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

[jdaggett at gate.net]

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. 

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. 
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