RX01 Floppies

Johnny Billquist bqt at update.uu.se
Sun Nov 23 00:09:46 CST 2014

On 2014-11-22 17:14, Jerome H. Fine wrote:
>  >Johnny Billquist wrote:
>> >On 2014-11-21 19:48, Jerome H. Fine wrote:
>>> I should have realized that the situation needs to be clarified:
>>> (a)  As far as I know, (almost?) all soft sectored 8" floppy media
>>> are formatted as either single-density (which is the IBM standard -
>>> also known as the DEC RX01 option) or double-density.(which
>>> is the additional DEC RX02 option).  In the latter case, the format
>>> is essentially the same, but only the data portion of the sector is
>>> double-density.  In addition, while a DEC RX01 drive can use only
>>> single-density media, a DEC RX02 drive can use BOTH single-
>>> density (SSSD) AND double-density (SSDD) media which means
>>> that the DEC RX02 drive can read / write media from a DEC RX01
>>> drive - but can boot an RX01 media only if the boot block is first
>>> changed from DX to DY.
>> Incorrect. The RX02 cannot read standard 8" Double Density floppies.
>> It can only deal with single density floppies, just like the RX01.
> I am quite sure that your information with regard to Double-Density
> floppies is technically correct.  Your much greater level of experience
> must have acquainted you with actual double-density floppies at some
> point.  However, it would be helpful if you could provide a specific
> example of a real double-density 8" floppy so that we can all share
> the same information.  The information which would be needed is
> the name of the manufacturer of the floppies, the name of the computer
> system, the name of the double-density drive, the name of the controller,
> the name of the operating system and the name of the device driver used
> when a "standard 8" Double Density floppy" is present.  If you don't have
> all of the above information, present at least as much as you remember.
> Also, at least an estimate of the percentage of "standard 8" Double
> Density floppies" in use compared to the standard 8" IBM SSSD
> floppies (including both DEC RX01 and DEC RX02) would be
> useful so we can judge if "standard 8" Double Density floppies"
> were significant.  In my opinion, anything less that 1% would be
> considered insignificant.
> It is great to contradict what all DEC documentation on 8" RX02 floppies
> mentions with respect to double-density media (and given that the DEC
> documentation was usually complete and correct - DEC, in this case
> much have been providing false information or at least characterizing
> "double-density" incorrectly at least as far as you are concerned) and I
> am sure that you do have some competing example in mind.  What is it?
> And why is the statement that DEC makes that the media are double-density
> (when the sector size is 256 bytes) "incorrect"?

The RX02 is not a standard double density floppy. It is a single density 
format floppy with a double density encoding for the data area of each 
sector. That is a very different thing.

I have double density IBM floppies at home, and believe me, I have tried 
using them in RX01 and RX02 drives. They obviously do not work.

But it should be obvious even without much more than a cursory glance at 
the documentation for an RX02. There is no contradictions to what DEC 
documented compared to what I wrote.

DEC RX02 floppies are really the same low level format as RX01 floppies. 
There is a bit in the sector header for each block that is set to 1 if 
the sector is actually in double density for an RX02, or 0 for single 
density. I don't know if IBM themselves ever used that bit in the sector 
header for anything, but DEC used it for this purpose, and it 
documented. You can, on a per sector basis, use either single density or 
double density. The low level format is always the same.


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