Drive capacity names (Was: WTB: HP-85 16k RAM Module and HPIB Floppy Drive

Christian Corti cc at
Thu Nov 16 03:11:51 CST 2017

On Wed, 15 Nov 2017, Fred Cisin wrote:
> On Wed, 15 Nov 2017, Christian Corti via cctalk wrote:
>> No, the 9122C has two high-density, two-sided 80 cylinder drives. A drive 
>> has no capacity, this is the function of the on-disk format.
>> ;-)
> "high-density" is even more meaningless than referring to them by their 
> capacity in a given format.  It is a BOGUS marketing term!

Fred, you should know by now that you don't need to tell *me* the 
correct definitions and terms.
And with "high-density", I didn't mean the media capacity but the analog 
recording aspects like coercivity, write current, frequency and so on.

> configurations that result in the same final capacities, it is generally 
> accepted as to WHICH kind of drive/controller configuration is meant by each 
> of those names.    "400K" generally means Macintosh single sided, not DEC 
> Rainbow, etc.

I disagree, that is not generally accepted, at least not any more, and 
this is good!

> Unformatted capacity would be a more correct nomenclature, although not 
> always precise, and relatively meaningless to the majority of users, who 
> didn't CARE except for how much space was available to them.   Formatted 
> capacity is generally between 40 and 60 percent of unformatted capacity.

Unformatted capacity doesn't tell you much without reference to the 
recording layout, i.e. no. of tracks, modulation, frequency and so on.

> Some specifications:
> 8" FM "Single Density" was 360 RPM at 250,000 bits per second. (about 500K 
> unformatted per side)
> 8" MFM "Double Density" was 360 RPM at 500,000 bits per second.  (about 1M 
> unformatted per side)

I beg to differ. The raw bit rate is about the same. With FM, you have a
500kbits/s raw bit rate but half of the bits are clock bits. It is 
effectively the same density.

> 5.25" MFM "High Density" was 360 RPM at 500,000 bits per second. (about 1M 
> unformatted per side)

What about 5¼" FM "High Density" at 360 RPM?

> 3.5" MFM "High Density" (sometimes called "1.44M", due to the most common 
> formsat being 1.41 Mebibytes, or 1.44 of a unit of 1000*1024 bytes), were 300 
> RPM at 500,000 bits per second.  (1M unformatted per side)

The Amiga (more exactly, the "HD" Chinon FZ-357A drives used in Amigas) 
switched to 150 RPM to keep the raw bit rate at 250kbits/s.

> 3.5" MFM "ED" (vertical recording?/barrium ferrite) were 300 RPM at 1,000,000 
> bits per second.  (2M unformatted per side)  NeXT referred to theirs by the 
> unformatted capacity: 4M, further confusing their users.

What about FM?

Your list just mixes two aspects that are not strictly correlated, 
raw recording density (bit rate) and data modulation (e.g. FM, MFM).

> Can you name another 20 exceptions?   (Chuck and Tony probably can)

Do you want me to start with things like 100tpi drives, GCR, M²FM, 
hard-sectored and other crazy formats?

Just accept that I am not as dumb as you may think.


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