Commodore Free 6/Commodore_Free_Issue_06_2007_Commodore_Computer_Club.d64/t.pres.1

Text File | 2007-01-01 | 6.6 KB | 207 lines

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    Commodore Preservation Project
      http://rittwage.com/c64pp

PART 1

Welcome to the Commodore 64 (C64)
Preservation Project The main goal of
this project is to archive pristine
versions of original Commodore 64
software, including copy protection. A
secondary goal will benefit of this
will be to catalog and document all
the different copy protection methods
used. This information will be used to
improve emulation, as well as
remastering software onto disks for
you to enjoy on the real thing. These
goals are much like that of C.A.P.S.
project for the Amiga. Of course, to
reach these goals, we need your help.
This software exists only on magnetic
media from the 1980's, and as such has
been disappearing into attics, yard
sales, and landfills for almost 20
years. Floppy disks were also never
made to last a lifetime, as I have
found in purchasing them in auctions.
Even disks that were stored and cared
for by their owners all these years
have pretty high failure rate, some
where the magnetic material actually
wipes right off, making the drive
heads filthy. You can help preserve
them for our own and future
generations in a number of ways.

* If you have a 1541 or 1571 disk
drive and a XEP, XAP, or XMP cable
(serial/parallel combination), we can
send you the latest mastering
software. You can then send us the
resulting image and statistical data
for analysis and inclusion in the
archive.

* We will happily pay for postage to
get any original disks you have to
us! We will promptly image them and
send them back to you.

* If you don't have the cabling or
don't wish to send us your originals,
you can make us nibbled copies of the
originals by whatever the best means
you have and send us those. We will
pay for postage, as well as return
these disks to you, of course. We can
sometimes reconstruct the protection
onto the image by looking at what it
checks for, plus they can be used to
verify the original image if nothing
else.

Copy Protection Methods Many different
protection methods were developed over
the years in a cat-and-mouse game with
those that wanted to make a copy of
their own (or a friend's) disk. The
methods steadily increased in
complexity, but whether or not the
protection was copyable or not, a way
was always found to make a working
backup. Descriptions of the drive
hardware itself, as well as many of
the methods that different companies
employed to keep the disks from being
copied are found here.

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Background

The Commodore 1541 disk drive is a
stand-alone computer that talks to the
C64 through a somewhat slow serial
port. It is based on similar
technology to the C64 itself,
employing a 6502 CPU, two 6522 VIA I/O
chips, and only 2k of memory (the
limitation of which will be discussed
later). It came with either an Alps or
a Newtronics 5.25" double-density
floppy mechanism, both of which are
functionally equivalent. This
mechanism requires double-density 48
track-per-inch 5.25"
magnetically-coated Mylar disks.

Tracks Tracks on the disk are
organized as concentric circles, and
the drive's stepper motor can stop at
84 different locations (tracks) on a
disk. However, the read/write head on
the drive is too wide to use each one
separately, so every other track is
skipped for a total of 42 theoretical
tracks. The common terminology for the
step in between each track is a
"half-track" and a specific track
would be referred to as (for example)
"35.5" instead of the actual track
(which would be 71). Commodore limited
use to only the first 35 tracks in
their standard DOS, but commercial
software isn't limited by this. Most
floppy media is rated to use 40
tracks, and the drives usually have no
trouble reading out to track 41,
although some will bump and not get
past 40. Most software does not use
any track past 35 except for copy
protection, but alternative DOS
systems like Speed-DOS used all 40
tracks in it's own DOS implementation.

CBM drives have no way in hardware to
detect which track it is on, or where
it is on any particular track. The
software must handle these functions
which leads to many of the more
creative styles of copy protection.
5.25" disks contain an "index hole"
which is the little hole you see
diagonal to the hub ring on your
disks. If you spin the disk around in
it's shell, you'll see that there is a
hole in that, too. Other drive
manufacturers used an optical sensor
to detect when this hole passed by,
which would signal the start of a
track. Commodore didn't implement
this, so we have to use marks that are
written to the disk during formatting.
Later copy protection implementations
took advantage of this because the
devices used to master original
software are not typically a 1541.
They could master the disk using the
index hole so it was lined up
perfectly. As far as knowing which
track we are on, the only way to tell
is again in software. Each sector on a
track has a header that contains
(among other things) it's track
number. DOS will read this whenever it
needs to access a track so it knows
how many times to step the motor to
get there. The famous "drive-knock" is
the software hack that Commodore
employed to reset the drive to track 0
when we couldn't find any sectors. In
this case, the motor is stepped out as
far as it can go until it physically
stops (causing the knocking noise)
which guarantees it's at track 0.
Unfortunately, this behavior is what
is blamed for the terrible drive
alignment problems of the early
mechanisms.

Sectoring Tracks are further divided
into sectors, which are sections of
each track divided by the
forementioned software-generated sync
marks. The drive motor spins at 300rpm
and can store data at 4 different bit
densities (essentially 4 different
clock speed rates of the read/write
hardware). The different densities are
needed because being round and the
motor running at a constant speed, the
disk surface travels over the head at
different speeds depending on whether
the drive is accessing the outermost
or innermost tracks. Since the surface
is moving faster on the outermost
tracks, they can store more data, so
they use the highest density setting.
Consequently, the innermost tracks use
the slowest density setting. Because
it's recording at a higher density, of
course more sectors are stored on the
outer tracks, and fewer on the inner
tracks. There is nothing stopping the
hardware from reading/writing at the
highest density across the entire disk
surface, but it isn't generally done
due to media reliabilty, and slight
speed differences between drives. The
media itself is only rated for a
certain bitrate at a certain speed.

Continued in part 2

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