Track length issue when emulating 2 drives

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snhi...@gmail.com

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Aug 7, 2021, 12:13:27 PMAug 7
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After a bit of a battle discovering the correct command line syntax, I convinced mfm_emu to attempt startup with two emulation images from a Tandy Model 16B.  Although both nominally represented the same number of sectors per track, there turned out to be a small difference in total track length that emulation flagged as an error.  Is there any workaround for such cases?  I have a feeling the difference is in trailing fill at the end, so can I modify one of the image's metadata to convince it to look enough like the other to emulate both?

David Gesswein

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Aug 7, 2021, 3:10:18 PMAug 7
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Do you know how you got images with different lengths? Did one manually
specify track_words when creating?

You should be able to safely pad the smaller image to the longer image.
You can try truncating the longer image but I assume there was a reason
it was created with longer tracks.

There is no standard option to modify an image. Modifing the header will
not work since the track data length would be wrong for the header.

Looks like this thread has the best code for using to create a modifed file.
https://groups.google.com/g/mfm-discuss/c/YVww3OdRzu8/m/qfFlcW68BQAJ

snhi...@gmail.com

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Aug 7, 2021, 4:10:08 PMAug 7
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Yes, the problem image was created by a Tandy emulator 'trs80gp'.  It does a workable job of creating an emulation image, but the track length was specified as 20832.  A capture from the original (failing) Tandon disk and images created using mfm_emu --init all regard the track as 20836.  So, the problem is caused by trs80gp.  That code you linked gives me the basis to convert the shorter file, so it is indeed a good workaround.  I've dropped a note to the trs80gp developer to inform him of the issue. 

Once again, thanks much for your prompt response! 

I need to go carefully through the help screens and docs, but at first pass I didn't see a syntax example for setting up two drives.  Probably my oversight!

David Gesswein

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Aug 7, 2021, 5:48:34 PMAug 7
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On Sat, Aug 07, 2021 at 01:10:07PM -0700, snhi...@gmail.com wrote:
> So, the
> problem is caused by trs80gp.
>
Is is directly generating my emulator file or are you converting it to my
emulator file? Found a site for it but looked like disk images were its own
format.


> I need to go carefully through the help screens and docs, but at first pass
> I didn't see a syntax example for setting up two drives. Probably my
> oversight!
>
Organization isn't my strong suit. A little bit is there.
http://www.pdp8online.com/mfm/revb/revb_board_build.shtml

If you wish to try emulating two drives connect J6 to your controller and use --drive 1,2 --file file1,file2 on the command line and set P8 to the drive select you want the second drive to be detected as.

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