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160 lines
6.7 KiB
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<TITLE>ca65 Users Guide: Expressions</TITLE>
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<LINK HREF="ca65-5.html" REL=next>
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<LINK HREF="ca65.html#toc4" REL=contents>
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<A HREF="ca65-5.html">Next</A>
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<A HREF="ca65.html#toc4">Contents</A>
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<H2><A NAME="s4">4.</A> <A HREF="ca65.html#toc4">Expressions</A></H2>
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<H2><A NAME="ss4.1">4.1</A> <A HREF="ca65.html#toc4.1">Expression evaluation</A>
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</H2>
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<P>All expressions are evaluated with (at least) 32 bit precision. An
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expression may contain constant values and any combination of internal and
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external symbols. Expressions that cannot be evaluated at assembly time
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are stored inside the object file for evaluation by the linker.
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Expressions referencing imported symbols must always be evaluated by the
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linker.</P>
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<H2><A NAME="ss4.2">4.2</A> <A HREF="ca65.html#toc4.2">Size of an expression result</A>
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</H2>
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<P>Sometimes, the assembler must know about the size of the value that is the
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result of an expression. This is usually the case, if a decision has to be
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made, to generate a zero page or an absolute memory references. In this
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case, the assembler has to make some assumptions about the result of an
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expression:</P>
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<P>
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<UL>
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<LI> If the result of an expression is constant, the actual value is
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checked to see if it's a byte sized expression or not.</LI>
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<LI> If the expression is explicitly casted to a byte sized expression by
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one of the '>', '<' or '^' operators, it is a byte expression.</LI>
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<LI> If this is not the case, and the expression contains a symbol,
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explicitly declared as zero page symbol (by one of the .importzp or
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.exportzp instructions), then the whole expression is assumed to be
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byte sized.</LI>
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<LI> If the expression contains symbols that are not defined, and these
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symbols are local symbols, the enclosing scopes are searched for a
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symbol with the same name. If one exists and this symbol is defined,
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it's attributes are used to determine the result size.</LI>
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<LI> In all other cases the expression is assumed to be word sized.</LI>
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</UL>
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</P>
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<P>Note: If the assembler is not able to evaluate the expression at assembly
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time, the linker will evaluate it and check for range errors as soon as
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the result is known.</P>
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<H2><A NAME="ss4.3">4.3</A> <A HREF="ca65.html#toc4.3">Boolean expressions</A>
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</H2>
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<P>In the context of a boolean expression, any non zero value is evaluated as
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true, any other value to false. The result of a boolean expression is 1 if
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it's true, and zero if it's false. There are boolean operators with extreme
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low precedence with version 2.x (where x > 0). The <CODE>.AND</CODE> and <CODE>.OR</CODE>
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operators are shortcut operators. That is, if the result of the expression is
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already known, after evaluating the left hand side, the right hand side is
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not evaluated.</P>
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<H2><A NAME="ss4.4">4.4</A> <A HREF="ca65.html#toc4.4">Constant expressions</A>
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</H2>
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<P>Sometimes an expression must evaluate to a constant without looking at any
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further input. One such example is the <CODE>
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<A HREF="ca65-10.html#.IF">.IF</A></CODE> command
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that decides if parts of the code are assembled or not. An expression used in
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the <CODE>.IF</CODE> command cannot reference a symbol defined later, because the
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decision about the <CODE>.IF</CODE> must be made at the point when it is read. If the
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expression used in such a context contains only constant numerical values,
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there is no problem. When unresolvable symbols are involved it may get harder
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for the assembler to determine if the expression is actually constant, and it
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is even possible to create expressions that aren't recognized as constant.
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Simplifying the expressions will often help.</P>
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<P>In cases where the result of the expression is not needed immediately, the
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assembler will delay evaluation until all input is read, at which point all
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symbols are known. So using arbitrary complex constant expressions is no
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problem in most cases.</P>
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<H2><A NAME="operators"></A> <A NAME="ss4.5">4.5</A> <A HREF="ca65.html#toc4.5">Available operators</A>
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</H2>
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<P>Available operators sorted by precedence:</P>
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<P>
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<BR><CENTER>
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<TABLE BORDER><TR><TD>
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Op</TD><TD>Description</TD><TD>Precedence</TD></TR><TR><TD>
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</TD><TD>Builtin string functions</TD><TD>0</TD></TR><TR><TD>
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</TD><TD>Builtin pseudo variables</TD><TD>1</TD></TR><TR><TD>
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</TD><TD>Builtin pseudo functions</TD><TD>1</TD></TR><TR><TD>
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+</TD><TD>Unary plus</TD><TD>1</TD></TR><TR><TD>
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-</TD><TD>Unary minus</TD><TD>1</TD></TR><TR><TD>
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~</TD><TD>Unary bitwise not</TD><TD>1</TD></TR><TR><TD>
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.BITNOT</TD><TD>Unary bitwise not</TD><TD>1</TD></TR><TR><TD>
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<</TD><TD>Low byte operator</TD><TD>1</TD></TR><TR><TD>
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></TD><TD>High byte operator</TD><TD>1</TD></TR><TR><TD>
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^</TD><TD>Bank byte operator</TD><TD>1</TD></TR><TR><TD>
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*</TD><TD>Multiplication</TD><TD>2</TD></TR><TR><TD>
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/</TD><TD>Division</TD><TD>2</TD></TR><TR><TD>
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.MOD</TD><TD>Modulo operation</TD><TD>2</TD></TR><TR><TD>
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&</TD><TD>Bitwise and</TD><TD>2</TD></TR><TR><TD>
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.BITAND</TD><TD>Bitwise and</TD><TD>2</TD></TR><TR><TD>
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^</TD><TD>Bitwise xor</TD><TD>2</TD></TR><TR><TD>
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.BITXOR</TD><TD>Bitwise xor</TD><TD>2</TD></TR><TR><TD>
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<<</TD><TD>Shift left operator</TD><TD>2</TD></TR><TR><TD>
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.SHL</TD><TD>Shift left operator</TD><TD>2</TD></TR><TR><TD>
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>></TD><TD>Shift right operato</TD><TD>r</TD></TR><TR><TD>
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.SHR</TD><TD>Shift right operator</TD><TD>2</TD></TR><TR><TD>
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+</TD><TD>Binary plus</TD><TD>3</TD></TR><TR><TD>
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-</TD><TD>Binary minus</TD><TD>3</TD></TR><TR><TD>
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|</TD><TD>Binary or</TD><TD>3</TD></TR><TR><TD>
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.BITOR</TD><TD>Binary or</TD><TD>3</TD></TR><TR><TD>
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=</TD><TD>Compare operation (equal)</TD><TD>4</TD></TR><TR><TD>
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<></TD><TD>Compare operation (not equal)</TD><TD>4</TD></TR><TR><TD>
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<</TD><TD>Compare operation (less)</TD><TD>4</TD></TR><TR><TD>
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></TD><TD>Compare operation (greater)</TD><TD>4</TD></TR><TR><TD>
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<=</TD><TD>Compare operation (less or equal)</TD><TD>4</TD></TR><TR><TD>
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>=</TD><TD>Compare operation (greater or equal)</TD><TD>4</TD></TR><TR><TD>
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&&</TD><TD>Boolean and</TD><TD>5</TD></TR><TR><TD>
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.AND</TD><TD>Boolean and</TD><TD>5</TD></TR><TR><TD>
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.XOR</TD><TD>Boolean xor</TD><TD>5</TD></TR><TR><TD>
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||</TD><TD>Boolean or</TD><TD>6</TD></TR><TR><TD>
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.OR</TD><TD>Boolean or</TD><TD>6</TD></TR><TR><TD>
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!</TD><TD>Boolean not</TD><TD>7</TD></TR><TR><TD>
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.NOT</TD><TD>Boolean not</TD><TD>7</TD></TR><TR><TD>
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</TD></TR></TABLE>
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<CAPTION>Available operators sorted by precedence</CAPTION>
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</CENTER><BR>
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</P>
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<P>To force a specific order of evaluation, braces may be used as usual.</P>
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