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