[Unix-jun72] Anyone working on V1 mkfs?

Warren Toomey wkt at tuhs.org
Thu May 1 22:31:22 EDT 2008


On Fri, May 02, 2008 at 10:58:08AM +1000, Warren Toomey wrote:
> On another note, I thought of taking V6 mkfs.c as a start-point to rewrite
> V1 mkfs.

I gave up and decided to write mkfs from scratch :-)

I'm a bit baffled on the i-node map and list. The V1 filesystem(5) manual says:

	I-numbers below 41 (base 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 is (i-41)/8 and offset (i-41)%8 bits from the right...

	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 is located in block (i+31)/16 of the file system,
	and begins 32*((i+31)%16) from its start.

	I-node 41 (base 10) is reserved for the root directory.

Now, I am assuming that this means:

  a) the i-mode bitmap is missing bits 0 to 40, but
  b) the actual i-nodes 0 to 40 do exist, and
  c) the system can still "read i-node 7", even if it can't allocate i-node 7.

Following the manual, the root directory i-node is number 41, so its
bit in the bitmap is (41-41)/8==0, offset (41-41)%8== bit 0. But the
actual i-node used is still i-node 41, so that is on block (41+31)/16==block4,
offset 256.

Let me know if that sounds right, or if I'm off-base somewhere.

Thanks,
	Warren



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