Extracting CDOS and CP/M) files

Chuck Guzis cclist at sydex.com
Fri Oct 12 00:29:01 CDT 2007


On 11 Oct 2007 at 20:14, dwight elvey wrote:


>  Another way might be to keep a random number, create from
> a hash of the information on the disk, that would need to be read
> before writing.

Uh, that's what CP/M does--that value of CKS in the DPB tells CP/M 
how many directory entries to checksum--which it does on open calls.  
Unfortunately, one needn't do an open call to create a file--a valid 
filled-in FCB usually is good enough.  There were applications that 
swapped FCBs around--and before CP/M 2.0, there were no random-access 
positioning calls, so programs would do their own FCB fiddling.  A 
hole that you could drive a truck through.

All of that got pretty much taken care of with CP/M 3.0 and MP/M, but 
CDOS predates those.  But even if you could tell that a disk was 
changed, there was no system noficiation function to call in CP/M to 
tell the OS to check the disk.

Cheers,
Chuck






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