2 Jan
2010
2 Jan
'10
10:24 p.m.
On Fri, 1 Jan 2010, Chuck Guzis wrote:
Modern (1.2M 5.25 and 1.44M 3.5) floppy drives come
with a "disk
changed" detection feature (really a "the drive gate was opened--
you'd better check" status). 360K and some 720K drives do not.
Microsoft apparently wanted a volume tracking feature that would work
for all removable media and so devised the above-described method;
words in the KB say that the VSN in the boot sector isn't reliable
enough and the implementors apparently didn't want to take the CP/M
route of checksumming the first few directory entries of a disk.
A quick over-simplified reminder of WHY, . . .
If you switch diskettes, and then WRITE to it, the system can mess up the
diskette because it "remembered" the directory of the wrong diskette.
Particularly when SMARTDSK is on, with write cacheing!
Simple. Press ^C whenever you change diskettes. We've all seen plenty of
times when that was NECESSARY. But, I have NEVER seen ANY mention by
MICROS~1 about ^C to be used to signal disk change to the OS. NEVER.
OK, if they won't tell people to press ^C, (and people would forget
anyway), there needs to be some other way to know about disk change. The
hardware works reliably, WHEN IT IS THERE. Ever have to mask off pin 34?
("READY" V "DISK CHANGE")
VSN should work, but it was too easy to DISKCOPY and end up with two disks
with the same VSN.
Checksumming the DIRectory works marginally OK, but won't catch a
situation where a disk has been replaced by its backup, particularly when
there have been modifications to a file that didn't change the number of
clusters of its length. Instead of the first few DIR entries, the last few
are more likely to be changed with file additions. A USE for detecting 00
vs E5 in the first byte of a DIR entry!
Instead of messing with my boot sector, howzbout creating a bogus
directory entry, as is done for disk name, or long filenames, at the
physical end of the directory sectors.
--
Grumpy Ol' Fred cisin at xenosoft.com
One wonders why MS couldn't have picked somewhere
else to write if
they really needed to do it. We used to warn our customers that if
they were set on using Win95 to browse floppies to make sure that
write protect was set on every floppy they looked at--or to disable
writing (by clipping the WG* pin on the floppy connector)
permanently on the floppy drive.
--Chuck