[Coco] How does the cartridge port work, anyway?

[William Astle]

On 13-02-15 05:30 PM, jdaggett at gate.net wrote:
> I believe the NMI IRQ routine checks the first two bytes at $C000 and determines if there is a
> valid Disk ROM located there first. For the Floppy Disk Cartridge the first two bytes are $44
> and $4B respectively (DK). That tells basic that there is a disk cartridge there. I think that the
> rest do start execution from $C000.

Actually, the NMI has nothing to do with starting cartridges at all. The 
autostart game cartridges use the FIRQ as noted elsewhere in the thread 
and the FIRQ handler does a bit of setup and then jumps to C000.

The Disk ROM detection is done by the extended basic ROM during its 
initialization just before it transfers control to the basic main loop. 
There is no interrupt involved in this detection at all. This is the DK 
detection business.


>
> james
>
> On 15 Feb 2013 at 5:09, John Donaldson wrote:
>
>> Allen,
>>      I'm surprised at you. It is really simple. The Cart connector has a pin that
>> is connected to the NMI (Non Maskable Interrupt) of the 6809 CPU. This interrupt
>> is tripped by the Cart's when the COCO is powered up. The software routine
>> associated withe the NMI then calls Address C0000 and executes the instructions
>> starting at that address. Thus the Cart program then runs.
>>
>> John Donaldson
>>
>>
>>
>>
>> ________________________________
>> From: Allen Huffman <alsplace at pobox.com>
>> To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
>> Sent: Thu, February 14, 2013 11:03:58 PM
>> Subject: [Coco] How does the cartridge port work, anyway?
>>
>> I have wondered this for ages, and thought this might be a good place to ask...
>>
>> Could someone explain how the CoCo cartridge port works? How are cartridges able
>> to show up as memory locations in that final area of the 64K?
>>
>> And, perhaps, in terms a software guy with little hardware background could
>> understand?
>> -
>> Allen Huffman - PO Box 22031 - Clive IA 50325 - 515-999-0227 (vmail/TXT only)
>> Sent from my MacBook.
>>
>> 22nd Annual "Last" Chicago CoCoFEST! April 27-28, 2013. Lombard, IL.
>> http://www.glensideccc.com
>>
>>
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>
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