[Coco] Nitros9 & Mess

[John Collyer]

The JVC virtual dsk files created by any of Jeff's emulators use no such
header.
Never did and probably never will.  The header was only described by Jeff
but
not ever used. The only part of the header that was used is the sectors per
track
count and defaults to 18 sectors per track.

John Collyer

----- Original Message ----- 
From: "Nathan Woods" <npwoods at cybercom.net>
To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
Sent: Monday, February 02, 2004 10:57 AM
Subject: Re: [Coco] Nitros9 & Mess


> Bill Nobel wrote:
>
> > I discovered this.  I overlayed them as a test to see if it did need
> > Mame and yes it compiled.  I jumped the gun a little by saying it
> > wasn't complete sorry.
>
> No problem; it is a somewhat weird situation and you are not the first
> person to get confused by that.
>
> >  So far I have found a couple of things that could be a possible bug.
> > There is no way the JVC drivers can function correctly with a virtual
> > disk that is NOT an exact size of the disk (even 256 byte boundry).
> > If it is out by 1 byte it will use the very fist byte as a sector size
> > and will scramble disks. if it is not of the same value for the
> > original drive.
>
> This is an interesting issue that has been somewhat confusing for a good
> amount of time.  MESS supports the JVC file format as described here:
>
>     http://home.netcom.com/~tlindner/JVC.html
>
> The problem is that the JVC file format is somewhat fragmented.  At
> first the file format was a simply a straight dump of 256 byte sectors.
> When Jeff introduced his CoCo 3 emulator, he added support for headers
> that could be used to specify the disk geometry.  The size of these
> headers are determined by examining the file size modulo 256.  The
> problem is that other tools such as PORT.EXE do not support these
> headers and will create disk images with lengths that are not modulo 256.
>
> So the net result is that we have what are effectively different disk
> image formats and some tools and emulators will generate disk images
> that are incompatable with each other, and it isn't clear how these
> weird disk images should be handled.  When coding MESS, it was not my
> intention at all to fragment the JVC format, but it is unavoidable when
> there isn't a clear standard that all tools follow.
>
> > I also discovered on a reset the FIRQ lines aren't reset to null as
> > they should be, the code resets IRQ twice.  I don't think this is a
> > problem but it should be addressed.
>
> Can you elaborate on this?  To my knowledge, the call to pia_reset() in
> generic_init_machine() should invoke the callbacks that reset both FIRQ
> and IRQ.
>
>
>
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