Re: Apple2 disk controller questions

[sales@mainlinebbs.com (John H. Guillory)]

On Wed, 12 May 1999 09:11:37 +0200, "Fabrice Frances"
<frances@ensica.NOSPAM.fr> wrote:

>> Apple II double sided, double
>>density 3.5" disks are also GCR (800k).  Apple II double
>>sided, double density disks are MFM (1.44MB).
>>
>>GCR is no less efficient than MFM, unlike FM which is less
>>efficient.  In fact, Apple II and Macs squish 800k into a disk
>>that Win/DOS PCs format to 720k!
    The above, just for the record, has nothing to do with GCR vs MFM
effiency, its dealing with directory structure, and sector size, etc.
In fact, my Commodore FD-2000 uses MFM as its primary mode of
operation, and gets 800k when in 1581 emulation, or using a
double-density disk, and uses 1.6megs when in high-density, just like
the mac.  The 1571, unlike the FD-2000, 1581, and 1541 can read both
GCR and MFM diskettes, which is why I'd like to one day try to get the
directory layout, etc. on an Apple diskette, and  see if the commodore
could read it.  (It'd have to be the 5 1/4" disk though, because the 3
1/2" disk are standard MFM drives...)   From some personal test, and
what I've seen for myself with the 1571, it does appear like the GCR
is slightly more effiecient than MFM, and does seem overall faster.  

>there's still something confusing me...
>It seems 343 GCR-encoded bytes are transfered through the
>disk controller for a single 256-bytes sector... I would say
>GCR efficiency lies between FM (512) and MFM (256),
>am I wrong ?
I'd say above that of the MFM... ;-) But if so, not much...
One thing that confuses me is why on both the Apple and Commodore,
their 3 1/2" can get 800k where as the IBM 720k, yet the 5 1/4" disk
(single-sided) can get 145k as compared to the IBM's 160k/180k
depending on sector size....  Seems if they had the insite to do
everything else better from the start, why not make the 5 1/4" better
from the beginning ;-)

>This would mean more sectors are stored in the outer tracks
>than the inner tracks, is it the case ?
Hmmm, like the Commodore then... The Commodore went one step further
and guaranteed the programmer a set RPM on every track, such that when
it got to the inside tracks, the drive slowed down to maintain the
same speed.... (not one of their smartest moves, but made it tougher
for people to break copy protections....)  I say build a drive to go
Full Throttle all the time, if a given track can read at a rate of
2megs/sec, then do so, if it can only read at a rate of 1k/min. Then
fine, so be it, but read the fastest the drive is capable of for each
individual track, and don't try to limit the speed....  I have a
question for the guru's out there... When you format a disk, what does
the 1:2 and 1:4 mean?