[Coco] Cartridge memory - mapping in and out

[Mike Pepe]

Steve Bjork wrote:
> It's not a question of needing to write to a ROM, you can't.  (and not 
> for the reason your thing of.)
> 
> Think about the DATA bus.  If you write to a RAM chip the data bus (of 
> the RAM chip) goes into tri-state so it can receive the data from the CPU.
> 
> Not so with ROMs. If a ROM is selected it put its logic onto the data 
> bus, be it a read or write cycle.  In the case of a write cycle, you 
> could a logic level short if the data did not match.  (Yes, some ROMs 
> and EPROMs more than one chip select, but not all of the cheep ones.)
> 
> Now, the ROMs on a CoCo card only use the cart ROM enable line to select 
> the chip and the read/write does NOT come into play.  This makes the 
> cart and ROM chips less costly to manufacture.
> 
> The Bottom line: This was designed into the coco from day one to 
> safeguard the CPU and lower the cost of the ROM paks.
> 
> Steve (6809er) Bjork
> 

Steve and Phill are correct in their technical assessment of the situation.

If this were my project, I'd use an "indirect" method to program the 
flash. I'm thinking something like Tony D.'s ubiquitous auto-increment 
circuit he uses in his super floppy controller, ram disk, and eprom 
programmer. You can use a few addresses in the *SCS range to program the 
flash.