12 Jul
2013
12 Jul
'13
12:48 p.m.
On 2013-07-12 11:38, Cory Smelosky wrote:
On Fri, 12 Jul 2013, Johnny Billquist wrote:
BAD in turn only updates the bad block list on the pack, and do nothing
else. This is why these packs reserve the last track. That is where the
bad block list is expected to reside, along with pack serial number.
What if the last track ends up composed entirely of bad blocks? ;)
Then that pack is screwed. The bad block list is a little complex, but you need a couple
of ok blocks on the last track, or it really is a broken pack.
To be more specific about the last track, the blocks are divided into manufacturer bad
block lists, and user bad block lists. I seem to remember that it alternates between the
two, so block 0,2,4,... are manufacturer bad block list copies, and 1,3,5,... are replicas
of the user bad block list.
And the manufacturer bad block list also have some pack metadata stored, such as the pack
serial number.
If the first block with the list can't be read, you are expected to try one of the
alternates.
However, all that said (I'm using that phrase much right now), lots of software abuse
and fail on this. Many do not at all even look at the user bad block list, and some even
just ignore the manufacturer bad block list. There is no physical protection from anyone
just overwriting all those blocks anyway, and I think Unix make use of all the tracks, for
example.
John mentioned that the device driver in RT-11 handles the bad block remapping in the
driver itself. OS/8 do that too. However, they create their own bad block list adapted for
the device driver, placed somewhere else (like in block 1 for RT-11 if I understood John
right). That list should hopefully be based on what is stored in the last track, but this
is also something done at filesystem initialization.
But if RT-11 uses this device driver even when reading the disk without trying to handle
it as an RT-11 file system, then the RT-11 device driver can't be used at all if you
want to read foreign packs.
Like I said, the RSX driver do not do anything at all. If just gives you all blocks. Good
or bad.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic
trip
email: bqt at softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" -
B. Idol