Date   : Mon, 06 Nov 2006 00:39:17 +0000
From   : splodge@... (Richard Gellman)
Subject: DFS on 3.5inch - Is this supposed to work?

Jonathan Graham Harston wrote:
> Richard Gellman <splodge@...> wrote:
>   
>> My initial thought was that a modern PC floppy drive wouldn't be able to
>> handle the 200K single density format of DFS. But when I tried
>>
>> Now my question is, is this working perfectly, or is there some sort of
>> half-way format that I've created where single-density data is being
>> recorded in a double-density mode or some such silliness? I.e. if I put
>>     
>  
> Double-density *IS* single density. Single density *IS* double
> density. The only difference is how the bitstream is encoded. As
> far as the disk and the drive is concerned, it's just 50,000 bits
> of data. What the *controller* does with that data is completely
> irrelavant.
>   
> There's no such thing as a single density disk or a single density
> drive. Single density disks/drives are double density disks/drive
> and vis versa. You should call them "low density" in contrast to
> "high density" disks, the black ones with two holes.
>   
OK... I'm getting a teense tired of people barking information at me as 
if I'm a complete idiot for not knowing this stuff in advance. I freely 
confess to knowing exactly nothing about density differences, except 
that the sector header is different. This is why I post my question to 
the list... i.e. "I do not know what I'm doing here, please tell me if 
I'm too optimistic on this..." etc.

[highly technical information snipped]

I actually found this interesting reading, if a tad unnecessary. My 
question was simply "Will this work, or do I need something else as well?".
> Double density data format uses Modified Frequency (MFM) Encoding.
> This uses *THE* *SAME* *NUMBER* *OF* *BYTES* recorded at *THE*
> *SAME* *DENSITY* written on a disk with *THE* *SAME* *MAGNETIC*
> *SURFACE* with *THE* *SAME* *MAGNETIC* *GRANULARITY* to encode a
> greater amount of user data.
>   
And herein is the barking again. SD and DD are the same format. You made 
your point, now please calm down and refrain from excessive use of bold 
highlighting.

[more technical info snipped]
> Another matter to know is that very few PC disk controllers can
> actually do single density encoding. The vast majority only
> understand MFM and many PCs have crippled BIOSes that only allows
> reading and writing of 512-byte sectors.
>    
> When you cover up the hole in a HD disk the drive will attempt to
> write to it, the drive will record with the 600 oersted DD write
> current. 600 oersteds is insufficient to properly write the data
> and it will fail.
>   
I can report that I've successfully written to HD disks with covered 
holes and read them back again with no faults. I imagine this is down to 
the properties of either the disk or the drive, my thinking leaning 
towards the disk.

If I were to be as curiously pedantic (maybe not the right word?) as 
you, I might point out that not all HD floppies are black. Indeed I have 
a 1.44Mb DOS "rescue" floppy here that is distinctly yellow, but I 
digress....

Now that you've finished speaking to me in such tones...

After a quick test with my model B, it appears that I do indeed require 
something else to make my 3.5in drive work with the model B, as I do get 
the Drive Error 10  when trying to read discs.

I'm told (from another post) that you have prepared a patched DNFS ROM 
image that sorts this problem out. Forgive my being out of touch, but is 
this available ready-blown in a ROM (any cost?), or do I need to invest 
in a chip and dig out my EPROM programmer?

-- Richard