[Coco] reading the ROM on a CoCo 3

[Robert Gault]

Mike Pepe wrote:
> 
> 
> Robert Gault wrote:
> 
>> 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
>>>
>>
>> There should not be any problems but your question needs more details. 
>> Is there an MPI in use? If so, what slots are occupied and with what 
>> packs?
>>
>> Keep in mind that the default setting for a Coco3 is all RAM. That 
>> means you probably want to switch to ROM mode when reading a pack. 
>> That means Basic should not be used to do the reading. Write a short 
>> ml program that goes to ROM mode, reads the pack, goes back to RAM 
>> mode, and then returns to Basic.
>>
> 
> Hey Robert,
> 
> I'm trying out an EEPROM in the socket of an FD502. In order to modify 
> it, I need direct access to the ROM.
> 
> I of course realize that I can't do this in basic, and I am testing in 
> assembly.
> 
> Is there something I'm missing in the CoCo3's GIME registers that turns 
> the ROM on or off? I thought switching the MMU off was all I needed to do.
> 
> -Mike
> 
> 

As Art mentioned, the ROM/RAM switch for the Coco3 is $FFDE-$FFDF. But 
be careful if you POKE this because there is extensive patching of both 
upper and lower RAM for the Coco3 OS. There are no problems with 
assembly code if you kill interrupts as needed.

Are you really going to try to program the EEPROM from the disk 
controller? Don't you need special voltages to switch the chip from Read 
to Write mode? I'd have thought you would need a special EEPROM 
programmer as separate hardware.