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The CD-ROM readers will read what ever is physically crammed on the
discs. The actual amount of data varies from disc to disc. The volume
size reported on PCs is often just a nominal maximum of 650Mb (about
74 minutes running time) rather than the actual disc volume size.
The actual volume size of a disc depends on how much is burned on the
disc -- which types of disks are used to stage the material to be mastered,
which low and/or high volume equipment is used to create the media, etc.
The standard blank discs come rated in number of minutes running time. We
typically use 74 minute blanks, but also have some 68 minute ones.
I've seen some 60, 62, 63 minute blanks and there might be some 78
minute blanks too. I've burned 30 second disks contain one or two
files as well as burning full 74 minute discs.
The formula for determining size of the volume is:
(nn minutes * 60 secs/minute) * 75 * 2048 / 2**10 = Mb
(60 minutes * 60 secs/minute) * 75 * 2048 / 2**10 = 527.34Mb 552,960,000
(68 minutes * 60 secs/minute) * 75 * 2048 / 2**10 = 597.65Mb 626,688,000
(72 minutes * 60 secs/minute) * 75 * 2048 / 2**10 = 632.81Mb 663,552,000
(74 minutes * 60 secs/minute) * 75 * 2048 / 2**10 = 650.39MB 681,984,000
I often see a cdrom quoted as having a capacity of 660 megabytes (10**6,
72 minutes) and 650 Mb (2**10, 74 minutes).
There are practical considerations in burning longer and longer disks.
Its been awhile and equipment might have improved, but longer disks are
placing data near the outer edge of the disc. The outer edge, besides
being more subject to handling, also has more hard manufacturing
defects. For our purposes, we conservatively use 72 minutes as
our max volume size.
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No, you missed the point. VMS can support whatever is on the disc.
There is a physical limit of what can be crammed on a disc. So the
answer is yes.
I'll be happy to provide a DOS CD-ROM driver that will report back a
size of 20000MB if you want... but in reality only the amount of data
that was written on the disc can be read off and that amount is finite
and defined by the cd-rom physical standard and that physical standard
is supported by any/all cd-rom readers.
My point is that you never know if the disc size being reported is
based upon a fixed size hard coded within the driver, the value
returned from querying the device (which might have a hard coded value),
the TOC value from the disc, or a computed value from the data track.
For example, I've a disc containing ~500MB - that is what is reported
back and is derived from the TOC information. But in reality there is
only a 10MB data track (first track) and then a bunch of music tracks
(non-data). The returned value is the size of the whole disc, not the
data track. Put it into another drive/platform combination and it will
report 10MB. In another 600MB, and in another 650MB.... but its all
the same disc.
Reporting on the size of a cd-rom disc is completely relative/arbitrary --
and that was my point.
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