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.
As an aside, I have a machine using standed '360K' 5.25" drives -- that
is Tandon TM100s -- which does correclty implent a disk-changed feature.
How it does it is to have the 2 drives separately cabled to the
controller board and the drives always selected. It then looks for a
change-of-state of the write-protect line. This must change if a disk is
inserted or removed.
Anyone want to guess the machine? I have 3 of them, all different models
(one only has on disk drive).
> 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.
I much prefer having to mount and unmount disks than having an OS that
overwrites the boot sector without asking me....
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!
WOuldn't simply re-reading the directory into the cache on any
'open-for-writing' call get round this. Anyone who switches disks when
files are open is going to have problems anyway.
-tony