223cc6685e
git-svn-id: svn://svn.compuextreme.de/Viitor/V963/Viitor_cc65@5933 504e572c-2e33-0410-9681-be2bf7408885
202 lines
6.4 KiB
HTML
202 lines
6.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<HTML>
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<META NAME="GENERATOR" CONTENT="LinuxDoc-Tools 0.9.20">
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<TITLE>ca65 Users Guide: Symbols and labels</TITLE>
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<LINK HREF="ca65-6.html" REL=next>
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<LINK HREF="ca65-4.html" REL=previous>
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<LINK HREF="ca65.html#toc5" REL=contents>
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</HEAD>
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<BODY>
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<A HREF="ca65-6.html">Next</A>
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<A HREF="ca65-4.html">Previous</A>
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<A HREF="ca65.html#toc5">Contents</A>
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<HR>
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<H2><A NAME="s5">5.</A> <A HREF="ca65.html#toc5">Symbols and labels</A></H2>
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<P>The assembler allows you to use symbols instead of naked values to make
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the source more readable. There are a lot of different ways to define and
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use symbols and labels, giving a lot of flexibility.</P>
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<H2><A NAME="ss5.1">5.1</A> <A HREF="ca65.html#toc5.1">Numeric constants</A>
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</H2>
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<P>Numeric constants are defined using the equal sign or the label assignment
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operator. After doing</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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two = 2
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>may use the symbol "two" in every place where a number is expected, and it is
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evaluated to the value 2 in this context. The label assignment operator causes
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the same, but causes the symbol to be marked as a label, which may cause a
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different handling in the debugger:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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io := $d000
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>The right side can of course be an expression:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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four = two * two
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<H2><A NAME="ss5.2">5.2</A> <A HREF="ca65.html#toc5.2">Standard labels</A>
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</H2>
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<P>A label is defined by writing the name of the label at the start of the line
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(before any instruction mnemonic, macro or pseudo directive), followed by a
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colon. This will declare a symbol with the given name and the value of the
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current program counter.</P>
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<H2><A NAME="ss5.3">5.3</A> <A HREF="ca65.html#toc5.3">Local labels and symbols</A>
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</H2>
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<P>Using the <CODE>
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<A HREF="ca65-10.html#.PROC">.PROC</A></CODE> directive, it is possible to
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create regions of code where the names of labels and symbols are local to this
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region. They are not known outside of this region and cannot be accessed from
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there. Such regions may be nested like PROCEDUREs in Pascal.</P>
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<P>See the description of the <CODE>
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<A HREF="ca65-10.html#.PROC">.PROC</A></CODE>
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directive for more information.</P>
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<H2><A NAME="ss5.4">5.4</A> <A HREF="ca65.html#toc5.4">Cheap local labels</A>
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</H2>
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<P>Cheap local labels are defined like standard labels, but the name of the
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label must begin with a special symbol (usually '@', but this can be
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changed by the <CODE>
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<A HREF="ca65-10.html#.LOCALCHAR">.LOCALCHAR</A></CODE>
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directive).</P>
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<P>Cheap local labels are visible only between two non cheap labels. As soon as a
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standard symbol is encountered (this may also be a local symbol if inside a
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region defined with the <CODE>
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<A HREF="ca65-10.html#.PROC">.PROC</A></CODE> directive), the
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cheap local symbol goes out of scope.</P>
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<P>You may use cheap local labels as an easy way to reuse common label
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names like "Loop". Here is an example:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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Clear: lda #$00 ; Global label
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ldy #$20
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@Loop: sta Mem,y ; Local label
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dey
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bne @Loop ; Ok
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rts
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Sub: ... ; New global label
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bne @Loop ; ERROR: Unknown identifier!
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<H2><A NAME="ss5.5">5.5</A> <A HREF="ca65.html#toc5.5">Unnamed labels</A>
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</H2>
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<P>If you really want to write messy code, there are also unnamed labels. These
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labels do not have a name (you guessed that already, didn't you?). A colon is
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used to mark the absence of the name.</P>
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<P>Unnamed labels may be accessed by using the colon plus several minus or plus
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characters as a label designator. Using the '-' characters will create a back
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reference (use the n'th label backwards), using '+' will create a forward
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reference (use the n'th label in forward direction). An example will help to
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understand this:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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: lda (ptr1),y ; #1
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cmp (ptr2),y
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bne :+ ; -> #2
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tax
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beq :+++ ; -> #4
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iny
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bne :- ; -> #1
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inc ptr1+1
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inc ptr2+1
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bne :- ; -> #1
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: bcs :+ ; #2 -> #3
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ldx #$FF
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rts
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: ldx #$01 ; #3
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: rts ; #4
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>As you can see from the example, unnamed labels will make even short
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sections of code hard to understand, because you have to count labels
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to find branch targets (this is the reason why I for my part do
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prefer the "cheap" local labels). Nevertheless, unnamed labels are
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convenient in some situations, so it's your decision.</P>
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<H2><A NAME="ss5.6">5.6</A> <A HREF="ca65.html#toc5.6">Using macros to define labels and constants</A>
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</H2>
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<P>While there are drawbacks with this approach, it may be handy in some
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situations. Using <CODE>
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<A HREF="ca65-10.html#.DEFINE">.DEFINE</A></CODE>, it is
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possible to define symbols or constants that may be used elsewhere. Since
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the macro facility works on a very low level, there is no scoping. On the
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other side, you may also define string constants this way (this is not
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possible with the other symbol types).</P>
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<P>Example:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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.DEFINE two 2
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.DEFINE version "SOS V2.3"
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four = two * two ; Ok
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.byte version ; Ok
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.PROC ; Start local scope
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two = 3 ; Will give "2 = 3" - invalid!
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.ENDPROC
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<H2><A NAME="ss5.7">5.7</A> <A HREF="ca65.html#toc5.7">Symbols and <CODE>.DEBUGINFO</CODE></A>
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</H2>
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<P>If <CODE>
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<A HREF="ca65-10.html#.DEBUGINFO">.DEBUGINFO</A></CODE> is enabled (or
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<A HREF="ca65-2.html#option-g">-g</A> is given on the command line), global, local and
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cheap local labels are written to the object file and will be available in the
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symbol file via the linker. Unnamed labels are not written to the object file,
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because they don't have a name which would allow to access them.</P>
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