Forums

In Dega I'm finding with some SMS games (Chessmaster, Xenon 2), they halt but by pressing the pause button (i.e. cause a one-off NMI) they can be 'jolted' onto the next frame.

Is there a NMI or something similar (e.g. jump to 0x66 or thereabouts) which happens regularly?
Or is it only ever caused by the pause button?

thx

Quote

> In Dega I'm finding with some SMS games (Chessmaster, Xenon 2), they halt but by pressing the pause button (i.e. cause a one-off NMI) they can be 'jolted' onto the next frame.

> Is there a NMI or something similar (e.g. jump to 0x66
> or thereabouts) which happens regularly?
> Or is it only ever caused by the pause button?

On SMS, only the pause button can generate NMI.

Quote

> > In Dega I'm finding with some SMS games (Chessmaster, Xenon 2), they halt but by pressing the pause button (i.e. cause a one-off NMI) they can be 'jolted' onto the next frame.

> > Is there a NMI or something similar (e.g. jump to 0x66
> > or thereabouts) which happens regularly?
> > Or is it only ever caused by the pause button?

> On SMS, only the pause button can generate NMI.

Quote

> > > In Dega I'm finding with some SMS games (Chessmaster, Xenon 2), they halt but by pressing the pause button (i.e. cause a one-off NMI) they can be 'jolted' onto the next frame.

> > > Is there a NMI or something similar (e.g. jump to 0x66
> > > or thereabouts) which happens regularly?
> > > Or is it only ever caused by the pause button?

> > On SMS, only the pause button can generate NMI.

Thought so. I'll try forcing the normal interrupt then: Maybe 0x66 and 0x38 just happen to call the same code in these two games.
It's possible I have the wrong logic for activating an IRQ

Quote

> > > On SMS, only the pause button can generate NMI.

> Thought so. I'll try forcing the normal interrupt then: Maybe 0x66 and 0x38 just happen to call the same code in these two games.
> It's possible I have the wrong logic for activating an IRQ

Slight doh - I have my 'cause NMI' and 'cause IRQ' in the same place in the code, so if an NMI happens, the IRQ doesn't.
(I'm guessing this isn't a good idea since the NMI would happen at a random time in relation to the TV signal, right?)

So what is jolting these games is the IRQ *not* happening, because I pressed the pause button. Too many IRQs. Still haven't found why yet though.

Quote

>
> Slight doh - I have my 'cause NMI' and 'cause IRQ' in the same
> place in the code, so if an NMI happens, the IRQ doesn't.
> (I'm guessing this isn't a good idea since the NMI would
> happen at a random time in relation to the TV signal, right?)

> So what is jolting these games is the IRQ *not* happening,
> because I pressed the pause button. Too many IRQs. Still
> haven't found why yet though.

I've looked through your DOZE code (at least, I think it's yours) and I have a question:

In the case of an INT/NMI on HALT, how does a real Z80 behave? Does it push the PC for the next instruction after the HALT (which I believe is what DOZE does)? Or does it push the PC for the HALT, forcing the processor back into HALT mode on return from the handler?

--
Eric Quinn

Quote

> I've looked through your DOZE code (at least, I think it's yours) and I have a question:

> In the case of an INT/NMI on HALT, how does a real Z80 behave? Does it push the PC for the next instruction after the HALT (which I believe is what DOZE does)? Or does it push the PC for the HALT, forcing the processor back into HALT mode on return from the handler?

Wouldn't the latter mean it would never leave the HALT state?

Quote

> Wouldn't the latter mean it would never leave the HALT state?

Yes, until RESET. In my mind, HALT implies a drastic event (fatal to system), and shouldn't be recovered from easily.

I take it from your response, though, that this is not the case, and execution resumes with the instruction after the HALT.

Thanks.

--
Eric Quinn

Quote

>> Wouldn't the latter mean it would never leave the HALT state?
> Yes, until RESET. In my mind, HALT implies a drastic event (fatal to system), and shouldn't be recovered from easily.
> I take it from your response, though, that this is not the case, and execution resumes with the instruction after the HALT.

Yes, it is the case. HALT is basically used to wait for an interrupt.

Author	Message
Dave Guest	NMIs Posted: Sun Jul 29, 2001 3:08 pm
Dave Guest	In Dega I'm finding with some SMS games (Chessmaster, Xenon 2), they halt but by pressing the pause button (i.e. cause a one-off NMI) they can be 'jolted' onto the next frame. Is there a NMI or something similar (e.g. jump to 0x66 or thereabouts) which happens regularly? Or is it only ever caused by the pause button? thx

Bock Site Admin Joined: 08 Jul 2001 Posts: 8652 Location: Paris, France	Posted: Sun Jul 29, 2001 3:12 pm
	Quote > In Dega I'm finding with some SMS games (Chessmaster, Xenon 2), they halt but by pressing the pause button (i.e. cause a one-off NMI) they can be 'jolted' onto the next frame. > Is there a NMI or something similar (e.g. jump to 0x66 > or thereabouts) which happens regularly? > Or is it only ever caused by the pause button? On SMS, only the pause button can generate NMI.

Bock Site Admin Joined: 08 Jul 2001 Posts: 8652 Location: Paris, France	It's hardwired on the system board nt Posted: Sun Jul 29, 2001 3:13 pm
	Quote > > In Dega I'm finding with some SMS games (Chessmaster, Xenon 2), they halt but by pressing the pause button (i.e. cause a one-off NMI) they can be 'jolted' onto the next frame. > > Is there a NMI or something similar (e.g. jump to 0x66 > > or thereabouts) which happens regularly? > > Or is it only ever caused by the pause button? > On SMS, only the pause button can generate NMI.

Dave Guest	Re: It's hardwired on the system board nt Posted: Sun Jul 29, 2001 3:16 pm
Dave Guest	Quote > > > In Dega I'm finding with some SMS games (Chessmaster, Xenon 2), they halt but by pressing the pause button (i.e. cause a one-off NMI) they can be 'jolted' onto the next frame. > > > Is there a NMI or something similar (e.g. jump to 0x66 > > > or thereabouts) which happens regularly? > > > Or is it only ever caused by the pause button? > > On SMS, only the pause button can generate NMI. Thought so. I'll try forcing the normal interrupt then: Maybe 0x66 and 0x38 just happen to call the same code in these two games. It's possible I have the wrong logic for activating an IRQ

Dave Guest	Slight 'doh!' Posted: Sun Jul 29, 2001 3:29 pm
Dave Guest	Quote > > > On SMS, only the pause button can generate NMI. > Thought so. I'll try forcing the normal interrupt then: Maybe 0x66 and 0x38 just happen to call the same code in these two games. > It's possible I have the wrong logic for activating an IRQ Slight doh - I have my 'cause NMI' and 'cause IRQ' in the same place in the code, so if an NMI happens, the IRQ doesn't. (I'm guessing this isn't a good idea since the NMI would happen at a random time in relation to the TV signal, right?) So what is jolting these games is the IRQ not happening, because I pressed the pause button. Too many IRQs. Still haven't found why yet though.

Eric R. Quinn Joined: 18 Sep 1999 Posts: 498 Location: Portland, Oregon USA	Re: Slight 'doh!' Posted: Mon Jul 30, 2001 9:45 pm
	Quote > > Slight doh - I have my 'cause NMI' and 'cause IRQ' in the same > place in the code, so if an NMI happens, the IRQ doesn't. > (I'm guessing this isn't a good idea since the NMI would > happen at a random time in relation to the TV signal, right?) > So what is jolting these games is the IRQ not happening, > because I pressed the pause button. Too many IRQs. Still > haven't found why yet though. I've looked through your DOZE code (at least, I think it's yours) and I have a question: In the case of an INT/NMI on HALT, how does a real Z80 behave? Does it push the PC for the next instruction after the HALT (which I believe is what DOZE does)? Or does it push the PC for the HALT, forcing the processor back into HALT mode on return from the handler? -- Eric Quinn

Dave Guest	Re: Slight 'doh!' Posted: Mon Jul 30, 2001 10:02 pm
Dave Guest	Quote > I've looked through your DOZE code (at least, I think it's yours) and I have a question: > In the case of an INT/NMI on HALT, how does a real Z80 behave? Does it push the PC for the next instruction after the HALT (which I believe is what DOZE does)? Or does it push the PC for the HALT, forcing the processor back into HALT mode on return from the handler? Wouldn't the latter mean it would never leave the HALT state?

Eric R. Quinn Joined: 18 Sep 1999 Posts: 498 Location: Portland, Oregon USA	Re: Slight 'doh!' Posted: Mon Jul 30, 2001 10:30 pm
	Quote > Wouldn't the latter mean it would never leave the HALT state? Yes, until RESET. In my mind, HALT implies a drastic event (fatal to system), and shouldn't be recovered from easily. I take it from your response, though, that this is not the case, and execution resumes with the instruction after the HALT. Thanks. -- Eric Quinn

Bock Site Admin Joined: 08 Jul 2001 Posts: 8652 Location: Paris, France	Re: Slight 'doh!' Posted: Tue Jul 31, 2001 7:39 am
	Quote >> Wouldn't the latter mean it would never leave the HALT state? > Yes, until RESET. In my mind, HALT implies a drastic event (fatal to system), and shouldn't be recovered from easily. > I take it from your response, though, that this is not the case, and execution resumes with the instruction after the HALT. Yes, it is the case. HALT is basically used to wait for an interrupt.

Forums

View topic - NMIs