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<H1><A NAME="SECTION00480000000000000000"></A><A NAME="sub:Overlaying"></A><A NAME="1647"></A>
<BR>
3.8 Overlaying
</H1>
<P>
For non-reentrant<A NAME="1648"></A> functions SDCC will try to reduce
internal ram space usage by overlaying parameters and local variables
of a function (if possible). Parameters and local variables<A NAME="1649"></A>
of a function will be allocated to an overlayable segment if the function
has <I>no other function calls and the function is non-reentrant
and the memory model<A NAME="1650"></A> is small.</I> If an explicit
storage class<A NAME="1651"></A> is specified for a local variable,
it will NOT be overlaid.
<P>
Note that the compiler (not the linkage editor) makes the decision
for overlaying the data items. Functions that are called from an interrupt
service routine should be preceded by a #pragma nooverlay<A NAME="1653"></A>
if they are not reentrant.
<P>
Also note that the compiler does not do any processing of inline assembler
code, so the compiler might incorrectly assign local variables and
parameters of a function into the overlay segment if the inline assembler
code calls other c-functions that might use the overlay. In that case
the #pragma nooverlay should be used.
<P>
Parameters and local variables of functions that contain 16 or 32
bit multiplication<A NAME="1654"></A> or division<A NAME="1655"></A>
will NOT be overlaid since these are implemented using external functions,
e.g.:
<BLOCKQUOTE>
<TT>#pragma save </TT>
<BR><TT>#pragma nooverlay<A NAME="1658"></A>
</TT>
<BR><TT>void set_error(unsigned char errcd) </TT>
<BR><TT>{</TT>
<BR><TT> P3 = errcd;</TT>
<BR><TT>} </TT>
<BR><TT>#pragma restore </TT>
<BR>
<BR><TT>void some_isr () __interrupt<A NAME="1665"></A> (2)</TT>
<BR><TT>{</TT>
<BR><TT> ...</TT>
<BR><TT> set_error(10);</TT>
<BR><TT> ... </TT>
<BR><TT>}</TT>
</BLOCKQUOTE>
In the above example the parameter <I>errcd</I> for the function <I>set_error</I>
would be assigned to the overlayable segment if the #pragma nooverlay
was not present, this could cause unpredictable runtime behavior when
called from an interrupt service routine. The #pragma nooverlay
ensures that the parameters and local variables for the function are
NOT overlaid.
<P><P>
<BR>
<P>
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