Retro/pcjs
Watch
1
0
Fork
You've already forked pcjs
 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Updated DiskDump comments

Browse source
This commit is contained in:
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
Download patch file Download diff file Expand all files Collapse all files

8
modules/diskdump/lib/diskdump.js
View file

@ -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. * 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 * 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 * 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, * 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. * 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 * (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 * 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 * 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. * 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. * 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 * 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 [ // define BPB for 10Mb hard drive
0xEB, 0xFE, 0x90, // 0x00: JMP instruction, following by 8-byte OEM signature 0xEB, 0xFE, 0x90, // 0x00: JMP instruction, following by 8-byte OEM signature