Path: utzoo!utgpu!water!watmath!clyde!rutgers!rochester!PT.CS.CMU.EDU!B.GP.CS.CMU.EDU!Ralf.Brown@B.GP.CS.CMU.EDU
From: Ralf.Brown@B.GP.CS.CMU.EDU
Newsgroups: comp.sys.ibm.pc
Subject: Re: What are these?
Message-ID: <21ecc39b@ralf.home>
Date: 14 Jan 88 13:18:19 GMT
Sender: netnews@PT.CS.CMU.EDU
Lines: 28
In-Reply-To: <675@bucket.UUCP>

In article <675@bucket.UUCP>, leonard@bucket.UUCP (Leonard Erickson) writes:
}Single density disks (which do exist in 5.25!) use FM recording. This is
}data-bit clock-bit data-bit.....
}
}Double density disks (and QD & HD & "normal" hard disks) use MFM. This
}uses some sort of scheme (I forget the details) to make the data
}"self-clocking" and thus eliminate the clock-bits. This enabled twice
}the data density, thus the term double-density. 

MFM is really RLL 1,3 (i.e. at most 3 zero or one bits in a row).  The RLL
controllers use RLL 2,7 (at most 7 identical bits in a row), and ARLL uses
(I think) RLL 3,9.

}For what it is worth, if the PC had supported single density disks
}(which would take a different controller chip!) it would have had

Actually, the controller is perfectly capable of handling single
density.  One of the bits in the command code that is sent to the 
controller indicates whether to use FM or MFM.

}Leonard Erickson                ...!tektronix!reed!percival!bucket!leonard
}CIS: [70465,203]

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