223cc6685e
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130 lines
4.5 KiB
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130 lines
4.5 KiB
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<TITLE>cc65 Users Guide: Inline assembler</TITLE>
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<LINK HREF="cc65-10.html" REL=next>
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<LINK HREF="cc65.html#toc9" REL=contents>
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<A HREF="cc65-10.html">Next</A>
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<A HREF="cc65.html#toc9">Contents</A>
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<H2><A NAME="inline-asm"></A> <A NAME="s9">9.</A> <A HREF="cc65.html#toc9">Inline assembler</A></H2>
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<P>The compiler allows to insert assembler statements into the output file. The
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syntax is</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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asm (<string literal>[, optional parameters]) ;
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</PRE>
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</CODE></BLOCKQUOTE>
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or
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<BLOCKQUOTE><CODE>
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<PRE>
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__asm__ (<string literal>[, optional parameters]) ;
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>The first form is in the user namespace and is disabled by <CODE>
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<A HREF="cc65-2.html#option--standard">--standard</A></CODE> if the argument is not <CODE>cc65</CODE>.</P>
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<P>The asm statement may be used inside a function and on global file level. An
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inline assembler statement is a primary expression, so it may also be used as
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part of an expression. Please note however that the result of an expression
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containing just an inline assembler statement is always of type <CODE>void</CODE>.</P>
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<P>The contents of the string literal are preparsed by the compiler and inserted
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into the generated assembly output, so that the can be further processed by
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the backend and especially the optimizer. For this reason, the compiler does
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only allow regular 6502 opcodes to be used with the inline assembler. Pseudo
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instructions (like <CODE>.import</CODE>, <CODE>.byte</CODE> and so on) are <EM>not</EM> allowed,
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even if the ca65 assembler (which is used to translate the generated assembler
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code) would accept them. The builtin inline assembler is not a replacement for
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the full blown macro assembler which comes with the compiler.</P>
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<P>Note: Inline assembler statements are subject to all optimizations done by the
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compiler. There is currently no way to protect an inline assembler statement
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from being moved or removed completely by the optimizer. If in doubt, check
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the generated assembler output, or disable optimizations.</P>
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<P>The string literal may contain format specifiers from the following list. For
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each format specifier, an argument is expected which is inserted instead of
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the format specifier before passing the assembly code line to the backend.</P>
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<P>
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<UL>
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<LI><CODE>%b</CODE> - Numerical 8-bit value</LI>
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<LI><CODE>%w</CODE> - Numerical 16-bit value</LI>
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<LI><CODE>%l</CODE> - Numerical 32-bit value</LI>
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<LI><CODE>%v</CODE> - Assembler name of a (global) variable or function</LI>
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<LI><CODE>%o</CODE> - Stack offset of a (local) variable</LI>
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<LI><CODE>%g</CODE> - Assembler name of a C label</LI>
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<LI><CODE>%s</CODE> - The argument is converted to a string</LI>
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<LI><CODE>%%</CODE> - The % sign itself</LI>
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</UL>
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</P>
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<P>Using these format specifiers, you can access C <CODE>#defines</CODE>, variables or
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similar stuff from the inline assembler. For example, to load the value of
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a C <CODE>#define</CODE> into the Y register, one would use</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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#define OFFS 23
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__asm__ ("ldy #%b", OFFS);
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Or, to access a struct member of a static variable:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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typedef struct {
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unsigned char x;
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unsigned char y;
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unsigned char color;
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} pixel_t;
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static pixel_t pixel;
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__asm__ ("ldy #%b", offsetof(pixel_t, color));
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__asm__ ("lda %v,y", pixel);
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Note: Do not embed the assembler labels that are used as names of global
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variables or functions into your asm statements. Code like this</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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int foo;
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int bar () { return 1; }
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__asm__ ("lda _foo"); /* DON'T DO THAT! */
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...
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__asm__ ("jsr _bar"); /* DON'T DO THAT EITHER! */
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>may stop working if the way, the compiler generates these names is changed in
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a future version. Instead use the format specifiers from the table above:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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__asm__ ("lda %v", foo); /* OK */
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...
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__asm__ ("jsr %v", bar); /* OK */
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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