[atari7800] Re: [personal] why are IRQs disabled?

The only place where the hardware IRQ is connected in the 7800 is to a
pin on the cartridge connector which no existing cartridge uses. The IRQ
signal has a pullup resistor on it so it shouldn't ever activate, but
it's probably a good idea to keep the interrupt disabled anyway. As far
as I know the BRK instruction is not effected by the SEI which only
inhibits hardware interrupts. 

Dan

> -----Original Message-----
> From: atari7800-bounce@xxxxxxxxxxxxx 
> [mailto:atari7800-bounce@xxxxxxxxxxxxx] On Behalf Of Greg DeMent
> Sent: Thursday, November 18, 2004 9:09 PM
> To: atari7800@xxxxxxxxxxxxx
> Subject: [personal] [atari7800] why are IRQs disabled?
> 
> 
> I've recently noticed that all of the demos and examples I've 
> seen use the "sei" instruction before initializing the 7800.  
> This disables interrupts.  None of the examples use "cli" to 
> restore them.  I'm doing the same thing in my code, but I'm 
> not sure why.
> 
> 
> 1.  Why is it necessary to disable interrupts at this stage?
> 
> 2.  Is it safe to leave them disabled forever?  This causes 
> instability on x86 PCs, but granted the 7800 is a simpler machine.
> 
> 3.  Will the "brk" instruction be affected by this setting?
> 
> 4.  Is there any source of maskable interrupts in the 7800 
> besides the "brk" instruction?  If so, then what are they?  
> If not, then what's the point of calling "sei"?
> 
> Thanks for any information on this.  I'd like to make sure my 
> initialization is in proper form.
> 
> 
> -Greg
> -- 
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