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<H2><A NAME="SECTION004132000000000000000"></A><A NAME="sub:Naked-Functions"></A><A NAME="2247"></A>
<BR>
3.13.2 Naked Functions
</H2>
<P>
A special keyword may be associated with a function declaring it as
<I>_naked<A NAME="2248"></A><A NAME="2249"></A>.</I> The
<I>_naked</I> function modifier attribute prevents the compiler from
generating prologue<A NAME="2251"></A> and epilogue<A NAME="2252"></A>
code for that function. This means that the user is entirely responsible
for such things as saving any registers that may need to be preserved,
selecting the proper register bank, generating the <I>return</I> instruction
at the end, etc. Practically, this means that the contents of the
function must be written in inline assembler. This is particularly
useful for interrupt functions, which can have a large (and often
unnecessary) prologue/epilogue. For example, compare the code generated
by these two functions:
<BLOCKQUOTE>
<TT>volatile<A NAME="2255"></A> data unsigned char counter;</TT>
<BR>
<BR><TT>void simpleInterrupt(void) __interrupt<A NAME="2257"></A><A NAME="2258"></A>
(1)</TT>
<BR><TT>{</TT>
<BR><TT> counter++;</TT>
<BR><TT>}</TT>
<BR>
<BR><TT>void nakedInterrupt(void) __interrupt (2) __naked</TT>
<BR><TT>{</TT>
<BR><TT> __asm<A NAME="2265"></A><A NAME="2266"></A></TT>
<BR><TT> inc _counter ; does not change flags,
no need to save psw</TT>
<BR><TT> reti ; MUST explicitly include ret or
reti in _naked function.</TT>
<BR><TT> __endasm<A NAME="2269"></A><A NAME="2270"></A>;</TT>
<BR><TT>}</TT>
</BLOCKQUOTE>
For an 8051 target, the generated simpleInterrupt looks like:
<BLOCKQUOTE>
<TT>Note, this is an</TT> <TT><I>outdated</I></TT> <TT>example,
recent versions of SDCC generate</TT>
<BR><TT>the</TT> <TT><I>same</I></TT> <TT>code for simpleInterrupt()
and nakedInterrupt()!</TT>
<BR>
<BR><TT>_simpleInterrupt:</TT>
<BR><TT> push acc</TT>
<BR><TT> push b</TT>
<BR><TT> push dpl</TT>
<BR><TT> push dph</TT>
<BR><TT> push psw</TT>
<BR><TT> mov psw,#0x00</TT>
<BR><TT> inc _counter</TT>
<BR><TT> pop psw</TT>
<BR><TT> pop dph</TT>
<BR><TT> pop dpl</TT>
<BR><TT> pop b</TT>
<BR><TT> pop acc</TT>
<BR><TT> reti</TT>
</BLOCKQUOTE>
whereas nakedInterrupt looks like:
<BLOCKQUOTE>
<TT>_nakedInterrupt:</TT>
<BR><TT> inc _counter ; does not change flags, no
need to save psw</TT>
<BR><TT> reti ; MUST explicitly include
ret or reti in _naked function</TT>
</BLOCKQUOTE>
The related directive #pragma exclude<A NAME="2301"></A>
allows a more fine grained control over pushing & popping<A NAME="2302"></A>
the registers.
<P>
While there is nothing preventing you from writing C code inside a
<TT>_naked</TT> function, there are many ways to shoot yourself in
the foot doing this, and it is recommended that you stick to inline
assembler.
<P>
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