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Minor README tweaks

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Jeff Parsons 2017-09-21 09:11:25 -07:00 • committed by Jeff Parsons
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25
disks/pcx86/fixed/10mb/README.md
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@ -51,54 +51,55 @@ will load the following PC AT controller configuration and disk image:
### Notes Regarding 10Mb Disks ### Notes Regarding 10Mb Disks
The [PC-DOS 2.00 Formatted Disk (Empty)](pcdos200-empty.xml) is *completely* empty. It was partitioned with the PC-DOS 2.00 The [PC-DOS 2.00 Formatted Disk (Empty)](pcdos200-empty.xml) is *completely* empty. It was partitioned with the PC-DOS
**FDISK** utility, allocating the entire disk to a single DOS partition, and then it was formatted with the PC-DOS 2.00 2.00 **FDISK** utility, allocating the entire disk to a single DOS partition, and then it was formatted with the PC-DOS
**FORMAT** utility. Neither the **FORMAT** "/S" option nor the **SYS** command were used, so no system files were 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. transferred, leaving the disk completely empty and unbootable.
After formatting, PC-DOS 2.00 reports: After formatting, PC-DOS 2.00 reports:
10592256 bytes total disk space 10592256 bytes total disk space
10592256 bytes available on disk 10592256 bytes available on disk
As explained in the [DiskDump source code](/modules/diskdump/lib/diskdump.js), in its description of the 10Mb As explained in our [DiskDump source code](/modules/diskdump/lib/diskdump.js), in its description of the 10Mb
BPB, a 10Mb "Type 3" PC XT fixed disk contains: BPB, a 10Mb "Type 3" PC XT fixed disk contains:
306 cylinders 306 cylinders
4 heads 4 heads
17 sectors/track 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 for a total of 20808 sectors or 10,653,696 bytes. However, as page 1-179 of the PC XT Technical Reference Manual
(April 1983) notes: (April 1983) notes:
WARNING: The last cylinder on the fixed disk drive is reserved for diagnostic use. WARNING: The last cylinder on the fixed disk drive is reserved for diagnostic use.
Diagnostic write tests will destroy any data on this cylinder. 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: And this is confirmed by the PC XT BIOS, page A-94, in the code for the "GET PARAMETERS (AH = 8)" function:
C800:03B2 268B07 MOV AX,ES:[BX] ; MAX NUMBER OF CYLINDERS 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 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 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 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 are a total of 69 reserved sectors. 20808 - 69 = 20739 sectors (0x5103), which is exactly what's stored in the
sectors" field of the disk's BPB, yielding a total partition size of 10,618,368 bytes. "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, 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: which, in this case, consists of:
- Boot sector (1) - DOS Boot sector (1)
- FAT sectors (2 * 8 = 16) - FAT sectors (2 * 8 = 16)
- Root directory sectors (32) - Root directory sectors (32)
for a total of 49 sectors, leaving (20739 - 49) = 20690 sectors. Moreover, free space is measured in clusters, 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 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. 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 Thus, actual free space is (20739 - 51) * 512, or 10,592,256 bytes, which is exactly what DOS reports as the available
space. space.
Some sources on the internet (eg,[http://www.wikiwand.com/en/Timeline_of_DOS_operating_systems](http://www.wikiwand.com/en/Timeline_of_DOS_operating_systems)) Some sources on the internet (eg,
[http://www.wikiwand.com/en/Timeline_of_DOS_operating_systems](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: 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 "The fixed disk has 10,618,880 bytes of raw space: 305 cylinders (the equivalent of tracks) × 2 platters

2
pubs/pc/programming/README.md
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@ -5,7 +5,7 @@ permalink: /pubs/pc/programming/
--- ---
PC Programming Guides PC Programming Guides
--- ---------------------
The following is a list of programming guides that were useful during the development of PCjs. Additional The following is a list of programming guides that were useful during the development of PCjs. Additional
information (errata, disks, etc) is provided where available. information (errata, disks, etc) is provided where available.