[Coco] The DOS command

[Phill Harvey-Smith]

On 08/11/2019 00:57, RETRO Innovations wrote:
> is there a comparable story about self starting carts that do NOT tie 
> CART to Q on the cart port? I tried to explain that such a thing must be 
> happening for things like CoCoSDC and the FD controller, as they need 
> CART IRQ available, but they autostart...  I assume it must be a special 
> set of values in the ROM on the cart.

First two charcaters of the ROM must be 'DK' and the cart is entered at 
$C002.

 From the Dragon ROM, I'm assuming the CoCo would be much the same :

	LDX     #$444B		; Check for 'DK' flag for DOS cart
	CMPX    $C000
         LBEQ    $C002		; yes, found, jump to Dos init
	
         ANDCC   #$AF		; Enable IRQ and FIRQ
At this point if FIRQ is tied to Q, then the FIRQ vector will be 
followed, which ultimately enters the cart at $C000

It should also be noted that if a 'DOS' cart is entered at $C002, the 
IRQ and FIRQ are still disabled to give the DOS rom a chance to change 
the vectors, and clear any interrupt sources it may be using before 
enabling them.

I do know of at least one cart for the Dragon that enters the code at 
$C002 that is not a DOS cart, namely Compusense's Edit+.

Cheers.

Phill.

-- 
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.