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Annotation of researchv10dc/dist/man/v3/man5/fs.5, revision 1.1

1.1     ! root        1: .pa 1
        !             2: .he 'FILE SYSTEM (V)'3/15/72'FILE SYSTEM (V)'
        !             3: .ti 0
        !             4: NAME           format of file system
        !             5: .sp
        !             6: .ti 0
        !             7: DESCRIPTION
        !             8: .br
        !             9: .in 8
        !            10: Every
        !            11: file system storage volume
        !            12: (e.g. RF disk, RK disk, DECtape reel)
        !            13: has a common format for certain vital information.
        !            14: 
        !            15: Every such volume is divided into a certain number
        !            16: of 256 word (512 byte) blocks.  Blocks 0 and 1 are
        !            17: collectively known as the super_____-block_____ for the device;
        !            18: they define its extent and contain an i-node map
        !            19: and a free-storage map.
        !            20: The first word
        !            21: contains the number of bytes in the free-storage
        !            22: map; it is always even.
        !            23: It is followed by the map.
        !            24: There is one bit for each block on the device; the bit is "1"
        !            25: if the block is free.
        !            26: Thus if the
        !            27: number of free-map bytes is n_, the blocks on the device
        !            28: are numbered 0 through 8n_-1.
        !            29: The free-map count is followed by the free map
        !            30: itself.
        !            31: The bit for block k_ of the device
        !            32: is in byte k_/8 of the map; it is offset k_(mod 8) bits from the right.
        !            33: Notice that bits exist for the superblock and the i-list,
        !            34: even though they are never allocated or freed.
        !            35: 
        !            36: After the free map is a word containing the byte count
        !            37: for the i-node map.  It too is always even.
        !            38: I-numbers below 41(10) are reserved
        !            39: for special files, and are
        !            40: never allocated; the first bit in the i-node
        !            41: free map refers to i-number 41.
        !            42: Therefore the byte number in the i-node map
        !            43: for i-node i_ is (i_-41)/8.
        !            44: It is offset (i_-41) (mod 8) bits from the right; unlike
        !            45: the free map, a "0" bit indicates an available i-node.
        !            46: 
        !            47: I-numbers begin at 1, and the storage for i-nodes
        !            48: begins at block 2.
        !            49: Also, i-nodes are 32 bytes long, so 16 of them fit into a block.
        !            50: Therefore, i-node i_ is located in block (i_+31)/16 of
        !            51: the file system, and begins 32\u.\d((i_+31)(mod 16)) bytes
        !            52: from its start.
        !            53: 
        !            54: There is always one file system which is always mounted;
        !            55: in standard UNIX it resides on the RF disk.
        !            56: This device is also used for swapping.
        !            57: On the primary file system device, there are
        !            58: several pieces of information following that previously
        !            59: discussed.
        !            60: There are two words with the calendar time
        !            61: (measured since 00:00 Jan 1, 1972);
        !            62: two words with the time spent executing in the system;
        !            63: two words with the time spent waiting for I/O on the RF and RK
        !            64: disks; two words with the time spent executing in a user's
        !            65: core; one byte with the count of errors on the RF
        !            66: disk; and one byte with the count of errors on the RK disk.
        !            67: All the times are measured in sixtieths of a second.
        !            68: 
        !            69: I-node 41(10) is reserved for the root directory of the
        !            70: file system.
        !            71: No i-numbers other than this one and those from
        !            72: 1 to 40 (which represent special files) have a built-in
        !            73: meaning.
        !            74: Each i-node represents one file.
        !            75: The format of an i-node is as follows, where the
        !            76: left column represents the offset from the beginning
        !            77: of the i-node:
        !            78: .sp
        !            79: .nf
        !            80: .in +3
        !            81: 0-1    flags (see below)
        !            82: 2      number of links
        !            83: 3      user ID of owner
        !            84: 4-5    size in bytes
        !            85: 6-7    first indirect block or contents block
        !            86: .li
        !            87: ...
        !            88: 20-21  eighth indirect block or contents block
        !            89: 22-25  creation time
        !            90: 26-29  modification time
        !            91: 30-31          unused
        !            92: .fi
        !            93: .sp
        !            94: .ti -3
        !            95: The flags are as follows:
        !            96: .sp
        !            97: .nf
        !            98: 100000 i-node is allocated
        !            99: 040000 directory
        !           100: 020000 file has been modified (always on)
        !           101: 010000 large file
        !           102: 000040 set user ID on execution
        !           103: 000020 executable
        !           104: 000010 read, owner
        !           105: 000004 write, owner
        !           106: 000002 read, non-owner
        !           107: 000001 write, non-owner
        !           108: .sp
        !           109: .in -3
        !           110: .fi
        !           111: The allocated bit (flag 100000) is believed even
        !           112: if the i-node map says the i-node is free;
        !           113: thus corruption of the map may cause i-nodes
        !           114: to become unallocatable, but will not cause active
        !           115: nodes to be reused.
        !           116: 
        !           117: Byte number n_ of a file is accessed as follows:
        !           118: n_ is divided by 512 to find its logical block number (say b_)
        !           119: in the file.
        !           120: If the file is small (flag 010000 is 0),
        !           121: then b_ must be less than 8, and the physical block
        !           122: number corresponding to b_ is the b_th entry
        !           123: in the address portion of the i-node.
        !           124: 
        !           125: Even if the file is large, b_ will be less than
        !           126: 128 (128*512 = 2^16).
        !           127: The first number in the i-node address
        !           128: portion
        !           129: gives the physical block number of the indirect block.
        !           130: b_ is doubled
        !           131: to give a byte offset in the indirect block
        !           132: and the word there found is the physical address of
        !           133: the block corresponding to b_.
        !           134: 
        !           135: For block b_ in a file to exist, it
        !           136: is not necessary that all blocks less than b_ exist.
        !           137: A zero block number either in the address words of
        !           138: the i-node or in an indirect block indicates that the
        !           139: corresponding block has never been allocated.
        !           140: Such a missing block reads as if it contained all zero words.
        !           141: 
        !           142: .in 16
        !           143: .ti 0
        !           144: BUGS           Two blocks are not enough to handle
        !           145: the i- and free-storage maps for an RP02
        !           146: disk pack, which contains around 10 million words.
        !           147: .sp

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