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Updated DiskDump comments

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Jeff Parsons 2017-06-15 09:19:47 -07:00 • committed by Jeff Parsons
commit 37eab35953
1 changed files with 5 additions and 5 deletions
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10
modules/diskdump/lib/diskdump.js
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@ -434,7 +434,7 @@ DiskDump.aDefaultBPBs = [
[ // define BPB for 320Kb diskette
0xEB, 0xFE, 0x90, // 0x00: JMP instruction, following by 8-byte OEM signature
0x50, 0x43, 0x4A, 0x53, 0x2E, 0x4F, 0x52, 0x47, // PCJS_OEM
// 0x49, 0x42, 0x4D, 0x20, 0x20, 0x31, 0x2E, 0x30, // "IBM 1.0" (this is a real OEM signature)
// 0x49, 0x42, 0x4D, 0x20, 0x20, 0x31, 0x2E, 0x30, // "IBM 1.0" (this is a real OEM signature)
0x00, 0x02, // 0x0B: bytes per sector (0x200 or 512)
0x02, // 0x0D: sectors per cluster (2)
0x01, 0x00, // 0x0E: reserved sectors; ie, # sectors preceding the first FAT--usually just the boot sector (1)
@ -530,15 +530,15 @@ 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
* The BPB for a 10Mb "type 3" PC XT hard disk specifies 0x5103 or 20739 for TOTAL_SECS, which is the 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. That overhead is the boot sector
* That 69-sector overhead is NOT overhead incurred by the FAT file system. The FAT 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,
* 20690 sectors. Moreover, 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.
@ -562,7 +562,7 @@ DiskDump.aDefaultBPBs = [
* and then they conflated the two values to produce a single (incorrect) result.
*
* Even if one were to assume that the disk had only 305 cylinders, that would only change the partitioning overhead
* to 1 sector; the file system overhead would still be 51 sectors.
* to 1 sector; the FAT file system overhead would still be 51 sectors.
*/
[ // define BPB for 10Mb hard drive
0xEB, 0xFE, 0x90, // 0x00: JMP instruction, following by 8-byte OEM signature