Re: 6502 Multitasking OS announce

[supercat@MCS.COM (John Payson)]

[crossposted because I'm suggesting a technique useable on all 6502 machines]

In article <4iml11$s69@narses.hrz.tu-chemnitz.de>,
Andre Fachat <fachat@physik.tu-chemnitz.de> wrote:
>They disable Multitasking during their critical sections. The problem
>with the C64 is, that is doesn't have atomic test&set operations. So all
>task switches have to be disabled to test a semaphore, or when you enter
>a critical region.

Sure the 6502 has an atomic test-and-set style operation; my preferred
implementation [for example, to ensure that MyRoutine can only be called
once] is...

MyFlag:
	.db	$FF

MyRoutine:
	inc	MyFlag
	bne	MyRtnEnd
	.. do my routine
MyRtnEnd:
	dec	MyFlag
	rts

If MyFlag is $FF when I start MyRoutine, then either the inc will happen
before another instance of MyRoutine is started [in which case, I'll be
able to enter] or else it won't [in which case, the second instance will
get in].  Other variations include shift or rotate instructions but I
don't remember the flag semantics for those; if ROR set the overflow flag
with the old value of bit 7, that would be nice but I don't think it does.
-- 
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 supercat@mcs.com    |  "Je crois que je ne vais jamais voir...  |   J\_/L
 John Payson         |   Un animal aussi beau qu'un chat."       |  ( o o )