Interdata_v6/usr/doc/man/man5/fs.5

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including files from this version of Unix. 
.th "FILESYSTEM" V 8/12/77
.sh NAME
fs - format of file system volume
.sh DESCRIPTION
The format of a file system volume is slightly different from
that described in The Book,
because of the Interdata's 32-bit
word length.

Differences are:

.lp +16 16
super-block	\c
The
.it free[]
and
.it inode[]
arrays each hold 50 fullword pointers, not 100.

.lp +16 16
freelist	\c
Free list index blocks contain 50 free-block pointers, not 100.

.lp +16 16
inode	\c
The
.it iaddr[]
array holds 8 fullword block numbers, so the inode is 64 bytes long.
Thus inode
.it i
is located in block (i+15)/8,
at offset 64*(i+15)(mod 8) bytes from the beginning
of the block.

.lp +16 16
indirect block	\c
Each indirect block holds 128 fullword block numbers, not 256.
The large file argorithm is changed accordingly.

.lp +16 16
directory	\c
A directory entry is 16 bytes long: a
.it halfword
i-number, and a 14-character filename.
.i0
.sh "SEE ALSO"
The Book