[Coco] Cartridge memory - mapping in and out
6809er at bjork-huffman.net
Tue Oct 30 01:44:26 EDT 2007
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.
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