ca65-6.html
来自「cc65 的编译器文档」· HTML 代码 · 共 323 行
HTML
323 行
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><HTML><HEAD> <META NAME="GENERATOR" CONTENT="LinuxDoc-Tools 0.9.20"> <TITLE>ca65 Users Guide: Scopes</TITLE> <LINK HREF="ca65-7.html" REL=next> <LINK HREF="ca65-5.html" REL=previous> <LINK HREF="ca65.html#toc6" REL=contents></HEAD><BODY><A HREF="ca65-7.html">Next</A><A HREF="ca65-5.html">Previous</A><A HREF="ca65.html#toc6">Contents</A><HR><H2><A NAME="scopes"></A> <A NAME="s6">6.</A> <A HREF="ca65.html#toc6">Scopes</A></H2><P>ca65 implements several sorts of scopes for symbols.</P><H2><A NAME="ss6.1">6.1</A> <A HREF="ca65.html#toc6.1">Global scope</A></H2><P>All (non cheap local) symbols that are declared outside of any nested scopesare in global scope.</P><H2><A NAME="ss6.2">6.2</A> <A HREF="ca65.html#toc6.2">A special scope: cheap locals</A></H2><P>A special scope is the scope for cheap local symbols. It lasts from one nonlocal symbol to the next one, without any provisions made by the programmer.All other scopes differ in usage but use the same concept internally.</P><H2><A NAME="ss6.3">6.3</A> <A HREF="ca65.html#toc6.3">Generic nested scopes</A></H2><P>A nested scoped for generic use is started with <CODE><A HREF="ca65-10.html#.SCOPE">.SCOPE</A></CODE> and closed with <CODE><A HREF="ca65-10.html#.ENDSCOPE">.ENDSCOPE</A></CODE>.The scope can have a name, in which case it is accessible from the outside byusing <A HREF="#scopesyntax">explicit scopes</A>. If the scope does nothave a name, all symbols created within the scope are local to the scope, andaren't accessible from the outside.</P><P>A nested scope can access symbols from the local or from enclosing scopes byname without using explicit scope names. In some cases there may beambiguities, for example if there is a reference to a local symbol that is notyet defined, but a symbol with the same name exists in outer scopes:</P><P><BLOCKQUOTE><CODE><PRE> .scope outer foo = 2 .scope inner lda #foo foo = 3 .endscope .endscope</PRE></CODE></BLOCKQUOTE></P><P>In the example above, the <CODE>lda</CODE> instruction will load the value 3 into theaccumulator, because <CODE>foo</CODE> is redefined in the scope. However:</P><P><BLOCKQUOTE><CODE><PRE> .scope outer foo = $1234 .scope inner lda foo,x foo = $12 .endscope .endscope</PRE></CODE></BLOCKQUOTE></P><P>Here, <CODE>lda</CODE> will still load from <CODE>$12,x</CODE>, but since it is unknown to theassembler that <CODE>foo</CODE> is a zeropage symbol when translating the instruction,absolute mode is used instead. In fact, the assembler will not use absolutemode by default, but it will search through the enclosing scopes for a symbolwith the given name. If one is found, the address size of this symbol is used.This may lead to errors:</P><P><BLOCKQUOTE><CODE><PRE> .scope outer foo = $12 .scope inner lda foo,x foo = $1234 .endscope .endscope</PRE></CODE></BLOCKQUOTE></P><P>In this case, when the assembler sees the symbol <CODE>foo</CODE> in the <CODE>lda</CODE>instruction, it will search for an already defined symbol <CODE>foo</CODE>. It willfind <CODE>foo</CODE> in scope <CODE>outer</CODE>, and a close look reveals that it is azeropage symbol. So the assembler will use zeropage addressing mode. If<CODE>foo</CODE> is redefined later in scope <CODE>inner</CODE>, the assembler tries to changethe address in the <CODE>lda</CODE> instruction already translated, but since the newvalue needs absolute addressing mode, this fails, and an error message "Rangeerror" is output.</P><P>Of course the most simple solution for the problem is to move the definitionof <CODE>foo</CODE> in scope <CODE>inner</CODE> upwards, so it preceeds its use. There may berare cases when this cannot be done. In these cases, you can use one of theaddress size override operators:</P><P><BLOCKQUOTE><CODE><PRE> .scope outer foo = $12 .scope inner lda a:foo,x foo = $1234 .endscope .endscope</PRE></CODE></BLOCKQUOTE></P><P>This will cause the <CODE>lda</CODE> instruction to be translated using absoluteaddressing mode, which means changing the symbol reference later does notcause any errors.</P><H2><A NAME="ss6.4">6.4</A> <A HREF="ca65.html#toc6.4">Nested procedures</A></H2><P>A nested procedure is created by use of <CODE><A HREF="ca65-10.html#.PROC">.PROC</A></CODE>. Itdiffers from a <CODE><A HREF="ca65-10.html#.SCOPE">.SCOPE</A></CODE> in that it must have aname, and a it will introduce a symbol with this name in the enclosing scope.So</P><P><BLOCKQUOTE><CODE><PRE> .proc foo ... .endscope</PRE></CODE></BLOCKQUOTE></P><P>is actually the same as</P><P><BLOCKQUOTE><CODE><PRE> foo: .scope foo ... .endscope</PRE></CODE></BLOCKQUOTE></P><P>This is the reason why a procedure must have a name. If you want a scopewithout a name, use <CODE><A HREF="ca65-10.html#.SCOPE">.SCOPE</A></CODE>.</P><P><B>Note:</B> As you can see from the example above, scopes and symbols live indifferent namespaces. There can be a symbol named <CODE>foo</CODE> and a scope named<CODE>foo</CODE> without any conflicts (but see the section titled <A HREF="#scopesearch">"Scope search order"</A>).</P><H2><A NAME="ss6.5">6.5</A> <A HREF="ca65.html#toc6.5">Structs, unions and enums</A></H2><P>Structs, unions and enums are explained in a <A HREF="ca65-13.html#structs">separate section</A>, I do only cover them here, because if they are declared with aname, they open a nested scope, similar to <CODE><A HREF="ca65-10.html#.SCOPE">.SCOPE</A></CODE>. However, when no name is specified, the behaviour isdifferent: In this case, no new scope will be opened, symbols declared withina struct, union, or enum declaration will then be added to the enclosing scopeinstead.</P><H2><A NAME="scopesyntax"></A> <A NAME="ss6.6">6.6</A> <A HREF="ca65.html#toc6.6">Explicit scope specification</A></H2><P>Accessing symbols from other scopes is possible by using an explicit scopespecification, provided that the scope where the symbol lives in has a name.The namespace token (<CODE>::</CODE>) is used to access other scopes:</P><P><BLOCKQUOTE><CODE><PRE> .scope foo bar: .word 0 .endscope ... lda foo::bar ; Access foo in scope bar</PRE></CODE></BLOCKQUOTE></P><P>The only way to deny access to a scope from the outside is to declare a scopewithout a name (using the <CODE><A HREF="ca65-10.html#.SCOPE">.SCOPE</A></CODE> command).</P><P>A special syntax is used to specify the global scope: If a symbol or scope ispreceeded by the namespace token, the global scope is searched:</P><P><BLOCKQUOTE><CODE><PRE> bar = 3 .scope foo bar = 2 lda #::bar ; Access the global bar (which is 3) .endscope</PRE></CODE></BLOCKQUOTE></P><H2><A NAME="scopesearch"></A> <A NAME="ss6.7">6.7</A> <A HREF="ca65.html#toc6.7">Scope search order</A></H2><P>The assembler searches for a scope in a similar way as for a symbol. First, itlooks in the current scope, and then it walks up the enclosing scopes untilthe scope is found.</P><P>However, one important thing to note when using explicit scope syntax is, thata symbol may be accessed before it is defined, but a scope may <B>not</B> beused without a preceeding definition. This means that in the followingexample:</P><P><BLOCKQUOTE><CODE><PRE> .scope foo bar = 3 .endscope .scope outer lda #foo::bar ; Will load 3, not 2! .scope foo bar = 2 .endscope .endscope</PRE></CODE></BLOCKQUOTE></P><P>the reference to the scope <CODE>foo</CODE> will use the global scope, and not thelocal one, because the local one is not visible at the point where it isreferenced.</P><P>Things get more complex if a complete chain of scopes is specified:</P><P><BLOCKQUOTE><CODE><PRE> .scope foo .scope outer .scope inner bar = 1 .endscope .endscope .scope another .scope nested lda #outer::inner::bar ; 1 .endscope .endscope .endscope .scope outer .scope inner bar = 2 .endscope .endscope</PRE></CODE></BLOCKQUOTE></P><P>When <CODE>outer::inner::bar</CODE> is referenced in the <CODE>lda</CODE> instruction, theassembler will first search in the local scope for a scope named <CODE>outer</CODE>.Since none is found, the enclosing scope (<CODE>another</CODE>) is checked. There isstill no scope named <CODE>outer</CODE>, so scope <CODE>foo</CODE> is checked, and finallyscope <CODE>outer</CODE> is found. Within this scope, <CODE>inner</CODE> is searched, and inthis scope, the assembler looks for a symbol named <CODE>bar</CODE>.</P><P>Please note that once the anchor scope is found, all following scopes(<CODE>inner</CODE> in this case) are expected to be found exactly in this scope. Theassembler will search the scope tree only for the first scope (if it is notanchored in the root scope). Starting from there on, there is no flexibility,so if the scope named <CODE>outer</CODE> found by the assembler does not contain ascope named <CODE>inner</CODE>, this would be an error, even if such a pair does exist(one level up in global scope).</P><P>Ambiguities that may be introduced by this search algorithm may be removed byanchoring the scope specification in the global scope. In the example above,if you want to access the "other" symbol <CODE>bar</CODE>, you would have to write:</P><P><BLOCKQUOTE><CODE><PRE> .scope foo .scope outer .scope inner bar = 1 .endscope .endscope .scope another .scope nested lda #::outer::inner::bar ; 2 .endscope .endscope .endscope .scope outer .scope inner bar = 2 .endscope .endscope</PRE></CODE></BLOCKQUOTE></P><HR><A HREF="ca65-7.html">Next</A><A HREF="ca65-5.html">Previous</A><A HREF="ca65.html#toc6">Contents</A></BODY></HTML>⌨️ 快捷键说明
复制代码Ctrl + C
搜索代码Ctrl + F
全屏模式F11
增大字号Ctrl + =
减小字号Ctrl + -
显示快捷键?