305 lines
7 KiB
Groff
305 lines
7 KiB
Groff
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V6/usr/man/man5/fs.5
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.TH "FILE SYSTEM" V 2/9/75
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.SH NAME
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fs \*- format of file system volume
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.SH DESCRIPTION
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Every
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file system storage volume
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(e.g. RF disk, RK disk, RP disk, DECtape reel)
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has a common format for certain vital information.
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Every such volume is divided into a certain number
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of 256 word (512 byte) blocks.
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Block 0 is unused and is available to contain
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a bootstrap program, pack label, or other information.
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.s3
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Block 1 is the
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.IB "super block."
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Starting from its first word, the format of a super-block is
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.s3
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.nf
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struct {
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int isize;
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int fsize;
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int nfree;
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int free[100];
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int ninode;
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int inode[100];
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char flock;
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char ilock;
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char fmod;
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int time[2];
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};
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.s3
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.fi
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.IP Isize
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is the number of blocks devoted to the i-list,
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which starts just after the super-block, in block 2.
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.IP Fsize
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is the first block not potentially available for allocation
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to a file.
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These numbers are used by the system to
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check for bad block numbers;
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if an ``impossible'' block number is allocated from the free list
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or is freed,
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a diagnostic is written on the on-line console.
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Moreover, the free array is cleared, so as to prevent further
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allocation from a presumably corrupted free list.
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.s3
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The free list for each volume is maintained as
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follows.
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The
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.IB free
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array contains, in
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.IB "free[1], ... , free[nfree\*-1],"
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up to 99 numbers of free blocks.
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.IB Free[0]
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is the block number of the head
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of a chain of blocks constituting the free list.
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The first word in each free-chain block is the number
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(up to 100) of free-block numbers listed in the
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next 100 words of this chain member.
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The first of these 100 blocks is the link to the
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next member of the chain.
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To allocate a block:
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decrement
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.IB nfree,
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and the new block is
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.IB free[nfree].
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If the new block number is 0,
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there are no blocks left, so give an error.
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If
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.IB nfree
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became 0,
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read in the block named by the new block number,
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replace
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.IB nfree
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by its first word,
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and copy the block numbers in the next 100 words into the
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.IB free
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array.
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To free a block, check if
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.IB nfree
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is 100; if so,
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copy
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.IB nfree
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and the
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.IB free
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array into it,
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write it out, and set
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.IB nfree
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to 0.
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In any event set
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.IB free[nfree]
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to the freed block's number and
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increment
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.IB nfree.
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.s3
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.IB Ninode
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is the number of free i-numbers in the
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.IB inode
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array.
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To allocate an i-node:
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if
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.IB ninode
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is greater than 0,
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decrement it and return
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.IB inode[ninode].
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If it was 0, read the i-list
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and place the numbers of all free inodes
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(up to 100) into the
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.IB inode
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array,
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then try again.
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To free an i-node,
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provided
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.IB ninode
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is less than 100,
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place its number into
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.IB inode[ninode]
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and increment
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.IB ninode.
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If
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.IB ninode
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is already 100, don't bother to enter the freed i-node into any table.
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This list of i-nodes is only to speed
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up the allocation process; the information
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as to whether the inode is really free
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or not is maintained in the inode itself.
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.s3
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.IB Flock
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and
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.IB ilock
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are flags maintained in the core
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copy of the file system
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while it is mounted
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and their values on disk are immaterial.
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The value of
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.IB fmod
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on disk is likewise immaterial;
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it is used as a flag to indicate that the super-block has
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changed and should be copied to
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the disk during the next periodic update of file
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system information.
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.s3
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.IB Time
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is the last time the super-block of the file system was changed,
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and is a double-precision representation
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of the number of seconds that have elapsed
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since
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0000 Jan. 1 1970 (GMT).
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During a reboot, the
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.IB time
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of the super-block for the root file system
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is used to set the system's idea of the time.
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.s3
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I-numbers begin at 1, and the storage for i-nodes
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begins in block 2.
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.tr |
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Also, i-nodes are 32 bytes long, so 16 of them fit into a block.
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Therefore, i-node
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.IB i
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is located in block (\fIi\fR|+|31)|/|16, and begins
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32\u\fB.\fR\d((\fIi\fR|+|31)|(mod 16) bytes from its start.
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I-node 1 is reserved for the root directory of the file
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system, but no other i-number has a built-in
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meaning.
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Each i-node represents one file.
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The format of an i-node is as follows.
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.s3
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.nf
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.if t .ta .5i 1.i 2.5i
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struct {
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int flags; /* +0: see below */
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char nlinks; /* +2: number of links to file */
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char uid; /* +3: user ID of owner */
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char gid; /* +4: group ID of owner */
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char size0; /* +5: high byte of 24-bit size */
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int size1; /* +6: low word of 24-bit size */
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int addr[8]; /* +8: block numbers or device number */
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int actime[2]; /* +24: time of last access */
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int modtime[2]; /* +28: time of last modification */
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};
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.dt
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.fi
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.s3
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The flags are as follows:
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.s3
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.lp +10 9
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100000 i-node is allocated
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.lp +10 9
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060000 2-bit file type:
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.lp +15 9
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000000 plain file
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.lp +15 9
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040000 directory
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.lp +15 9
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020000 character-type special file
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.lp +15 9
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060000 block-type special file.
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.lp +10 9
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010000 large file
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.lp +10 9
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004000 set user-ID on execution
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.lp +10 9
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002000 set group-ID on execution
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.lp +10 9
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000400 read (owner)
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.lp +10 9
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000200 write (owner)
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.lp +10 9
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000100 execute (owner)
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.lp +10 9
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000070 read, write, execute (group)
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.lp +10 9
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000007 read, write, execute (others)
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.s3
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.i0
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Special files are recognized by their flags
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and not by i-number.
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A block-type special file is basically one which
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can potentially be mounted as a file system;
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a character-type special file cannot, though it is
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not necessarily character-oriented.
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For special files the high byte of the first address word
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specifies the type of device; the low byte specifies
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one of several devices of
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that type.
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The device type numbers
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of block and character special files overlap.
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.s3
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The address words of ordinary files and directories
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contain the numbers of the blocks in the
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file (if it is small)
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or the numbers of indirect blocks (if the file
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is large).
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Byte number
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.IB n
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of a file is accessed as follows.
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.IB N
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is divided by 512 to find its logical block number
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(say
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.IB b
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)
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in the file.
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If the file is small (flag 010000 is 0),
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then
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.IB b
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must be less than 8, and the physical
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block number is
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.IB addr[b].
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.s3
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If the file is large,
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.IB b
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is divided by 256 to yield
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.IB i.
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If
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.IB i
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is less than 7, then
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.IB addr[i]
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is the physical block number of
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the indirect block.
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The remainder from the division
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yields the word in the indirect block
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which contains the number of the block for
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the sought-for byte.
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.s3
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If
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.IB i
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is equal to 7,
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then the file has become extra-large (huge),
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and
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.IB addr[7]
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is the address of a first indirect block.
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Each word in this block
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is the number of a second-level indirect block;
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each word in the second-level indirect blocks points to a data block.
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Notice that extra-large files are not marked by any mode
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bit, but only by having
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.IB addr[7]
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non-zero;
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and that although this scheme allows for more than
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256\*X256\*X512 = 33,554,432 bytes per file,
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the length of files is stored in 24 bits
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so in practice a file can be at most
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16,777,216 bytes long.
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.s3
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For block
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.IB b
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in a file to exist,
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it
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is not necessary that all blocks less than
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.IB b
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exist.
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A zero block number either in the address words of
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the i-node or in an indirect block indicates that the
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corresponding block has never been allocated.
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Such a missing block reads as if it contained all zero words.
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.SH "SEE ALSO"
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icheck, dcheck (VIII)
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