+.pa 1
+.he 'FILE SYSTEM (V)'3/15/72'FILE SYSTEM (V)'
+.ti 0
+NAME format of file system
+.sp
+.ti 0
+DESCRIPTION
+.br
+.in 8
+Every
+file system storage volume
+(e.g. RF disk, RK disk, DECtape reel)
+has a common format for certain vital information.
+
+Every such volume is divided into a certain number
+of 256 word (512 byte) blocks. Blocks 0 and 1 are
+collectively known as the super\b\b\b\b\b_____-block\b\b\b\b\b_____ for the device;
+they define its extent and contain an i-node map
+and a free-storage map.
+The first word
+contains the number of bytes in the free-storage
+map; it is always even.
+It is followed by the map.
+There is one bit for each block on the device; the bit is "1"
+if the block is free.
+Thus if the
+number of free-map bytes is n\b_, the blocks on the device
+are numbered 0 through 8n\b_-1.
+The free-map count is followed by the free map
+itself.
+The bit for block k\b_ of the device
+is in byte k\b_/8 of the map; it is offset k\b_(mod 8) bits from the right.
+Notice that bits exist for the superblock and the i-list,
+even though they are never allocated or freed.
+
+After the free map is a word containing the byte count
+for the i-node map. It too is always even.
+I-numbers below 41(10) are reserved
+for special files, and are
+never allocated; the first bit in the i-node
+free map refers to i-number 41.
+Therefore the byte number in the i-node map
+for i-node i\b_ is (i\b_-41)/8.
+It is offset (i\b_-41) (mod 8) bits from the right; unlike
+the free map, a "0" bit indicates an available i-node.
+
+I-numbers begin at 1, and the storage for i-nodes
+begins at block 2.
+Also, i-nodes are 32 bytes long, so 16 of them fit into a block.
+Therefore, i-node i\b_ is located in block (i\b_+31)/16 of
+the file system, and begins 32\u.\d((i\b_+31)(mod 16)) bytes
+from its start.
+
+There is always one file system which is always mounted;
+in standard UNIX it resides on the RF disk.
+This device is also used for swapping.
+On the primary file system device, there are
+several pieces of information following that previously
+discussed.
+There are two words with the calendar time
+(measured since 00:00 Jan 1, 1972);
+two words with the time spent executing in the system;
+two words with the time spent waiting for I/O on the RF and RK
+disks; two words with the time spent executing in a user's
+core; one byte with the count of errors on the RF
+disk; and one byte with the count of errors on the RK disk.
+All the times are measured in sixtieths of a second.
+
+I-node 41(10) is reserved for the root directory of the
+file system.
+No i-numbers other than this one and those from
+1 to 40 (which represent special files) have a built-in
+meaning.
+Each i-node represents one file.
+The format of an i-node is as follows, where the
+left column represents the offset from the beginning
+of the i-node:
+.sp
+.nf
+.in +3
+0-1 flags (see below)
+2 number of links
+3 user ID of owner
+4-5 size in bytes
+6-7 first indirect block or contents block
+.li
+...
+20-21 eighth indirect block or contents block
+22-25 creation time
+26-29 modification time
+30-31 unused
+.fi
+.sp
+.ti -3
+The flags are as follows:
+.sp
+.nf
+100000 i-node is allocated
+040000 directory
+020000 file has been modified (always on)
+010000 large file
+000040 set user ID on execution
+000020 executable
+000010 read, owner
+000004 write, owner
+000002 read, non-owner
+000001 write, non-owner
+.sp
+.in -3
+.fi
+The allocated bit (flag 100000) is believed even
+if the i-node map says the i-node is free;
+thus corruption of the map may cause i-nodes
+to become unallocatable, but will not cause active
+nodes to be reused.
+
+Byte number n\b_ of a file is accessed as follows:
+n\b_ is divided by 512 to find its logical block number (say b\b_)
+in the file.
+If the file is small (flag 010000 is 0),
+then b\b_ must be less than 8, and the physical block
+number corresponding to b\b_ is the b\b_th entry
+in the address portion of the i-node.
+
+Even if the file is large, b\b_ will be less than
+128 (128*512 = 2^16).
+The first number in the i-node address
+portion
+gives the physical block number of the indirect block.
+b\b_ is doubled
+to give a byte offset in the indirect block
+and the word there found is the physical address of
+the block corresponding to b\b_.
+
+For block b\b_ in a file to exist, it
+is not necessary that all blocks less than b\b_ exist.
+A zero block number either in the address words of
+the i-node or in an indirect block indicates that the
+corresponding block has never been allocated.
+Such a missing block reads as if it contained all zero words.
+
+.in 16
+.ti 0
+BUGS Two blocks are not enough to handle
+the i- and free-storage maps for an RP02
+disk pack, which contains around 10 million words.
+.sp
projects / unix-history / commitdiff