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Forums > C64 Coding > How to flags?
2019-09-27 09:52

Registered: Oct 2010
Posts: 66
How to flags?

I want to set a flag in NMI, basically
lda flags
ora #setbits
sta flags

then I want to check & clear the flag in main routine, and branch if the flag was not set. But since it's not possible to suppress nmi by SEI, I can't make a critical section around the handler. There will be always several instructions.

It's preferrable that the checking code would be only in one place. Currently i have
lda flags
and #testbits
eor flags
sta flags
beq skip

I suspect there's a way to do this without the critical section but just wondering if someone knows already.
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2019-09-28 14:07

Registered: Apr 2002
Posts: 4608
thats the same problem again, what is the problem that you want to solve with the flagging ?
2019-09-28 14:38

Registered: Jan 2014
Posts: 47
Often enough in such situations, you can get away with a "don't care" approach, because your code will always pick up the flag way before the next NMI that would potentially set it is triggered. Did you check that, is there an *actual* (not only potential) race condition?
2019-09-28 15:15

Registered: Oct 2010
Posts: 66
lda flags    
             # if nmi sets the tested flag here, it will be
             # overwritten by zero, others get eor'red in
eor flags
             # if nmi sets any flag here it's forgotten
sta flags
2019-09-28 19:59

Registered: Jun 2002
Posts: 93
The NMI should access and set a flag anytime, and the main loop will process a flag when it has time. It does not matter if the NMI has set the flag already twice before it is processed?

Maybe you can duplicate all flags and make it an array of 2 elements? One element is read by the mainloop, the other is written by NMI, and the main loop switches the index when done.

But using single bits of a byte as flags creates quite some overhead in code. If you don't have a "real" array of flags... maybe it is not worth it at all?

I would prefer CJs first solution.
Here's a real lock, but it's of no use here. Here it does nothing more than a flag that denies NMI access to the flags.

lda#$4d ; eor abs
lock sta *
beq locked
lda#8d ;sta abs
sta lock
2019-09-29 08:05

Registered: Oct 2014
Posts: 409
your main code never touches the flags, only the NMI does..

so you have
MainLastState .byte ?
NMIFlags .byte ?

the NMI toggles the flags rather than set and forget.

and then you do

lda MainLastState
cmp NMIFlags
beq -
lda NMIFlags
sta MainLastState

and thus your main code looks for a state change, rather than a value.
2019-09-29 13:09

Registered: Oct 2010
Posts: 66
I like your idea oziphantom! Downside of this is that if nmi can't "set" the flag twice, but it shouldn't be a problem.

And hoogo, I already settled on cj's idea. (it's true it creates overhead in code too) But just wanted to see if it's possible.
2019-09-29 17:23

Registered: Oct 2014
Posts: 409
then get the NMI it set it to the opposite of last flags, thus if it "sets it again" it will still be read as "set" by the main routine.
2019-09-30 19:58

Registered: Oct 2010
Posts: 66
Also one suggestion from TNT/BF (his account is deleted):

This method has a byte per flag, but allows it to queue max 255 sets.

INC flag

LDA oldstate
CMP flag
BEQ noflagset
INC oldstate
JSR do_something
2019-10-02 21:25

Registered: Jun 2002
Posts: 93
For that, a DEC should do, too.

INC flag

LDA flag
BEQ noflagset
DEC flag
JSR do_something
2019-11-04 21:20

Registered: Nov 2019
Posts: 1
I would suggest to use a semaphore which protects a bit field of flags. The semaphore protects in the main program the manipulation of the bit field and prevents a race condition on it.
The semaphore could be realized by means of a byte location where a value of 0 represents a set semaphore. It's some limited style of concurrent programming because the "NMI" task is always atomic, not interruptible by the "main program" task.
Therefore the main program does not need to busy wait on the semaphore or to skip. If it manipulates the bit field it sets the semaphore to enter the critical section with mutual exclusive access to the bit field. If the NMI appears while in the critical section, the bit manipulation from NMI is "masked". The main program task is the dominant part.
        lda #$ff        ; initialize semaphore
        sta sema

        inc sema        ; atomic set & test
        bne skip        ; if sema = 1, semaphore was set (sema=0)
                        ; and main program is in critical section!
                        ; otherwise:
        lda #1          ; bit 0 flag of a flag set
        ora flags
        sta flags
skip    dec sema        ; restore (regarded as atomic with inc sema)

        inc sema        ; atomic set & test (sema = 0)
                        ; semaphore set
        lda flags       ; mutual exclusive
        and #1
        php             ; get flag's state
        lda flags       ; clear bit 0 flag
        and #255-1
        sta flags
        dec sema        ; (sema = $ff)
                        ; semaphore cleared as soon as possible
        plp             ; restore bit 0 state
        beq not_set
        jsr do_something ; if bit 0 was set
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