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base:nmi_lock [2015-04-17 04:33] (current)
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 +====== NMI lock ======
 +From Go64!/CW-issue 09/1999
 +By Wolfram Sang (Ninja/The Dreams -
 +Final section added by Frantic/HT after some confused discussions on the CSDb forum.
 +Don't rely on names!
 +NMI is short for "Non-Maskable Interrupt" what means, you can't disable
 +it. But as we talk about C64, there is of course a way to do so.
 +Some demo-effects or transmission routines have a very critical timing.
 +Hit RESTORE once and everything is gone. Even if the NMI-vector points
 +to a RTI, so the Interrupt will be quit immediately, it will "cost"
 +13 cycles, but just one would be far too much. So it can be quite useful
 +to disable the NMI, though it is a little against its purpose. First
 +I am going to talk about the theory, if you are just interested in the
 +result, then copy the corresponding lines from the example source code.
 +To avoid confusion I want to mention before, that IRQ and NMI are both
 +interrupts, where NMI has higher priority. As the IRQ is more often
 +used, some people call it just "interrupt". Please don't mix it up!
 +I will use exact definitions in this article.
 +======  Theory... ======
 +First question is, how are interrupts on the C64 generated? Let us
 +examine the "standard" IRQ. The CPU checks its IRQ-line for a LOW-signal.
 +If there is one, an interrupt will be initiated. It has to be
 +acknowledged via registers $D019 (for VIC) or $DC0D (for CIA1), so the
 +IRQ-line goes HIGH again. If you forget that, there will be another
 +interrupt right after the first one, as there is still a LOW-signal!
 +Okay? Good.
 +Concerning NMI this procedure is a little different. Not
 +the LOW-signal forces this interrupt, but the change from HIGH to LOW.
 +Of course, the NMI-line has to be raised as well, register $DD0D does
 +that job for CIA2, and some hardware logic for the RESTORE-key. And
 +here is the trick: Don't do that, and nothing special will happen. We
 +still have a LOW-signal, but remember, it is the changing from HIGH to
 +LOW which initiates an interrupt. Best thing is, other incoming
 +HIGH-signals (e.g. from RESTORE) will be absorbed. Aforementioned
 +change is not possible anymore, the CPU will never get to know, if an
 +NMI is requested. I guess, this can be called "disabled NMI", to
 +reenable it, just acknowledge via $DC0D, so do what you have
 +intentionally "forgotten" before.
 +====== ... and praxis ======
 +For a better understanding I'll give you some comments on the
 +example-sourcecode. If you are not familiar with CIA-Timers, I recommend
 +getting some descriptions of them, because it would be too much for this
 +article, if I should explain them here. At the beginning the programm
 +disables IRQ-interrupts and changes the NMI-vector to our own routine.
 +Then Timer A of CIA2 is stopped and loaded with 0, so after setting it
 +as NMI-source and starting it, a NMI is going to occur. The interrupt
 +routine increases the border-color, which is only for illustration
 +purpose, and then exits the interrupt without reading $DD0D. The
 +interrupt is not acknowledged! The main programm now continuously
 +changes the upper-left corner of the text-screen, again only for
 +illustration. Try pressing RESTORE, which would normally cause a NMI,
 +but nothing will happen, the border-color stays the same. For a
 +counterexample just press SPACE. The Timer A will be cleared as
 +NMI-source (otherwise it would initiate another NMI and disable it
 +again) and NMI-line will be set HIGH by acknowledging through reading
 +$DD0D. As NMI is allowed again, pressing RESTORE will increase
 +Well, that's it! Like most good tricks, it is not much, you just have
 +to know, how it works. If you have problems in understading at first,
 +take your time and try it again. It might look complicated, but it is
 +not, honestly.
 +====== Source Code ======
 +    ; 'Disable NMI' by Ninja/The Dreams/TempesT
 +        SEI             ; disable IRQ
 +        LDA #LO(NMI)    ;
 +        STA $0318       ; change NMI vector
 +        LDA #HI(NMI)    ; to our routine
 +        STA $0319       ;
 +        LDA #$00        ; stop Timer A
 +        STA $DD0E       ;
 +        STA $DD04       ; set Timer A to 0, after starting
 +        STA $DD05       ; NMI will occur immediately
 +        LDA #$81        ;
 +        STA $DD0D       ; set Timer A as source for NMI
 +        LDA #$01        ;
 +        STA $DD0E       ; start Timer A -> NMI
 +                        ; from here on NMI is disabled
 +         INC $0400      ; change screen memory, proves computer is alive
 +        LDA #$10        ; SPACE pressed?
 +        AND $DC01       ;
 +        BNE NOSPC       ; if not, branch
 +        LDA #$01        ; if yes, clear Timer A
 +        STA $DD0D       ; as NMI source
 +        LDA $DD0D       ; acknowledge NMI, i.e. enable it
 +        JMP LOOP        ; endless loop
 +         INC $D020      ; change border colour, indication for a NMI
 +        RTI             ; exit interrupt
 +                        ; (not acknowledged!)
 +====== Aren't NMI interrupts blocking IRQ interrupts? ======
 +Some of you might have learned that IRQ interrupts are not served when NMI interrupts are being served. ...and you are right! Then a reasonable question is: when using the method to block NMI interrupts described above, why aren't IRQ's disabled too if NMI's are disabled? After all, the method relies on trigging an interrupt, but never acknowledging it. The answer is that IRQ interrupts are not disabled just because NMI interrupts are not acknowledged. As soon as the RTI of the NMI interrupt is executed, IRQ interrupts will be served again. The crucial thing to note here is that you should not mix up the execution of an RTI instruction and acknowledging interrupts. You can do one of them, without doing the other. This means that if you want to be able to use IRQ interrupts when NMI interrupts are disabled, you can safely "neglect" (intentionally) to acknowledge the NMI interrupt, as described in the article above, but that you should make sure that you execute an RTI instruction.
 +To be even more precise IRQs are not allowed until the interrupt mask flag of the CPU is cleared. When an NMI starts, the I flag is set automatically and RTI clears the I flag among other things. If you clear the mask manually (using CLI) before quitting the NMI code with RTI, you can serve IRQs while executing your NMI.
base/nmi_lock.txt ยท Last modified: 2015-04-17 04:33 (external edit)