[Simh] Overwrite last track and set badblocks

[Mark Pizzolato]

Hi Will,

On Thursday, January 21, 2016 at 7:01 AM, Will Senn wrote:
> A quick couple of questions...
> 
> 1. Why does SimH prompt to overwrite the last track on some images every
> time it runs, even if I let it, it will ask on the next run.
> 
> 2. What is a use case for setting bad blocks and is it a one shot deal or does it
> need to stay in the .ini?

DEC shipped disk devices which were formatted at the factory.  Almost all
disk media had a very small percentage of the media which didn't perfectly 
store data.  Certain modern, disk devices (MSCP) had spare sectors built into 
the internal format information on the drives and they presented a full disk 
of clean blocks to the system.  Older disks shipped with factory with a defect
table written in the last track of the device.  When an operating system 
initially wrote file system data structures on a disk volume, it would allocate
the defective sectors to a BADBLOCK file on the disk and therefore those 
sectors would be avoided for normal user data.  During the life of the disk
some areas might subsequently become bad.  If the OS detected these 
later the newly identified areas would also be allocated to the BADBLOCK
disk file and the rest of the disk could still be used.

So, back to your original question.  The prompt about "overwriting the last 
track" is intended to create an essentially empty list of defective sectors 
for a newly created disk image (for the disk types which actually had this
'feature').  The intention is that you should be prompted for this (or could 
provide a -Y switch on the attach command) ONLY when you are creating
a new disk image.  If you are being prompted with this question each 
time you attach an existing disk image then there could be a bug.  Please 
identify the simulator and the specific commands which generate this 
prompt.

Thanks.

- Mark