formatting MFM drives on a IBM PC

Jules Richardson jules.richardson99 at gmail.com
Fri Sep 29 12:35:08 CDT 2017


On 09/29/2017 11:20 AM, Jon Elson via cctalk wrote:
> I can only imagine a real AT would be even less likely to handle a drive
> over about 40 MB.

I think the limit was normally 512MB in the old c/h/s addressing days, 
wasn't it? For a connected drive, the BIOS set aside 4 bits for the number 
of heads per cylinder, 6 bits for the sectors per track, and 10 bits for 
the number of cylinders - i.e. maximums of 16, 64 and 1024. At 512 bytes 
per sector, that came out as 512MB.

Older BIOS firmware provided no means for the user to define the geometry 
of a connected drive - just a list of predefined types, and those often 
maxed out at far less than any 512MB limit. There were various software 
solutions to get around it, though.

Of course operating systems had various limits on the maximum size of a 
partition on top of that - e.g. I think it was 32MB in earlier versions of 
MS-DOS.

cheers

Jules



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