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Minix/CD-ROM-2.0/DOSUTILS/FIPS/TECHINFO.TXT
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Minix/CD-ROM-2.0/DOSUTILS/FIPS/TECHINFO.TXT
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Technical Info on FIPS
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----------------------
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FIPS was written in C++ V2.0 with the Turbo C++ 1.0 and Borland C++ 3.1
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compilers.
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It should compile with any newer C++ compiler (perhaps after minor changes
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to the BIOS calls).
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If you're a C++ wizard, don't look too closely at the code, this is my first
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C++ program, so it is far from acceptable (too much public data, some ini-
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tializers and assignment operators are missing etc.). Constructive critizism
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is always welcome however.
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How FIPS works:
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FIPS uses the BIOS interrupts 13h 00h (reset disks), 13h 02h (read sector),
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13h 08h (get drive parameters), 13h 03h (write sector) and 13h 04h (verify
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sector).
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Here is the sequence of function calls in main:
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evaluate_argument_vector
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read the commandline arguments and set the global variables accordingly
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notice
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display copyright notice and version number
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ask_for_drive_number
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let the user choose the drive (if more than 1)
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harddrive.reset
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reset harddrive
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harddrive.rootsector->read
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read the sector 0,0,1 of the chosen drive into an array of unsigned char
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hd.partition_table().get
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extract the necessary information from the root sector (see below - The
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root sector)
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hd.print_partition_table
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print the information
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hd.check
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check if everything is ok (see below - The root sector)
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ask_for_partition_number
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let the user choose the partition
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partition->bootsector->read
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read the first sector of the chosen partition to another array
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partition->bpb().get
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extract info from the boot sector (see below - The boot sector)
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partition->print_bpb
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print the info
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partition->info().get
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calculate no. of clusters, starting sector of FATs etc.
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partition->check
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check boot sector (see below - The boot sector)
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fat1.check_against(fat2)
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check if FAT 1 is identical to FAT 2 (see below - The FAT)
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save_root_and_boot
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write root- and boot sector to floppy disk (optional)
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ask_for_new_start_cylinder
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ask the user for the first cylinder of the new partition
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fat2.check_empty
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check if chosen part of partition is empty (see below - The FAT)
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hd.calculate_new_root
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from the chosen start cylinder calculate the new partition table
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Note that the partition entries will be moved to the beginning of the par-
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tition table, so that the new partition will be the last one and the drive
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names will not change.
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hd.partition_table.put
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write the new partition table into the root sector buffer
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hd.partition_table.get,hd.print_partition_table,hd.check
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check if new root sector is ok
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partition->calculate_new_boot
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put new number of sectors in boot sector info
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partition->bpb()->put
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write new boot sector info into boot sector buffer
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partition->bpb()->get,partition->print_bpb,partition->check
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check if new boot sector is ok
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ask_for_write_permission
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ask if user wants to proceed
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harddrive.rootsector->write
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write the changed root sector to the disk
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partition->bootsector->write
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write the changed boot sector to the disk
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The root sector
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---------------
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The root sector is the first sector on every hard disk. It contains the
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program that loads the boot sector of the bootable partition and the
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partition table. The last two bytes of the root sector must be 55 aa (hex).
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The partition table begins at 1be. It contains 4 * 16 Bytes for the four
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possible partitions.
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All numbers are zero based except the start/end-sector number (may be 1-63).
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One partition entry contains the following:
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1 Byte - Bootable Flag. Must be 0 (not bootable) or 80h (bootable).
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At most one Partition may be bootable at a time.
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(somewhere I read the bootable flag may also be 81h for the
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second drive - does anybody know anything about that?)
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1 Byte - Start Head. The number of the head of the first sector of the
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partition.
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2 Bytes - Start Sector + Cylinder. The Bits are as follows:
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CCSSSSSS CCCCCCCC
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where the first byte contains the sector number (1 - 63), and
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the high two bits of the cylinder number. The second byte con-
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tains the low eight bits of the cylinder number.
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1 Byte - System Indicator. For DOS this may be:
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1 - 12-bit FAT, 16-bit sector number
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4 - 16-bit FAT, 16-bit sector number
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5 - Extended Partition
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6 - 16-bit FAT, 32-bit sector number
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1 Byte - End Head. Head Number of the last sector of the partition
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2 Bytes - End Sector + Cylinder. Same format as Start Sector + Cylinder
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4 Bytes - First Sector. Number of the first sector of the partition. This
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corresponds to the Start Head, Sector + Cylinder. High Byte
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comes first.
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4 Bytes - Total number of Sectors.
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The function check_rootsector_validity checks the following:
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- Signature Bytes (55 aa) in the last two bytes of the sector
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- not more than one bootable partition
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- Bootable flag is 0 or 80h
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- Start/End sector of a partition is not 0
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- Start/End sector & head are not greater than drive geometry allows
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- Start cylinder * sectors * heads + start head * sectors + start sector - 1
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= first sector (where sectors is no. of sectors per track, heads is
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no. of heads of the drive)
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- End cylinder * sectors * heads + end head * sector + end sector = first
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sector + number of sectors
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- if System Indicator is 0, all other bytes of partition entry are 0
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- all partitions except the first begin on cylinder boundaries (head = 0,
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sectors = 1)
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- all partition end on cylinder boundaries
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- partitions don't overlap
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- no free space between partitions
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The boot sector
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---------------
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The boot sector is the first sector of every partition. It contains the
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program that boots the operating system and the bios parameter block.
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The last two bytes must again contain 55 aa. The information in the
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boot sector is the following:
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00 3 bytes jump instruction ('eb xx 90' or 'e9 xx xx')
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03 8 bytes OEM name and version - e.g. MSDOS5.0
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0b 2 bytes bytes per sector - should be 512
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0d 1 byte sectors per cluster - power of two
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0e 2 bytes reserved sectors - typically 1 (boot sector)
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10 1 byte number of FATs - must be 2
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11 2 bytes number of rootdirectory entries - typically 512
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13 2 bytes number of sectors (short) - 0, if BIGDOS partition
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15 1 byte media descriptor - typically f8h
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16 2 bytes sectors per FAT - varies
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18 2 bytes sectors per track
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1a 2 bytes number of heads
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1c 2 bytes number of hidden sectors (low)
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- extended BPB since DOS 4.0 -
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1e 2 bytes number of hidden sectors (high)
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20 4 bytes number of sectors (long)
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24 1 byte physical drive number - 80h or 81h
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25 1 byte reserved
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26 1 byte signature - 29h
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The function check_bootsector_validity checks the following:
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- correct jump instruction
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- signature bytes 55 aa in the last two bytes of the sector
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- bytes per sector = 512
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- sectors per cluster is power of two
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- reserved sectors = 1
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- number of FATs = 2
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- number of rootdirectory entries is multiple of 16
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- media descriptor = f8h
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- sectors per fat <= 256
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- sectors per fat big enough to hold complete FAT
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- sectors per track matches BIOS info
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- number of heads matches BIOS info
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- hidden sectors = start sector
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- signature = 29h, if BIGDOS
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- physical drive number = actual drive number
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- number of sectors matches partition info
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- system indicator byte in root sector matches partition type
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The FAT
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-------
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The File Allocation Table contains the information how the clusters of the
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disk are linked to files. Every directory entry contains a pointer to the
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first cluster of the file. The corresponding cluster entry in the FAT con-
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tains a pointer to the next cluster, or an EOF marker (FFFF for 16-bit FATs,
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FFF for 12-bit FATs) if the cluster is the last one of the file.
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Bad clusters are marked with FFF7 or FF7. Empty clusters are marked with 0.
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The first cluster on the disk is cluster number 2, it begins at the first
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sector after the root directory. The FAT entries for the clusters 0 and 1
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contain the media descriptor byte (usually F8h for harddisk) and two or
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three FFh bytes.
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There exist two copies of the FAT on a normal DOS partition, these two
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copies must be identical. FAT 2 is the primary FAT.
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The function check_fat_validity checks if the two FATs are identical and if
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the entries 0 and 1 contain what they are supposed to.
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The function check_fat_empty checks if the cluster entries that cover the
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new partition contain either 0 (empty) or FFF7 (Bad cluster).
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------------------------------------------------------------------------------
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I hope you find this information useful. If you found anything not to be
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exact or if you have additions, please let me know asap.
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Arno Schaefer
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schaefer@rbg.informatik.th-darmstadt.de
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