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<H2><A NAME="SECTION005615000000000000000">
4.6.15 Interrupts</A>
</H2>
<P>
An interrupt<A NAME="3329"></A> service routine (ISR) is declared
using the <I>interrupt</I> keyword.
<P>
<DL COMPACT>
<DT>
<DD>void isr(void) interrupt <I>n</I>
<P>
{
<P>
...
<P>
}
</DD>
</DL><I>n</I> is the interrupt number, which for PIC18F devices can be:
<P>
<DIV ALIGN="CENTER">
<TABLE CELLPADDING=3 BORDER="1">
<TR><TD ALIGN="CENTER"><I>n</I></TD>
<TD ALIGN="CENTER">Interrupt Vector</TD>
<TD ALIGN="CENTER">Interrupt Vector Address</TD>
</TR>
<TR><TD ALIGN="CENTER">0</TD>
<TD ALIGN="CENTER">RESET vector</TD>
<TD ALIGN="CENTER">0x000000</TD>
</TR>
<TR><TD ALIGN="CENTER">1</TD>
<TD ALIGN="CENTER">HIGH priority interrupts</TD>
<TD ALIGN="CENTER">0x000008</TD>
</TR>
<TR><TD ALIGN="CENTER">2</TD>
<TD ALIGN="CENTER">LOW priority interrupts</TD>
<TD ALIGN="CENTER">0x000018</TD>
</TR>
</TABLE>
</DIV>
<P>
<DIV ALIGN="CENTER">
</DIV>
<P>
When generating assembly code for ISR the code generator places a
<SMALL>GOTO </SMALL>instruction at the <I>Interrupt Vector Address</I> which
points at the generated ISR. This single GOTO instruction is part
of an automatically generated <I>interrupt entry point</I> function.
The actuall ISR code is placed as normally would in the code space.
Upon interrupt request, the GOTO instruction is executed which jumps
to the ISR code. When declaring interrupt functions as _naked this
GOTO instruction is <B>not</B> generated. The whole interrupt functions
is therefore placed at the Interrupt Vector Address of the specific
interrupt. This is not a problem for the LOW priority interrupts,
but it is a problem for the RESET and the HIGH priority interrupts
because code may be written at the next interrupt's vector address
and cause indeterminate program behaviour if that interrupt is raised.<A NAME="tex2html54"
HREF="footnode.html#foot6001"><SUP>4.3</SUP></A>
<P>
<I>n</I> may be omitted. This way a function is generated similar
to an ISR, but it is not assigned to any interrupt.
<P>
When entering an interrupt, currently the PIC16<A NAME="3349"></A> port
automatically saves the following registers:
<UL>
<LI>WREG
</LI>
<LI>STATUS
</LI>
<LI>BSR
</LI>
<LI>PROD (PRODL and PRODH)
</LI>
<LI>FSR0 (FSR0L and FSR0H)
</LI>
</UL>
These registers are restored upon return from the interrupt routine.<A NAME="tex2html55"
HREF="footnode.html#foot3352"><SUP>4.4</SUP></A>
<P>
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