151 lines
6.8 KiB
Plaintext
151 lines
6.8 KiB
Plaintext
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SAFEFORMAT
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=======================================================================
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The following new source files have been added to the format
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directory.
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SAFE.ASM - Additional program source code
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SAFE.INC - Extrn definitions to be included in other modules
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SAFEDEF.INC - Structure and equates used in SAFE.ASM
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NEWFMT.DOC - This information and specification file
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All changes to the existing format program have been incorporated
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within conditionals. The defined value SAFE will determine whether
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the new safe format is built. The build the safe format add the
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command line switch for the assembler "-DSAFE=1". To disable
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creation of the safe format and build the normal format program
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change the switch to -DSAFE=0.
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=======================================================================
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The design specification for the MS-DOS 4.x safe format and unformat
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utility is based on the these underlying assumptions.
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FORMAT assumptions:
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1. The utility is version dependent and will alway fail to run
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under versions of DOS (as reported by DOS function 30h)
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other the version it was designed for.
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2. The utility will only attempt to preserve and restore existing
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DOS 4.x compatible parititions.
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3. The utililty will incorporate general safty features but will
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rely on the integrety of the underlying operating system for
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detecting error conditions while accessing the hard disk.
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4. There will be no attempt to allow restoration of a partition
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which was recognized as being damaged before the format
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operation had begun.
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5. If for any reason the restoration files cannot be created
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the user will notified and allowed to decide if the format
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should continue without the restoration ability. There is
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one exception to this assumption and that is the case where
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a partition is detected to be unformated as described later.
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6. The default operation during format will be to create the
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restoration file. A command line switch will allow the user
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to disable the feature.
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7. All disk access will be done via DOS file handles and interrupts
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25h and 26h to allow DOS to detect and return any error
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conditions.
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8. Only the first copy of the FAT will be saved. If any sectors
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in the first copy of the FAT cannot be successfully read
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using int 25h, an attempt will be made to read the corresponding
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sectors from the second copy of the FAT. Only if the
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corresponding sectors in both copies of the FAT cannot be read
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will it be assumed the the FAT is bad and a restoration file
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cannot be created.
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9. The restoration file must contain all information neccessary
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to allow the restoration program to restore the original
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partition immediately after a format. If the original partition
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was a boot partition it must be restored so that it will boot
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the original operating system which was present before the
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format operation.
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10. The system files will consist of the either of the following
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2 groups of files plus COMMAND.COM.
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(IO.SYS and MSDOS.SYS) or (IBMBIO.SYS and IBMDOS.SYS)
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11. It will be assumed that a partition has never been formatted
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if the boot record is not valid using the conditions that the
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last 2 bytes of a boot sector contain a boot signature and
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the first byte is near or short jump opcode. When a partition
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has never been formatted a restoration file will not be
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created. There will be no need to alert the user to this action
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and formating will proceed as it does with the existing DOS 4.x
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utility.
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12. The restoration file will be created in the root directory
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before formating begins and this directory entry will be
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restored to the root directory as the 4th entry in the new
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root directory with an E5h placed in the first byte of the
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first 3 directory entries to allow room for operating system
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files. The restoration file will use the name "UNFORMAT.DAT"
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and will be created with HIDDEN, SYSTEM and READONLY
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attributes set.
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13. Any previous restoration file will be deleted at the time
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a new restoration file is created.
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WARNING: This will mean that if a user reformats a hard
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disk immediately after a previous format, the original
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contents of the hard disk will be lost.
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14. The restoration file will be designed so that it can allow
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restoration of the hard disk reguardless of the condition of
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boot sector, FAT or root directory after completion of the
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format operation. This feature is only for possible future
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enhancements and in this implementation there will be no
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attempt to restore a hard disk if the restoration file
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cannot be successfully read from the disk using the normal
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DOS file and sector read and write operations.
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15. The FAT which is written to the restoration file will also
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contain the FAT allocation chain for the restoration file.
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16. The DOS control C check will be disabled while the system areas
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of the disk are being written to help minimize the chance of
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losing the restoration file's directory entry and FAT chain.
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NOTE:
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The user will still be allowed to CTRL C out of the format at
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any time previous to this without damaging the integrity of
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the existing partition other than the deletion of any
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previously existing restoration file.
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17. There will be 2 checksums in the restoration file header. One
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will be a checksum of the sector containing the file header
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and the other will be a checksum of the entire restoration
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file. The file header checksum will be used to confirm the
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file header is the valid starting sector of a restoration
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file and the file checksum will be used to insure the
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integrety of entire restoration file before a restoration
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is done.
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18. The information contained in the restoration file is intended
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to only allow the restoration of a hard disk partition if the
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UNFORMAT utility is used before any new files are written to
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the partition after a format operation. If any new files are
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written the partition is formatted it will void the integrety
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of the original FAT and the partition will be corrupt.
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19. The FORMAT utility must allow restoration of the original
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partition even in the event that the /S option is used to
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install a new operating system on the partition. This will
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be accomplished by assuring that the new OS does not
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overwrite any existing allocated areas other than the first
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2 clusters of the disk which have been saved in the restoration
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file.
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20. The fatal disk error handler will be changed so that IGNORE is
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not an option for handling of fatal disk errors. Only RETRY,
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FAIL and ABORT will be valid options.
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