RX01 Floppies

Johnny Billquist bqt at update.uu.se
Fri Nov 21 15:23:47 CST 2014

On 2014-11-21 12:57, Noel Chiappa wrote:
>      > On 11/21/2014 2:26 PM, Glen Slick wrote:
>      > Is it correct that RX01 format floppies can be created from blank
>      > floppies with a standard PC floppy controller (or at least some
>      > controllers / motherboards, depending on the controller chip) while
>      > RX02 format floppies cannot?
> My understanding (perhaps incorrect, if so, someone please correct me) is that
> the wart-level-detail answer to this question is that '_blank_ RX02 floppies
> can be created on anything that can create RX01 floppies', but that RX02
> floppies _with data on them_ can only be created on RX02 drives. I.e. an RX02
> is fed RX01 floppies, which it then writes in the unique-to-DEC double density
> RX02 format. (As Chuck explained, the _headers_ are in the standard IBM form,
> but the _data_ part of the sectors is written in a DEC-specific double-density
> format.)

Pretty much correct.

You could also say that there is no RX02 format. You cannot format a 
floppy in RX02 format.
An RX02 floppy is formatted the same way as an RX01.
When an RX02 drive want to do double density, it just writes a bit in 
the sector header saying that this sector is in double density, and the 
actual data part of the sector is written in double density. The format 
of the floppy as such is still the same as an RX01.
And of course, an RX02 reading that sector will notice the bit in the 
header saying that it is double density, and will read the actual data 
part of the sector in double density mode.

So, any floppy that you use in an RX01 or RX02 is *always* formatted as 
an RX01. Which is the same as the original IBM SD SS 128 bytes per 
sector format.

Only an RX02 can actually do the DEC double density format as done by 
the RX02. You cannot read that floppy on other drivers with other 
controllers generally, since they aren't actually capable of switching 
between different densities in the middle of a sector.

But the RX02 is not a separate format as far as the floppy goes. It's 
just the data in each sector so written that is "special".


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