
* DOS's FAT is eight sectors (16 sectors for two copies) + 32 sectors for the root directory, room for 512
* directory entries + 2 sectors (one master and one DOS boot sector) = 50 sectors...."
*
* However, that's incorrect. First, the disk has 306 cylinders, not 305. Second, there are TWO overhead values:
* the overhead OUTSIDE the partition (69 sectors) and the overhead INSIDE the partition (51 sectors). They failed
* to account for the reserved cylinder in the first calculation and the fractional cluster in the second calculation,
* and then they conflated the two values to produce a single (incorrect) result.
*/
[ // define BPB for 10Mb hard drive
0xEB, 0xFE, 0x90, // 0x00: JMP instruction, following by 8-byte OEM signature