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105 lines
4.4 KiB
Groff
105 lines
4.4 KiB
Groff
11/3/70 FILE SYSTEM (V)
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NAME format of file system
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SYNOPSIS
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DESCRIPTION The RB09 fixed head disk has 8,000 64-word blocks on
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each of its two surfaces. Unix uses the second surface
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(physical blocks 8,000 to 15,999) and numbers them
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from logical block 0 to logical block 7,999. The following
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discussion refers to logical blocks.
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The Unix filesystem is divided into 8,000 blocks of 64 words
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per block. Block 0 is the "sysdata" block. The first word
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points to the first block of the free-storage map. Each block
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in the free-storage map is structured as follows: the first
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word is the block number of the next block in the free-storage
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map, or zero if this is the end of the free-storage map. The
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next nine words hold free block numbers, or zero (no block number).
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Block 1 on the filesystem is unused. (?)
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I-numbers begin at 1, and the storage for i-nodes begins at
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block 2. Blocks 2 to 711 contain the i-nodes, with five
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consecutive i-nodes per block. Certain i-nodes are reserved for
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special files, directories and devices:
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1 The core file written by "sys save" or a bad system call
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2 The "dd" directory directory
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3 The "system" directory
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6 The "ttyin" special file
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7 The "keyboard" (graphic-2) special file
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8 The "pptin" (paper tape reader) special file
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10 The "ttyout" special file
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11 The "display" (graphic-2) special file
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12 The "pptout" (paper tape punch) special file
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There is only one file system which is always mounted;
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it resides on the RB disk. This device is also used for
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swapping. The swap areas are at the high blocks on this device:
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blocks 6,400 to 7,999. These blocks do not appear in the free list.
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Each i-node represents one file. The format of
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an i-node is as follows, where the left column represents
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the offset in words from the beginning of the i-node:
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0 flags (see below)
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1 first indirect block or contents block
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...
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7 seventh indirect block or contents block
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8 user ID of owner
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9 link count
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10 size in words
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11 unique value assigned at creation
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The flags are as follows:
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400000 i-node is allocated
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200000 large file
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?????? allocated bit (always on)
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000040 special file
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000020 directory
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000010 read, owner
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000004 write, owner
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000002 read, non-owner
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000001 write, non-owner
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The allocated bit (flag ??????) is believed even if the i-node
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map says the i-node is free; thus corruption of the map may
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cause i-nodes to become unallocatable, but will not cause active
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nodes to be reused.
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Word number n of a file is accessed as follows: n is
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divided by 64 to find its logical block number (say b) in
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the file. If the file is small (flag 200000 is 0), then b
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must be less than 7, and the physical block number corresponding
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to b is the bth entry in the address portion of the i-node.
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If the file is large, b is divided by 64 to yield a number
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which must be less than 8 (or the file is too large for
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UNIX to handle). The corresponding slot in the i-node
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address portion gives the physical block number of an
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indirect block. The residue mod 64 gives a word offset
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in the indirect block, and the word found there is the
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physical address of the block corresponding to b.
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If block b in a file exists, it is not necessary that all
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blocks less than b exist. A zero block number either in the
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address words of the i-node or in an indirect block
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indicates that the corresponding block has never been
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allocated. Such a missing block reads as if it contained
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all zero words.
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FILES
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SEE ALSO format of directories
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DIAGNOSTICS
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BUGS
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OWNER
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