[atari7800] Re: [personal] why are IRQs disabled?
- From: "Dan Boris" <dboris@xxxxxxxxxxx>
- To: <atari7800@xxxxxxxxxxxxx>
- Date: Fri, 19 Nov 2004 16:33:08 -0500
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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