[Coco] CoCo 3 patch question

[Robert Gault]

Roger Taylor wrote:

>> Exactly what were you doing to cause the crash?
> 
> I never said I was doing this from the BASIC prompt.  I don't give the 
> command cursor a chance to do it's job.  All along I've said that my 
> "pretend Disk ROM" itself is running, printing text to the VDG screen, 
> and some of the text is corrupt on a CoCo 3.  We figured out why, now 
> I'm trying to switch to ROM mode from within my ROM code since the CoCo 
> 3 has patched over my own ROM thinking it's Disk BASIC.  From M.E.S.S., 
> it doesn't work without a crash.  I have yet to try it from a real 3 or 
> another emulator.
> 
> As we know, any ROM can be put in a disk controller or EPROM Pak and as 
> long as the first two bytes start with "DK", ECB will jump to it.  The 
> appearance is that you've inserted an auto-start program pak, but behind 
> the scenes, the BASIC environment has been set up (without Disk BASIC 
> yet) and is ready to give the OK prompt *IF* the "Disk ROM" doesn't take 
> control.  So on my "embedded system", I don't get any BASIC copyright 
> titles or prompts.  I get my own title screen and running ML code.
> 
> 

The Coco3 cold boot routine will not patch anything near the start of 
$C000. I'd suggest that you have something like:

  org $C002
  bra $C00C
  org $C00C
  sta $FFDE
* Run your won code.

That should be completely safe from any patching. What was patched will 
be back after switching to ROM mode.