Raymond Check discusses MS DOS Floppy Disk Cache

Chuck Guzis cclist at sydex.com
Tue Sep 24 14:51:02 CDT 2019

On 9/24/19 12:40 PM, Christian Liendo via cctalk wrote:
> I meant to say Raymond Chen and it didn't notice the spell check change
> Sometimes modern technology sucks.
> On Tue, Sep 24, 2019 at 3:18 PM Christian Liendo <cliendo at gmail.com> wrote:
>> How did MS-DOS decide that two seconds was the amount of time to keep
>> the floppy disk cache valid?
>> https://devblogs.microsoft.com/oldnewthing/20190924-00/?p=102915

I don't quite get the bit about the NAND gate, but back in the days of
5.25" floppies there was a way around that one.

One need only monitor the drive write-protect status every tick or so.
If it toggles, invalidate the cache.

I implemented it on a 8085 system, checking the write protect status
every 250 msec.  The drive needn't be spinning.  If you insert or remove
a 5.25" disk, the status will change.

We even went a bit further and checked to see if the disk contained any
files open for writing.  If so, we sounded an alert and froze the system
until the subject disk was re-inserted.  This reduced the number of
disk-swapping related errors essentially to zero.


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