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Aucbvax.4161 fa.unix-wizards utzoo!decvax!ucbvax!unix-wizards Fri Oct 2 00:42:46 1981 RM80 and its skip sector feature >From ARPAVAX.wnj@Berkeley Fri Oct 2 00:36:56 1981 On an RM80 disk each sector either is or isn't marked with a special bit. Sectors which are preceded on their track by a bad sector have this bit set; other sectors do not.

There is a bit in the controller interface, which must have the same value as this disk bit for data to be read.

Skip sector works because there are really 32 sectors per track, while the controller pretends there are 31. You normally don't set the bit in the interface and sector 32 is ignored. If a sector marked on the disk is encountered, an error occurs. You set the bit in the interface and read the sectors after the skip sector. You have to do all the arithmetic, the controller does nothing, except to complain.

This is all necessary because of the limited number of bad blocks which can be represented under standard 144.

Bill Joy

P.S. This is all rather confusing, but the answer is you probably didn't really want to know anyways. All this was explained to me by Bill Shannon at DEC. I can hardly believe anyone would have done this, but if I had about 10 operating systems to support, I probably wouldn't change the bad block format either.

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