[Simh] Difference Disk Files

[Rob Jarratt]

I have attached the source files for the difference disk functionality I
mentioned. I have compiled it on 32 and 64 bit Windows and on 32-bit Linux
(an oldish Slackware distribution). I have only tested it for VAX 780. I
have no easy way to test on 64-bit Linux and do not have a MAC or knowledge
how to use a MAC. Therefore if someone would be prepared to test the code on
those platforms that would be great. The sources are based on SIMH 3.8-1.
Here are the commands:

DIFF ATTACH <Unit> <DifferenceFile> <BlockSize>
Attaches a difference disk file to a unit. The unit must already be attached
and be a random access virtual disk. The <DifferenceFile> is the name of the
file to attach that contains the differences. If the file does not exist a
new one is created. The <BlockSize> is only used when the difference disk
file is created. The unit's virtual disk is divided into blocks of this size
and differences are recorded at this block level. The <BlockSize> parameter
is ignored if the difference file already exists.
DIFF DETACH <Unit>
Detaches the difference file from the unit. Normal operation without
differencing is resumed. However it is not advisable to continue running the
simulated machine after doing this as it is likely to invalidate the
difference disk. There is no mechanism to tell that the underlying disk has
been changed since the difference disk was last used.
DIFF MERGE <Unit>
Merges the attached difference disk back for the <Unit> into the original
disk and then deletes the difference disk file. Requires the difference disk
to have already been attached.
DIFF COMPARE <Unit> <VirtualDiskFile> <DifferenceFile> <BlockSize>
Compares the virtual disk file attached to the <Unit> against another
<VirtualDiskFile> and creates a new <DifferenceFile> with the given
<BlockSize>. If the unit has a difference disk attached the differences in
that difference disk are ignored, the base virtual disk is used.
DIFF STATS
Prints some statistics about the difference disk files.

 
Regards

Robert Jarratt
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