Minor README tweaks
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Jeff Parsons
Jeff Parsons
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2 changed files with 14 additions and 13 deletions
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@ -51,54 +51,55 @@ will load the following PC AT controller configuration and disk image:
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### Notes Regarding 10Mb Disks
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The [PC-DOS 2.00 Formatted Disk (Empty)](pcdos200-empty.xml) is *completely* empty. It was partitioned with the PC-DOS 2.00
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**FDISK** utility, allocating the entire disk to a single DOS partition, and then it was formatted with the PC-DOS 2.00
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**FORMAT** utility. Neither the **FORMAT** "/S" option nor the **SYS** command were used, so no system files were
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transferred, leaving the disk completely empty and non-bootable.
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The [PC-DOS 2.00 Formatted Disk (Empty)](pcdos200-empty.xml) is *completely* empty. It was partitioned with the PC-DOS
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2.00 **FDISK** utility, allocating the entire disk to a single DOS partition, and then it was formatted with the PC-DOS
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2.00 **FORMAT** utility. Neither the **FORMAT** "/S" option nor the **SYS** command were used, so no system files were
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transferred, leaving the disk completely empty and unbootable.
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After formatting, PC-DOS 2.00 reports:
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10592256 bytes total disk space
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10592256 bytes available on disk
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As explained in the [DiskDump source code](/modules/diskdump/lib/diskdump.js), in its description of the 10Mb
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As explained in our [DiskDump source code](/modules/diskdump/lib/diskdump.js), in its description of the 10Mb
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BPB, a 10Mb "Type 3" PC XT fixed disk contains:
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306 cylinders
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4 heads
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17 sectors/track
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for a total of 20808 sectors or 10,653,696 bytes. However, as p.1-179 of the PC XT Technical Reference Manual
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for a total of 20808 sectors or 10,653,696 bytes. However, as page 1-179 of the PC XT Technical Reference Manual
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(April 1983) notes:
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WARNING: The last cylinder on the fixed disk drive is reserved for diagnostic use.
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Diagnostic write tests will destroy any data on this cylinder.
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And this is confirmed by the PC XT BIOS, p.A-94, in the code for the "GET PARAMETERS (AH = 8)" function:
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And this is confirmed by the PC XT BIOS, page A-94, in the code for the "GET PARAMETERS (AH = 8)" function:
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C800:03B2 268B07 MOV AX,ES:[BX] ; MAX NUMBER OF CYLINDERS
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C800:03B5 2D0200 SUB AX,2 ; ADJUST FOR 0-N AND RESERVE LAST TRACK
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By "RESERVE LAST TRACK", they really meant "RESERVE LAST CYLINDER", because 68 (not 17) sectors are reserved at
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the end of the disk. In addition, the first sector of the disk is reserved for the Master Boot Record (MBR), so there
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are a total of 69 reserved sectors. (20808 - 69) = 20739 or 0x5103, which is exactly what's stored in the "total
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sectors" field of the disk's BPB, yielding a total partition size of 10,618,368 bytes.
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are a total of 69 reserved sectors. 20808 - 69 = 20739 sectors (0x5103), which is exactly what's stored in the
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"total sectors" field of the disk's BPB, yielding a total partition size of 10,618,368 bytes.
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However, that 69-sector overhead is not the end of the story. There is also overhead incurred by the FAT file system,
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which, in this case, consists of:
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- Boot sector (1)
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- DOS Boot sector (1)
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- FAT sectors (2 * 8 = 16)
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- Root directory sectors (32)
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for a total of 49 sectors, leaving (20739 - 49) = 20690 sectors. Moreover, free space is measured in clusters,
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for a total of 49 sectors, leaving 20739 - 49 = 20690 sectors. Moreover, free space is measured in clusters,
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not sectors, and the partition uses 8 sectors/cluster, leaving room for 2586.25 clusters. Since a fractional cluster
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is not allowed, another 2 sectors are lost, bringing the total FAT file system overhead to 51 sectors.
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Thus, actual free space is (20739 - 51) * 512, or 10,592,256 bytes, which is exactly what DOS reports as the available
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space.
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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))
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Some sources on the internet (eg,
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[http://www.wikiwand.com/en/Timeline_of_DOS_operating_systems](http://www.wikiwand.com/en/Timeline_of_DOS_operating_systems))
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claim that the FAT file system overhead for the XT's 10Mb disk is "50 sectors". As they explain:
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"The fixed disk has 10,618,880 bytes of raw space: 305 cylinders (the equivalent of tracks) × 2 platters
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@ -5,7 +5,7 @@ permalink: /pubs/pc/programming/
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---
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PC Programming Guides
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---
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---------------------
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The following is a list of programming guides that were useful during the development of PCjs. Additional
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information (errata, disks, etc) is provided where available.
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