[Coco] reading the ROM on a CoCo 3

[jdaggett at gate.net]

Mike 

Try enabling the ROM mode, clear $FFDE then set the INIT0 ($FF90) register with $8C. 
Also why are you disabling the MMU? Try $CC instead, ,leaving the MMU active!!!!! May be 
your cause for crashes! 

I went back thru some of the listing code for the SECB rom and it seems that a write to 
FFDE is done before $FF90. 

Looks as if t here maybe a small quirk in the way the GIME works. 

james


On 28 Jul 2006 at 6:54, Mike Pepe wrote:

> Oh, duh!
> 
> Forgot about the old SAM ROM/RAM switch.
> 
> Interestingly enough, when I do that, the program immediately crashes.
> 
> I'm loading the short code in $0E00, the machine has 512k. First thing 
> it does is disable the interrupts.
> 
> Soon as I sta $ffde, the machine locks up and the screen fills with crap.
> 
> maybe I need to move the code to a different location.
> 
> 
> 
> 
> Arthur Flexser wrote:
> > This is doable under BASIC by POKE&HFFDE,0, which switches in the ROMs, prior to
> > doing the read.  (You need to do this from a 32-column screen, since you'll be
> > switching out Super Extended Basic's RAM drivers for the 40/80 column screens.)
> > That will get you $C000-FDFF.  If you need $FE00-FEFF as well, there's a bit in
> > one of the GIME registers ($FF90? $FF91?) that enables you to switch off "always
> > RAM at $FE00" mode, though you probably need to disable the IRQ first to avoid a
> > crash (POKE &HFF03,&H34), since you'll be switching out interrupt vectors.
> > 
> > Art
> > 
> > On Thu, 27 Jul 2006, Mike Pepe wrote:
> > 
> >> Hi guys,
> >>
> >> Does anyone know how to read the cartridge ROM directly on a CoCo 3?
> >>
> >> Whenever I try, I read the RAM copy. There is no activity on the cart's 
> >> ROM select.
> >>
> >> I tried setting $FF90 to $8C, thinking that would switch off the MMU and 
> >> go directly to the hardware, but doing reads results in nothing.
> >>
> >> The only time I get read activity on that cart select pin is after reset.
> >>
> >> What's the trick?
> >>
> >> -Mike
> >>
> >> -- 
> >> Coco mailing list
> >> Coco at maltedmedia.com
> >> http://five.pairlist.net/mailman/listinfo/coco
> >>
> > 
> > 
> 
> 
> -- 
> Coco mailing list
> Coco at maltedmedia.com
> http://five.pairlist.net/mailman/listinfo/coco
> 
> 
> -- 
> No virus found in this incoming message.
> Checked by AVG Free Edition.
> Version: 7.1.394 / Virus Database: 268.10.4/402 - Release Date: 7/27/2006
>