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Annotation of researchv10dc/dist/man/v4/man5/fs.5, revision 1.1

1.1     ! root        1: .th "FILE SYSTEM" V 9/7/73
        !             2: .sh NAME
        !             3: fs \*- format of file system volume
        !             4: .sh DESCRIPTION
        !             5: .ft I
        !             6: Caution: this information applies only to
        !             7: the latest versions of the UNIX system.
        !             8: .s3
        !             9: .ft R
        !            10: Every
        !            11: file system storage volume
        !            12: (e.g. RF disk, RK disk, RP disk, DECtape reel)
        !            13: has a common format for certain vital information.
        !            14: Every such volume is divided into a certain number
        !            15: of 256 word (512 byte) blocks.
        !            16: Block 0 is unused and is available to contain
        !            17: a bootstrap program, pack label, or other information.
        !            18: .s3
        !            19: Block 1 is the
        !            20: .it "super block."
        !            21: Starting from its first word, the format of a super-block is
        !            22: .s3
        !            23: .nf
        !            24: struct {
        !            25:        int     isize;
        !            26:        int     fsize;
        !            27:        int     nfree;
        !            28:        int     free[100];
        !            29:        int     ninode;
        !            30:        int     inode[100];
        !            31:        char    flock;
        !            32:        char    ilock;
        !            33:        char    fmod;
        !            34:        int     time[2];
        !            35: };
        !            36: .s3
        !            37: .fi
        !            38: .it Isize
        !            39: is the number of blocks devoted to the i-list,
        !            40: which starts just after the super-block, in block 2.
        !            41: .it Fsize
        !            42: is the first block not potentially available for allocation
        !            43: to a file.
        !            44: This number is unused by the system, but is used by
        !            45: programs like
        !            46: .it "check (I)"
        !            47: to test for bad block numbers.
        !            48: The free list for each volume is maintained as
        !            49: follows.
        !            50: The
        !            51: .it free
        !            52: array contains, in
        !            53: .it "free[1], ... , free[nfree\*-1],"
        !            54: up to 99 numbers of free blocks.
        !            55: .it Free[0]
        !            56: is the block number of the head
        !            57: of a chain of blocks constituting the free list.
        !            58: The first word in each free-chain block is the number
        !            59: (up to 100) of free-block numbers listed in the
        !            60: next 100 words of this chain member.
        !            61: The first of these 100 blocks is the link to the
        !            62: next member of the chain.
        !            63: To allocate a block:
        !            64: decrement
        !            65: .it nfree,
        !            66: and the new block is
        !            67: .it free[nfree].
        !            68: If the new block number is 0,
        !            69: there are no blocks left, so give an error.
        !            70: If
        !            71: .it nfree
        !            72: became 0,
        !            73: read in the block named by the new block number,
        !            74: replace
        !            75: .it nfree
        !            76: by its first word,
        !            77: and copy the block numbers in the next 100 words into the
        !            78: .it free
        !            79: array.
        !            80: To free a block, check if
        !            81: .it nfree
        !            82: is 100; if so,
        !            83: copy
        !            84: .it nfree
        !            85: and the
        !            86: .it free
        !            87: array into it,
        !            88: write it out, and set
        !            89: .it nfree
        !            90: to 0.
        !            91: In any event set
        !            92: .it free[nfree]
        !            93: to the freed block's number and
        !            94: increment
        !            95: .it nfree.
        !            96: .s3
        !            97: .it Ninode
        !            98: is the number of free i-numbers in the
        !            99: .it inode
        !           100: array.
        !           101: To allocate an i-node:
        !           102: if
        !           103: .it ninode
        !           104: is greater than 0,
        !           105: decrement it and return
        !           106: .it inode[ninode].
        !           107: If it was 0, read the i-list
        !           108: and place the numbers of all free inodes
        !           109: (up to 100) into the
        !           110: .it inode
        !           111: array,
        !           112: then try again.
        !           113: To free an i-node,
        !           114: provided
        !           115: .it ninode
        !           116: is less than 100,
        !           117: place its number into
        !           118: .it inode[ninode]
        !           119: and increment
        !           120: .it ninode.
        !           121: If
        !           122: .it ninode
        !           123: is already 100, don't bother to enter the freed i-node into any table.
        !           124: This list of i-nodes is only to speed
        !           125: up the allocation process; the information
        !           126: as to whether the inode is really free
        !           127: or not is maintained in the inode itself.
        !           128: .s3
        !           129: .it Flock
        !           130: and
        !           131: .it ilock
        !           132: are flags maintained in the core
        !           133: copy of the file system
        !           134: while it is mounted
        !           135: and their values on disk are immaterial.
        !           136: The value of
        !           137: .it fmod
        !           138: on disk is likewise immaterial;
        !           139: it is used as a flag to indicate that the super-block has
        !           140: changed and should be copied to
        !           141: the disk during the next periodic update of file
        !           142: system information.
        !           143: .s3
        !           144: .it Time
        !           145: is the last time the super-block of the file system was changed,
        !           146: and is a double-precision representation
        !           147: of the number of seconds that have elapsed
        !           148: since
        !           149: 0000 Jan. 1 1970 (GMT).
        !           150: During a reboot, the
        !           151: .it time
        !           152: of the super-block for the root file system
        !           153: is used to set the system's idea of the time.
        !           154: .s3
        !           155: I-numbers begin at 1, and the storage for i-nodes
        !           156: begins in block 2.
        !           157: .tr |
        !           158: Also, i-nodes are 32 bytes long, so 16 of them fit into a block.
        !           159: Therefore, i-node
        !           160: .it i
        !           161: is located in block (\fIi\fR|+|31)|/|16, and begins
        !           162: 32\u\fB.\fR\d((\fIi\fR|+|31)|(mod 16) bytes from its start.
        !           163: I-node 1 is reserved for the root directory of the file
        !           164: system, but no other i-number has a built-in
        !           165: meaning.
        !           166: Each i-node represents one file.
        !           167: The format of an i-node is as follows.
        !           168: .s3
        !           169: .nf
        !           170: .if t .ta .5i 1.i 2.5i
        !           171: struct {
        !           172:        int     flags;  /* +0: see below */
        !           173:        char    nlinks; /* +2: number of links to file */
        !           174:        char    uid;    /* +3: user ID of owner */
        !           175:        char    gid;    /* +4: group ID of owner */
        !           176:        char    size0;  /* +5: high byte of 24-bit size */
        !           177:        int     size1;  /* +6: low word of 24-bit size */
        !           178:        int     addr[8];        /* +8: block numbers or device number */
        !           179:        int     actime[2];      /* +24: time of last access */
        !           180:        int     modtime[2];     /* +28: time of last modification */
        !           181: };
        !           182: .dt
        !           183: .fi
        !           184: .s3
        !           185: The flags are as follows:
        !           186: .s3
        !           187: .lp +10 9
        !           188: 100000 i-node is allocated
        !           189: .lp +10 9
        !           190: 060000 2-bit file type:
        !           191: .lp +15 9
        !           192: 000000 plain file
        !           193: .lp +15 9
        !           194: 040000 directory
        !           195: .lp +15 9
        !           196: 020000 character-type special file
        !           197: .lp +15 9
        !           198: 060000 block-type special file.
        !           199: .lp +10 9
        !           200: 010000 large file
        !           201: .lp +10 9
        !           202: 004000 set user-ID on execution
        !           203: .lp +10 9
        !           204: 002000 set group-ID on execution
        !           205: .lp +10 9
        !           206: 000400 read (owner)
        !           207: .lp +10 9
        !           208: 000200 write (owner)
        !           209: .lp +10 9
        !           210: 000100 execute (owner)
        !           211: .lp +10 9
        !           212: 000070 read, write, execute (group)
        !           213: .lp +10 9
        !           214: 000007 read, write, execute (others)
        !           215: .s3
        !           216: .i0
        !           217: Special files are recognized by their flags
        !           218: and not by i-number.
        !           219: A block-type special file is basically one which
        !           220: can potentially be mounted as a file system;
        !           221: a character-type special file cannot, though it is
        !           222: not necessarily character-oriented.
        !           223: For special files the high byte of the first address word
        !           224: specifies the type of device; the low byte specifies
        !           225: one of several devices of
        !           226: that type.
        !           227: The device type numbers
        !           228: of block and character special files overlap.
        !           229: .s3
        !           230: The address words of ordinary files and directories
        !           231: contain the numbers of the blocks in the
        !           232: file (if it is small)
        !           233: or the numbers of indirect blocks (if the file
        !           234: is large).
        !           235: .s3
        !           236: Byte number
        !           237: .it n
        !           238: of a file is accessed as follows.
        !           239: .it N
        !           240: is divided by 512 to find its logical block number
        !           241: (say
        !           242: .it b
        !           243: )
        !           244: in the file.
        !           245: If the file is small (flag 010000 is 0),
        !           246: then
        !           247: .it b
        !           248: must be less than 8, and the physical
        !           249: block number is
        !           250: .it addr[b].
        !           251: .s3
        !           252: If the file is large,
        !           253: .it b
        !           254: is divided by 256 to yield
        !           255: .it i,
        !           256: and
        !           257: .it addr[i]
        !           258: is the physical block number of
        !           259: the indirect block.
        !           260: The remainder from the division 
        !           261: yields the word in the indirect block
        !           262: which contains the number of the block for
        !           263: the sought-for byte.
        !           264: .s3
        !           265: For block
        !           266: .it b
        !           267: in a file to exist,
        !           268: it
        !           269: is not necessary that all blocks less than
        !           270: .it b
        !           271: exist.
        !           272: A zero block number either in the address words of
        !           273: the i-node or in an indirect block indicates that the
        !           274: corresponding block has never been allocated.
        !           275: Such a missing block reads as if it contained all zero words.
        !           276: .sh "SEE ALSO"
        !           277: check (VIII)

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