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<H2><A NAME="SECTION004131000000000000000"></A><A NAME="sub:A-Step-by_Assembler_Introduction"></A>
<BR>
3.13.1 A Step by Step Introduction
</H2>
<P>
Starting from a small snippet of c-code this example shows for the
MCS51 how to use inline assembly, access variables, a function parameter
and an array in xdata memory. The example uses an MCS51 here but is
easily adapted for other architectures. This is a buffer routine which
should be optimized:
<BLOCKQUOTE>
<TT><FONT SIZE="-1">unsigned char __far<A NAME="2110"></A><A NAME="2111"></A>
__at<A NAME="2112"></A><A NAME="2113"></A>(0x7f00) buf[0x100];<A NAME="2116"></A></FONT></TT>
<BR><TT><FONT SIZE="-1">unsigned char head, tail; /*
if interrupts<A NAME="2118"></A> are involved see</FONT></TT>
<BR><TT><FONT SIZE="-1"> section
<A HREF="node68.html#sub:Common-interrupt-pitfall-volatile">3.9.1.1</A> about</FONT></TT><TT><B><FONT SIZE="-1">volatile</FONT></B></TT><FONT SIZE="-1"> </FONT><TT><FONT SIZE="-1">*/</FONT></TT>
<BR><TT></TT>
<BR><TT><FONT SIZE="-1">void to_buffer( unsigned char c ) </FONT></TT>
<BR><TT><FONT SIZE="-1">{</FONT></TT>
<BR><TT><FONT SIZE="-1"> if( head != (unsigned char)(tail-1)
) /* cast</FONT></TT><FONT SIZE="-1"> </FONT><TT><B><FONT SIZE="-1">needed</FONT></B></TT><TT><FONT SIZE="-1">to avoid promotion<A NAME="2131"></A><A NAME="2132"></A>
to integer */</FONT></TT>
<BR><TT><FONT SIZE="-1"> buf[ head++ ] = c; /*
access to a 256 byte aligned array */</FONT></TT>
<BR><TT><FONT SIZE="-1">} </FONT></TT></BLOCKQUOTE>
<P>
<BLOCKQUOTE>
</BLOCKQUOTE>
If the code snippet (assume it is saved in buffer.c) is compiled with
SDCC then a corresponding buffer.asm file is generated. We define
a new function <TT>to_buffer_asm()</TT> in file buffer.c in which
we cut and paste the generated code, removing unwanted comments and
some ':'. Then add ''<B>__asm</B>'' and ''<B>__endasm;</B>''<A NAME="tex2html37"
HREF="footnode.html#foot2146"><SUP>3.6</SUP></A> to the beginning and the end of the function body:
<BLOCKQUOTE>
<TT><FONT SIZE="-1">/* With a cut and paste from the .asm file,
we have something to start with.</FONT></TT>
<BR><TT><FONT SIZE="-1"> The function is not yet OK! (registers
aren't saved) */ </FONT></TT>
<BR><TT><FONT SIZE="-1">void to_buffer_asm( unsigned char c ) </FONT></TT>
<BR><TT><FONT SIZE="-1">{ </FONT></TT>
<BR><TT><FONT SIZE="-1"> __asm<A NAME="2152"></A><A NAME="2153"></A></FONT></TT>
<BR><TT><FONT SIZE="-1"> mov r2,dpl </FONT></TT>
<BR><TT><FONT SIZE="-1">;buffer.c if( head != (unsigned char)(tail-1)
) /* cast</FONT></TT><FONT SIZE="-1"> </FONT><TT><B><FONT SIZE="-1">needed</FONT></B></TT><TT><FONT SIZE="-1">to avoid promotion<A NAME="2159"></A><A NAME="2160"></A>
to integer */</FONT></TT>
<BR><TT><FONT SIZE="-1"> mov a,_tail </FONT></TT>
<BR><TT><FONT SIZE="-1"> dec a </FONT></TT>
<BR><TT><FONT SIZE="-1"> mov r3,a </FONT></TT>
<BR><TT><FONT SIZE="-1"> mov a,_head </FONT></TT>
<BR><TT><FONT SIZE="-1"> cjne a,ar3,00106$ </FONT></TT>
<BR><TT><FONT SIZE="-1"> ret</FONT></TT>
<BR><TT><FONT SIZE="-1">00106$: </FONT></TT>
<BR><TT><FONT SIZE="-1">;buffer.c buf[ head++ ] = c; /* access
to a 256 byte aligned array */<A NAME="2173"></A></FONT></TT>
<BR><TT><FONT SIZE="-1"> mov r3,_head </FONT></TT>
<BR><TT><FONT SIZE="-1"> inc _head </FONT></TT>
<BR><TT><FONT SIZE="-1"> mov dpl,r3 </FONT></TT>
<BR><TT><FONT SIZE="-1"> mov dph,#(_buf >>
8) </FONT></TT>
<BR><TT><FONT SIZE="-1"> mov a,r2 </FONT></TT>
<BR><TT><FONT SIZE="-1"> movx @dptr,a </FONT></TT>
<BR><TT><FONT SIZE="-1">00103$: </FONT></TT>
<BR><TT><FONT SIZE="-1"> ret</FONT></TT>
<BR><TT><FONT SIZE="-1"> __endasm<A NAME="2182"></A><A NAME="2183"></A>;</FONT></TT>
<BR><TT><FONT SIZE="-1">} </FONT></TT></BLOCKQUOTE>
<P>
<BLOCKQUOTE>
</BLOCKQUOTE>
The new file buffer.c should compile with only one warning about the
unreferenced function argument 'c'. Now we hand-optimize the assembly
code and insert an #define USE_ASSEMBLY (1) and finally have:
<BLOCKQUOTE>
<TT><FONT SIZE="-1">unsigned char __far __at(0x7f00) buf[0x100];</FONT></TT>
<BR><TT><FONT SIZE="-1">unsigned char head, tail;</FONT></TT>
<BR><TT><FONT SIZE="-1">#define USE_ASSEMBLY (1)</FONT></TT>
<BR><TT></TT>
<BR><TT><FONT SIZE="-1">#if !USE_ASSEMBLY</FONT></TT>
<BR><TT></TT>
<BR><TT><FONT SIZE="-1">void to_buffer( unsigned char c )</FONT></TT>
<BR><TT><FONT SIZE="-1">{</FONT></TT>
<BR><TT><FONT SIZE="-1"> if( head != (unsigned char)(tail-1)
)</FONT></TT>
<BR><TT><FONT SIZE="-1"> buf[ head++ ] = c;</FONT></TT>
<BR><TT><FONT SIZE="-1">}</FONT></TT>
<BR><TT></TT>
<BR><TT><FONT SIZE="-1">#else</FONT></TT>
<BR><TT></TT>
<BR><TT><FONT SIZE="-1">void to_buffer( unsigned char c )</FONT></TT>
<BR><TT><FONT SIZE="-1">{</FONT></TT>
<BR><TT><FONT SIZE="-1"> c; // to avoid warning: unreferenced
function argument</FONT></TT>
<BR><TT><FONT SIZE="-1"> __asm<A NAME="2207"></A><A NAME="2208"></A></FONT></TT>
<BR><TT><FONT SIZE="-1"> ; save used registers here. </FONT></TT>
<BR><TT><FONT SIZE="-1"> ; If we were still using
r2,r3 we would have to push them here. </FONT></TT>
<BR><TT><FONT SIZE="-1">; if( head != (unsigned char)(tail-1) )</FONT></TT>
<BR><TT><FONT SIZE="-1"> mov a,_tail</FONT></TT>
<BR><TT><FONT SIZE="-1"> dec a</FONT></TT>
<BR><TT><FONT SIZE="-1"> xrl a,_head</FONT></TT>
<BR><TT><FONT SIZE="-1"> ; we could do an ANL a,#0x0f
here to use a smaller buffer (see below)</FONT></TT>
<BR><TT><FONT SIZE="-1"> jz t_b_end$</FONT></TT>
<BR><TT><FONT SIZE="-1"> ;</FONT></TT>
<BR><TT><FONT SIZE="-1">; buf[ head++ ] = c;</FONT></TT>
<BR><TT><FONT SIZE="-1"> mov a,dpl ;
dpl holds lower byte of function argument</FONT></TT>
<BR><TT><FONT SIZE="-1"> mov dpl,_head ;
buf is 0x100 byte aligned so head can be used directly</FONT></TT>
<BR><TT><FONT SIZE="-1"> mov dph,#(_buf>>8)</FONT></TT>
<BR><TT><FONT SIZE="-1"> movx @dptr,a</FONT></TT>
<BR><TT><FONT SIZE="-1"> inc _head</FONT></TT>
<BR><TT><FONT SIZE="-1"> ; we could do an ANL _head,#0x0f
here to use a smaller buffer (see above)</FONT></TT>
<BR><TT><FONT SIZE="-1">t_b_end$:</FONT></TT>
<BR><TT><FONT SIZE="-1"> ; restore used registers
here </FONT></TT>
<BR><TT><FONT SIZE="-1"> _endasm<A NAME="2228"></A><A NAME="2229"></A>;</FONT></TT>
<BR><TT><FONT SIZE="-1">}</FONT></TT>
<BR><TT><FONT SIZE="-1">#endif</FONT></TT></BLOCKQUOTE>
<P>
<BLOCKQUOTE>
</BLOCKQUOTE>
The inline assembler code can contain any valid code understood by
the assembler, this includes any assembler directives and comment
lines. The assembler does not like some characters like ':' or '''
in comments. You'll find an 100+ pages assembler manual in sdcc/sdas/doc/asxhtm.html<A NAME="2234"></A><A NAME="2235"></A>
or online at <TT><A NAME="tex2html38"
HREF="http://sdcc.svn.sourceforge.net/viewvc/*checkout*/sdcc/trunk/sdcc/sdas/doc/asxhtm.html">http://sdcc.svn.sourceforge.net/viewvc/*checkout*/sdcc/trunk/sdcc/sdas/doc/asxhtm.html</A></TT>.
<P>
The compiler does not do any validation of the code within the <TT>__asm<A NAME="2237"></A><A NAME="2238"></A>
... __endasm<FONT SIZE="-1"><A NAME="2239"></A><A NAME="2240"></A></FONT>;</TT>
keyword pair. Specifically it will not know which registers are used
and thus register pushing/popping<A NAME="2242"></A> has to be done
manually.
<P>
It is recommended that each assembly instruction (including labels)
be placed in a separate line (as the example shows). When the --<I>peep-asm<A NAME="2243"></A></I>
command line option is used, the inline assembler code will be passed
through the peephole optimizer<A NAME="2244"></A>. There are
only a few (if any) cases where this option makes sense, it might
cause some unexpected changes in the inline assembler code. Please
go through the peephole optimizer rules defined in file <I>SDCCpeeph.def</I>
before using this option.
<P>
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