[Coco] Cartridge memory - mapping in and out

[Steve Bjork]

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

At 09:25 PM 10/29/2007, you wrote:

>Mark McDougall wrote:
>>Mike Pepe wrote:
>>
>>>Hey, keep in mind that unless you're doing your own complete 
>>>address decoding, the cart rom enable line is only active during a read cycle.
>>Oh crap! :( Really?!?
>
>'fraid so. The designers of the SAM and GIME apparently never saw 
>that anyone would have a need to write to a ROM.