[Coco] auto-starting game paks

[Roger Taylor]

At 09:57 PM 1/19/2009, you wrote:
>Roger Taylor wrote:
>>I've got a little question.
>>What keeps an auto-starting ROM Pak (Q signal tied to CART) from 
>>continuously looping to $C000 instead of running the ROM code?
>>If BASIC is responsible for resetting the FIRQ behavior before 
>>jumping into the ROM, I can understand how this would work right, 
>>but if the CPU itself is responsible by using the upper hardware 
>>vectors to jump directly to $C000 before BASIC even gets a chance 
>>to start, then if the ROM code doesn't start with ORCC #80 (mask 
>>IRQ/FIRQ), then I can't see the code ever getting past the first 
>>instruction, which in some cases is a NOP.
>>Any ideas?
>
>It's the PIA at $FF23 that controls whether the CPU responds to the 
>CART line. Basic turns this on, the ROM pak triggers the interrupt, 
>the interrupt vectors to $A0F6

I'm aware how to configure the interrupts using the PIAs or GIME.  My 
question was whether or not BASIC does the jump to the game pak code 
or if the CPU does it.

I'm trying to think of a way connect Q to CART to make an 
auto-starting pak, but also use a few logic gates or maybe a 
transistor to block the Q-to-CART connection once something in the 
pak is On and Ready, such as the 6551 ACIA which also uses CART.  So, 
once the CPU interrupts are enabled back by the pak ROM software, Q 
will be filtered from entering CART and only the 6551's interrupt pin 
will affect CART.

-- 
Roger Taylor

http://www.wordofthedayonline.com