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<H1><A NAME="SECTION00470000000000000000"></A><A NAME="1606"></A><A NAME="1607"></A><A NAME="1608"></A><A NAME="sec:Parameters-and-Local-Variables"></A>
<BR>
3.7 Parameters & Local
Variables
</H1>
<P>
Automatic (local) variables and parameters to functions can either
be placed on the stack or in data-space. The default action of the
compiler is to place these variables in the internal RAM (for small
model) or external RAM (for medium or large model). This in fact makes
them similar to <I>static<A NAME="1610"></A></I> so by default functions
are non-reentrant<A NAME="1611"></A>.
<BR>
<BR>
They can be placed on the stack<A NAME="1612"></A> by using the <I>--stack-auto<A NAME="1613"></A></I>
option, by using <I>#pragma stackauto</I><A NAME="1615"></A>
or by using the <I>reentrant<A NAME="1616"></A></I> keyword in the
function declaration, e.g.:
<BLOCKQUOTE>
<TT>unsigned char foo(char i) __reentrant </TT>
<BR><TT>{ </TT>
<BR><TT> ... </TT>
<BR><TT>}</TT>
</BLOCKQUOTE>
Since stack space on 8051 is limited, the <I>reentrant</I> keyword
or the <I>--stack-auto</I> option should be used sparingly. Note
that the reentrant keyword just means that the parameters & local
variables will be allocated to the stack, it <I>does not</I> mean
that the function is register bank<A NAME="1626"></A>
independent.
<BR>
<BR>
Local variables<A NAME="1627"></A> can be assigned storage
classes and absolute<A NAME="1628"></A> addresses, e.g.:
<BLOCKQUOTE>
<TT>unsigned char foo(__xdata int parm) </TT>
<BR><TT>{</TT>
<BR><TT> __xdata unsigned char i;</TT>
<BR><TT> __bit bvar;</TT>
<BR><TT> __data __at<A NAME="1634"></A> (0x31) unsigned char
j;</TT>
<BR><TT> ... </TT>
<BR><TT>}</TT>
</BLOCKQUOTE>
In the above example the parameter<A NAME="1638"></A> <I>parm</I>
and the variable <I>i</I> will be allocated in the external ram, <I>bvar</I>
in bit addressable space and <I>j</I> in internal ram. When compiled
with <I>--stack-auto</I> or when a function is declared as <I>reentrant</I>
this should only be done for static variables.
<P>
It is however allowed to use bit parameters in reentrant functions
and also non-static local bit variables are supported. Efficient use
is limited to 8 semi-bitregisters in bit space. They are pushed and
popped to stack<A NAME="1645"></A> as a single byte just like the normal
registers.
<P>
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