formatting MFM drives on a IBM PC

Guy Sotomayor Jr ggs at
Fri Sep 29 12:46:01 CDT 2017

> On Sep 29, 2017, at 10:35 AM, Jules Richardson via cctalk <cctalk at> wrote:
> 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.

Yes, and I can say that I’m the one responsible for that restriction.  :-o  At the time we needed to support our LBA controllers and we figured that the restriction was OK because we’d have a BIOS replacement long before disk drives got that big.  We were wrong on both counts: drives got bigger a whole lot faster than we expected and that a BIOS replacement took a whole lot longer to make it into systems.  *sigh*

> 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.

The above (512MB max disk size) came about with the PS/2.  Prior to that BIOS just had predefined types.

TTFN - Guy

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