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<H2><A NAME="SECTION00047000000000000000">
3.7 Interrupt Service Routines</A>
</H2>
<P>
SDCC allows interrupt service routines to be coded in C, with some
extended keywords.
<BR>
<BR>
<TT>void timer_isr (void) interrupt 2 using 1 </TT>&nbsp;
<BR>
<TT>{ </TT>&nbsp;
<BR>
<TT>.. </TT>&nbsp;
<BR>
<TT>}</TT>&nbsp;
<BR>&nbsp;
<BR>
The number following the <I>interrupt</I> keyword is the interrupt
number this routine will service. The compiler will insert a call
to this routine in the interrupt vector table for the interrupt number
specified. The <I>using</I> keyword is used to tell the compiler to
use the specified register bank (8051 specific) when generating code
for this function. Note that when some function is called from an
interrupt service routine it should be preceded by a #pragma&nbsp;NOOVERLAY
if it is not reentrant. A special note here, int (16 bit) and long
(32 bit) integer division, multiplication &amp; modulus operations are
implemented using external support routines developed in ANSI-C, if
an interrupt service routine needs to do any of these operations then
the support routines (as mentioned in a following section) will have
to be recompiled using the <I>-stack-auto</I> option and the source
file will need to be compiled using the <I>-int-long-ren</I>t compiler
option.
<P>
If you have multiple source files in your project, interrupt service
routines can be present in any of them, but a prototype of the isr
MUST be present or included in the file that contains the function
<I>main</I>.
<P>
Interrupt Numbers and the corresponding address &amp; descriptions for
the Standard 8051 are listed below. SDCC will automatically adjust
the interrupt vector table to the maximum interrupt number specified.
<BR>
<P>
<TABLE CELLPADDING=3 BORDER="1">
<TR><TD ALIGN="CENTER">Interrupt #</TD>
<TD ALIGN="CENTER">Description</TD>
<TD ALIGN="CENTER">Vector Address</TD>
</TR>
<TR><TD ALIGN="CENTER">0</TD>
<TD ALIGN="CENTER">External 0</TD>
<TD ALIGN="CENTER">0x0003</TD>
</TR>
<TR><TD ALIGN="CENTER">1</TD>
<TD ALIGN="CENTER">Timer 0</TD>
<TD ALIGN="CENTER">0x000B</TD>
</TR>
<TR><TD ALIGN="CENTER">2</TD>
<TD ALIGN="CENTER">External 1</TD>
<TD ALIGN="CENTER">0x0013</TD>
</TR>
<TR><TD ALIGN="CENTER">3</TD>
<TD ALIGN="CENTER">Timer 1</TD>
<TD ALIGN="CENTER">0x001B</TD>
</TR>
<TR><TD ALIGN="CENTER">4</TD>
<TD ALIGN="CENTER">Serial</TD>
<TD ALIGN="CENTER">0x0023</TD>
</TR>
</TABLE>
<BR>
<BR>
If the interrupt service routine is defined without <I>using</I> a
register bank or with register bank 0 (using 0), the compiler will
save the registers used by itself on the stack upon entry and restore
them at exit, however if such an interrupt service routine calls another
function then the entire register bank will be saved on the stack.
This scheme may be advantageous for small interrupt service routines
which have low register usage.
<P>
If the interrupt service routine is defined to be using a specific
register bank then only <I>a, b &amp; dptr</I> are save and restored,
if such an interrupt service routine calls another function (using
another register bank) then the entire register bank of the called
function will be saved on the stack. This scheme is recommended for
larger interrupt service routines.
<P>
Calling other functions from an interrupt service routine is not recommended,
avoid it if possible.
<BR>
<BR>
Also see the _naked modifier.
<P>
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<ADDRESS>
<I>Johan Knol</I>
<BR><I>2001-07-13</I>
</ADDRESS>
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