[HECnet] Copying RL02 image under RSX

[Johnny Billquist]

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