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>Hardware Interrupts (Hard IRQs)</H1
><P
> Timer ticks, <TT
CLASS="HARDWARE"
><B
>network cards</B
></TT
> and
<TT
CLASS="HARDWARE"
><B
>keyboard</B
></TT
> are examples of real
hardware which produce interrupts at any time. The kernel runs
interrupt handlers, which services the hardware. The kernel
guarantees that this handler is never re-entered: if another
interrupt arrives, it is queued (or dropped). Because it
disables interrupts, this handler has to be fast: frequently it
simply acknowledges the interrupt, marks a `software interrupt'
for execution and exits.
</P
><P
> You can tell you are in a hardware interrupt, because
<TT
CLASS="FUNCTION"
>in_irq()</TT
> returns <SPAN
CLASS="RETURNVALUE"
>true</SPAN
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> Beware that this will return a false positive if interrupts are disabled
(see below).
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