On 2026-02-08 15:38, Martin Nicholas via Amforth-devel wrote:
Firstly, here's the docs:
http://amforth.sourceforge.net/TG/AVR8.html#interrupts
http://amforth.sourceforge.net/TG/recipes/Interrupt-Critical-Section.html
Source files:
amforth-6.9/avr8/lib/hardware/int-q.frt
etc.
Because of the way AmForth handles interrupts, the I-flag can be
cleared (disabling the interrupts globally) at any time during the
execution of a Forth primative. It can change to zero between any two
M/C code instructions.
So "int?" has to check both the I-flag and the value of "isrflag"
(T-flag) to come to a valid conclusion. "-int" cannot just perform a
CLI instruction as the next NEXT may process a pending (pending before
the CLI was executed) interrupt and re-enable them. Etc.
Reliable fetch of interrupt status.
Checks for pending interrupts.
I T*** int? Remarks
= = ==== =======
0 0 0 Interrupts disabled by the program.
0 1 -1 Interrupt pending*
1 0 -1 Normal operation.
1 1 -1 Shouldn't ever be**
* This will be serviced by the next NEXT and SREG:I will be
set once more.
** The interrupt will still get serviced, provided it isn't
overwritten.
*** The T-flag seems to be reserved as a holder for the ISR
status, but currently it isn't used. Location "isrflag" is used
instead and is a much slower.
Special care has to be taken to save and restore the I-flag. I have
written macro "no_ints". It uses MCUCR:IVCE to
disable interrupts without affecting the I-flag - see the docs.
Correctly preserve the interrupt status in @0:
.macro no_ints
in @0, MCUCR
ori @0, (1<<IVCE)
out MCUCR, @0
in @0, SREG
cli
.endmacro
Later Atmel cores automatically disable interrupts for a time if you
fiddle with the SP register, so "no_ints" is not required here. Maybe
there are other operations that do this too.
Hi Martin,
Thank you. Added to the list.
A preliminary check of the archives shows that the way interrupts were
handled by the VM changed between releases 6.4 and 6.5
In 6.4 there is
brts DO_INTERRUPT
In 6.5 there is
cp isrflag, zerol
brne DO_INTERRUPT
The release notes for 6.5 [1] state that there was a bug in the
interrupt handling on the avr8. Erich discovered and fixed the bug, and
so may be able to help. Current avr8 documentation references the T flag
but does not reference isrflag, so this is something that will need to
be updated.
The file avr8//drivers/generic-isr.asm shows considerable related
changes between the releases but I need more time to study it.
Best wishes,
Tristan
[1] https://sourceforge.net/p/amforth/mailman/message/35814877/
_______________________________________________
Amforth-devel mailing list for http://amforth.sf.net/
[email protected]
https://lists.sourceforge.net/lists/listinfo/amforth-devel
