GCR stands for "group Coded Record"
On Thu, 3 Nov 2022, Fred Cisin via cctalk wrote:
Also GCR, not MFM. NOT readable with a PC FDC.
On Thu, 3 Nov 2022, Grant Taylor via cctalk wrote:
Please expand "GCR".
Sure, . . . (GROSSLY OVER-SIMPLIFIED, such as "pulse" instead of flux
transition)
FM is "frequency modulated". Well, it is actually a regular clock pulse,
with data bit pulse, or no pulse, between each of the clock pulses.
There is, of course, a limit to how densely packed that can be on a track.
A signal with all zero bits of the data, and a signal with all one bits of
the data therefore are two different frequencies.
MFM is "Modified Frequency Modulated". Clock pulses really aren't necessary
when they fall between two consecutive data pulses. If we leave those out,
we end up with a much less dense pattern of pulses. (Over-simplified: MFM is
FM without any clock pulses deemed "unnecessary") We can get away with a
higher data transmission rate, even TWICE, and still not be much too
overcrowded on the track. Therefore, twice as much data per track. The
marketing people called that "DOUBLE DENSITY", and immediately started
calling FM, "SINGLE DENSITY", although some engineers would argue that FM was
"half density" and MFM would be "about single density". If you do historical
research, you will find the term "double density" was used in the literature
BEFORE the term "single density" was (Just like the phrase "WORLD WAR TWO"
was used in newspapers before "WORLD WAR I" was ever applied to the "great
war")
But, going back to FM, . . . if you look at all of the patterns of pulses,
you'll see that not ALL of them are dense. In fact, of the 256 possible
patterns for an 8 bit byte, you can find 32, or even 64, that are low enough
density that they could be compressed. We can use 5 or 6 bits to represent
those patterns. But, having only 5 or 6 bits usable to only use the specific
patterns that were low enough density means that we can't use 8 bit bytes
directly. but, we COULD recombine, to store 5 8 bit bytes as 8 5 bit
patterns, or 3 bytes as 4 6 bit patterns. THAT produced low enough "density"
of the signal that by upping the data transfer rate, about one and a half
times as much data scould be stored on a track, admittedly with some
additional processing overhead. Thus, the Apple2 got about 140K on a disk,
when the TRS80 got about under 100K (89,600). (Both were originally 35
track, using Shugart SA400 and SA390 drives)
"Beneath Apple DOS" has a decent description)
The FDC of PC can only directly handle WD/IBM sector and track structure, so
reading GCR, such as Apple (prior to 1.4M) Victor/Sirius, Commodore, etc.
calls for different hardware.
http://www.xenosoft.com/fmts.html has a list of a few of the different
machines that use formats that CAN be done using the PC FDC. They do still
have different file systems, with various sector sizes, and directory
structures.
--
Grumpy Ol' Fred ci...@xenosoft.com
