Burcon MITS Sector Skew and Sector Numbers

General discussions related to the Altair 8800 Clone

Burcon MITS Sector Skew and Sector Numbers

Postby jimmyorkin » July 20th, 2018, 1:24 pm

I have been looking at disk skew for a couple of days now. I see in the 8, 10, 3 skew pattern defined in the bios translation table and the routine altSkew that does the 17 sector skew that seems to be what is actually used.

My question is the disk images for Burcon CP/M seem to have the 17 sector skew built into the disk image. It also seems like the readSec and wrtSec BIOS routines apply the 17 sector skew to their operations. It seems to me that if the disk already has the 17 skew in the sector numbering and the read/write routines also do the 17 sector skew, there ends up being no skew at all. It seems to me that the format of the disk should not have any skew (sectors number sequentially) to make the skew work. Where am I off here?

Second question: on tracks 0-5 there are no sector numbers. I also don't see any start of track indicator in the disk images. How does the controller/software know where the start of a track is on tracks 0-5?

Third question: On tracks 0-5, there is no sector number. How does CP/M know what sector is being read or written on those tracks?

Thanks all,

Jimmy
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Re: Burcon MITS Sector Skew and Sector Numbers

Postby AltairClone » July 22nd, 2018, 3:17 pm

The Altair floppy controller is a hard-sector controller, and unlike a soft-sector controller, none of the sector information written on the disk is even required for the controller to know which sector it is reading. In fact, as you noticed, there are no sector numbers at all on tracks 0-5. The track and sector number data that is written on the disk is only used as part of verifying the data is read without error.

The Altair controller uses a TTL counter to keep track of the sector number under the head. On the diskette are 32 sector holes plus an index hole between sector hole 31 and 0. When the first sector hole following the index hole is detected, the sector register is reset to zero and then increments by one for each subsequent sector hole 0-31.

Mike
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Re: Burcon MITS Sector Skew and Sector Numbers

Postby jimmyorkin » July 22nd, 2018, 5:24 pm

Thanks Mike.

I had only been looking at the BIOS code and not the hardware. I had assumed that the controller was using the sector number written in the sector to know what sector was under the head.

I looked at the Altair Floppy disk manual and did see the sector counter hardware.

You confirmed what I was seeing in the hardware.

Thanks again.

Jimmy
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