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<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>

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<LI><A NAME="tex2html2149"
  HREF="node98.html#SECTION004181100000000000000">3.18.1.1 Small, Medium, Large and Huge</A>
<LI><A NAME="tex2html2150"
  HREF="node98.html#SECTION004181200000000000000">3.18.1.2 External Stack</A>
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<H2><A NAME="SECTION004181000000000000000"></A><A NAME="2576"></A><A NAME="2577"></A>
<BR>
3.18.1 MCS51 Memory Models
</H2>

<P>

<H3><A NAME="SECTION004181100000000000000">
3.18.1.1 Small, Medium, Large and Huge</A>
</H3>

<P>
SDCC allows four memory models for MCS51 code, <I>small, medium,
large</I> and <I>huge</I>. Modules compiled with different memory models
should <I>never</I> be combined together or the results would be unpredictable.
The library routines supplied with the compiler are compiled as small,
medium and large. The compiled library modules are contained in separate
directories as small, medium and large so that you can link to the
appropriate set.

<P>
When the medium, large or huge model is used all variables declared
without a storage class will be allocated into the external ram, this
includes all parameters and local variables (for non-reentrant<A NAME="2582"></A>
functions). Medium model uses pdata and large and huge models use
xdata. When the small model is used variables without storage class
are allocated in the internal ram.

<P>
The huge model compiles all functions as <I>banked<A HREF="node106.html#sub:Bankswitching">4.1.3</A></I>
and is otherwise equal to large for now. All other models compile
the functions without bankswitching by default.

<P>
Judicious usage of the processor specific storage classes<A NAME="2584"></A>
and the 'reentrant' function type will yield much more efficient code,
than using the large model. Several optimizations are disabled when
the program is compiled using the large model, it is therefore recommended
that the small model be used unless absolutely required.

<P>

<H3><A NAME="SECTION004181200000000000000"></A><A NAME="sub:External-Stack"></A><A NAME="2586"></A><A NAME="2587"></A>
<BR>
3.18.1.2 External Stack
</H3>

<P>
The external stack (--xstack option<A NAME="2588"></A>)
is located in pdata<A NAME="2589"></A> memory
(usually at the start of the external ram segment) and uses all unused
space in pdata (max. 256 bytes). When --xstack option is used to
compile the program, the parameters and local variables<A NAME="2590"></A>
of all reentrant functions are allocated in this area. This option
is provided for programs with large stack space requirements. When
used with the --stack-auto<A NAME="2591"></A>
option, all parameters and local variables are allocated on the external
stack (note: support libraries will need to be recompiled with the
same options. There is a predefined target in the library makefile).

<P>
The compiler outputs the higher order address byte of the external
ram segment into port P2<A NAME="2592"></A> (see also section
<A HREF="node103.html#sub:MCS51-variants">4.1</A>), therefore when using the External Stack
option, this port <I>may not</I> be used by the application program.

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2011-03-20
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