FDC Gap Length?

[Dave Dunfield]

Hi Jules,

>On Sun, 2005-07-17 at 07:28 -0400, Dave Dunfield wrote:
>> If no calculation is possible, can anyone point me at a more complete table of
>> suggested GPLs?  The NEC table has some large holes, for example 9x512 and 10x512
>> are missing.
>
>In case it's useful, I've got the following in the Torch Manta SCSI-
>floppy controller documentation (amongst others):
>
>512 byte sectors (5.25" or 3.5" media):
>
>  FM recording, 4 sectors/track, gap3 size is 175 bytes of FFh.
>  FM recording, 5 sectors/track, gap3 size is 31 bytes of FFh.
>  MFM recording, 9 sectors/track, gap3 size is 66 bytes of 4Eh.
>
>1024 byte sectors (5.25" or 3.5" media):
>
>  FM recording, 2 sectors/track, gap3 size is 255 bytes of FFh.
>  MFM recording, 4 sectors/track, gap3 size is 255 bytes of 4Eh.
>  MFM recording, 5 sectors/track, gap3 size is 66 bytes of 4Eh.
>
>512 byte sectors (8" media):
>
>  FM recording, 7 sectors/track, gap3 size is 149 bytes of FFh.
>  FM recording, 8 sectors/track, gap3 size is 62 bytes of FFh.
>  MFM recording, 14 sectors/track, gap3 size is 120 bytes of 4Eh.
>  MFM recording, 15 sectors/track, gap3 size is 73 bytes of 4Eh.
>  MFM recording, 16 sectors/track, gap3 size is 33 bytes of 4Eh.
>
>1024 byte sectors (8" media):
>
>  FM recording, 3 sectors/track, gap3 size is 255 bytes of FFh.
>  FM recording, 4 sectors/track, gap3 size is 157 bytes of FFh.
>  MFM recording, 7 sectors/track, gap3 size is 255 bytes of 4Eh.
>  MFM recording, 8 sectors/track, gap3 size is 128 bytes of 4Eh.
>
>
>can't remember what controller IC the board uses now though - I'm
>reasonably sure it's not a 765. 

Thanks - I'm not sure this helps me, as the numbers given are different
than the NEC table. Here's what I have:

(Sector-size  sectors/track  GPL1 GPL2 - all numbers are in decimal)
GPL1 = gap-length when reading/writing
GPL2 = gap=length when formatting

8" FM:
 128    26      7       27
 256    15      14      42
 512    8       27      58
1024    4       71      138
2048    2       200     255
4096    1       200     255

8" MFM:
 256    26      14      36
 512    15      27      84
1024    8       53      116
2048    4       153     255
4096    2       200     255
8192    1       200     255

5" FM:
 128    18      7       9
 128    16      16      25
 256    8       24      48
 512    4       70      135
1024    2       200     255
2048    1       200     255

5" MFM
 256    18      10      12
 256    16      32      50
 512    8       42      80
1024    4       128     240
2048    2       200     255
4096    1       200     255

Clearly the 200/255 are the maximum values. I have tried working the numbers
without these entries, and still cannot come up with a calculation.

As you can see the gap sizes differ quite a bit from the ones that you have
documented - with the 765, this is only one of several gaps, and the controller
does not provide complete access to the raw format like a 179x controller
would.

I assume the gap sizes need to decrease for formats which have an extra
sector (or two) over the NEC table, however I do not know by how much ...
Any additional table entries, or information on how to calculate this would
be much appreciated.


>The Manta docs only talk about gaps 1-4, however I know some of my Acorn
>docs reference a gap5 which sits between the gap4 at the end of the last
>sector on the track and the index marker - I have no idea what that's
>supposed to do as it's listed as being 0 bytes long in the Acorn
>examples!

Normally the last gap is filled from the end of the last sector until
the physical index mark occurs. The 765 appears to do this automatically
as part of the "Format Track" command. For a controller like the 179x,
you need to keep providing filler bytes until the physical end of track.

Regards,
Dave
-- 
dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:
                http://www.parse.com/~ddunfield/museum/index.html