New blog post: Adventures in Archiving
This commit is contained in:
Jeff Parsons
Jeff Parsons
parent
a1fced4f3c
commit
f6fc85e225
4 changed files with 160 additions and 40 deletions
|
|
@ -7,12 +7,80 @@ permalink: /disks/pcx86/fixed/10mb/
|
|||
10Mb Hard Drive (Fixed Disk) Images
|
||||
---
|
||||
|
||||
This folder contains the following 10Mb disk images:
|
||||
This folder contains the following 10Mb fixed disk images:
|
||||
|
||||
* Empty formatted disk ([PCDOS200-EMPTY](PCDOS200-EMPTY.json))
|
||||
* PC-DOS 2.00 with Windows 1.01 for CGA ([PCDOS200-WIN101-CGA](pcdos200-win101-cga.xml))
|
||||
* PC-DOS 2.00 with Windows 1.01 for EGA ([PCDOS200-WIN101-EGA](pcdos200-win101-ega.xml))
|
||||
|
||||
These disk images are used by various [Model 5160](/devices/pcx86/machine/5160/) machine configurations.
|
||||
These disk images are used by various IBM PC XT [Model 5160](/devices/pcx86/machine/5160/) machine configurations.
|
||||
|
||||
NOTE: The empty disk is *completely* empty. It was partitioned with the PC-DOS 2.00 **FDISK** utility, allocating
|
||||
the entire disk to a single DOS partition, and then it was formatted with the PC-DOS 2.00 **FORMAT** utility.
|
||||
Neither the **FORMAT** "/S" option nor the **SYS** command were used, so no system files were transferred, leaving
|
||||
the disk completely empty and non-bootable.
|
||||
|
||||
After formatting, PC-DOS 2.00 reports:
|
||||
|
||||
10592256 bytes total disk space
|
||||
10592256 bytes available on disk
|
||||
|
||||
As explained in the [DiskDump source code](/modules/diskdump/lib/diskdump.js), in its description of the 10Mb
|
||||
BPB, a 10Mb "Type 3" PC XT fixed disk contains:
|
||||
|
||||
306 cylinders
|
||||
4 heads
|
||||
17 sectors/track
|
||||
|
||||
for a total of 20808 sectors or 10,653,696 bytes. However, as p.1-179 of the PC XT Technical Reference Manual
|
||||
(April 1983) notes:
|
||||
|
||||
WARNING: The last cylinder on the fixed disk drive is reserved for diagnostic use.
|
||||
Diagnostic write tests will destroy any data on this cylinder.
|
||||
|
||||
And this is confirmed by the PC XT BIOS, p.A-94, in the code for the "GET PARAMETERS (AH = 8)" function:
|
||||
|
||||
C800:03B2 268B07 MOV AX,ES:[BX] ; MAX NUMBER OF CYLINDERS
|
||||
C800:03B5 2D0200 SUB AX,2 ; ADJUST FOR 0-N AND RESERVE LAST TRACK
|
||||
|
||||
By "RESERVE LAST TRACK", they really meant "RESERVE LAST CYLINDER", because 68 (not 17) sectors are reserved at
|
||||
the end of the disk. In addition, the first sector of the disk is reserved for the Master Boot Record (MBR), so there
|
||||
are a total of 69 reserved sectors. (20808 - 69) = 20739 or 0x5103, which is exactly what's stored in the "total
|
||||
sectors" field of the disk's BPB, yielding a total partition size of 10,618,368 bytes.
|
||||
|
||||
However, that 69-sector overhead is not the end of the story. There is also overhead incurred by the FAT file system,
|
||||
which, in this case, consists of:
|
||||
|
||||
- Boot sector (1)
|
||||
- FAT sectors (2 * 8 = 16)
|
||||
- Root directory sectors (32)
|
||||
|
||||
for a total of 49 sectors, leaving (20739 - 49) = 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, bringing the total FAT file system overhead to 51 sectors.
|
||||
|
||||
Thus, actual free space is (20739 - 51) * 512, or 10,592,256 bytes, which is exactly what DOS reports as the available
|
||||
space.
|
||||
|
||||
Some sources on the internet (eg, http://www.wikiwand.com/en/Timeline_of_DOS_operating_systems) claim that the (FAT)
|
||||
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 lost fractional cluster in the second
|
||||
calculation, and then they conflated the two values to produce a single (incorrect) result.
|
||||
|
||||
---
|
||||
|
||||
To use one of these disks with another machine configuration, include:
|
||||
|
||||
|
|
|
|||
Loading…
Reference in a new issue