🚀 go-pugleaf

exuptr@exu.ericsson.se (Patrick Taylor) writes:

>In article <1992Aug12.052719.4642@qiclab.scn.rain.com> leonard@qiclab.scn.rain.com (Leonard Erickson) writes:
>>user0006@student.anu.edu.au (Alex Stivala) writes:
>>>Unfortunately, it's not possible due to the fact that the 1541 uses GCR,
>>>which is *not* readable by IBMPC type drives.  You will either need to put
>>>the data onto MSDOS format disks from a Commodore 64 or 128 with a 1571 (or
>>>1581 if you have 3.5") or use some sort of RS232 connection.

>What's GCR?  Anyone?

Group Coded Recording.  It is the way the data is stored on the disk.

Bits stored on a floppy are stored as magnetic transitions:
1 for a change from n-s to s-n, and zero for no change.  The '1' transitions
are also used to keep the bit clock in sync, hence you must have at least one
'1' in every N bits, where 100/N is the percentage variance in disk rotation
speed you are willing to tolerate.  Usualy about 3 or 4.
Also, start of data and header blocks are marked by a series of 8 or more ones,
followed by a zero bit to mark the start of the first byte.  This syncs the
clock, and marks the byte boundaries.

Hence, you must encode data to be stored on a floppy in a form that does not
have large runs one ones, or more thatn a certain number of zeros in a row.
GCR does this by encoding each nybble of data as a set of 5 bits.  I have
not the code handy, but it is chosen such that there is at least enough
zero's to ensure no long runs of ones, and zeros carfully placed such that
no pair of code words placed next to each other produce a string of more
than 2 zeros.

--
Christopher Jam	__________________________________________________
'Another day, another try.  But remember, SHOOT OR DIE!' -turrican
phillips@swanee.ee.uwa.oz.au		c64 forever!!!!
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