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134 lines
3.8 KiB
HTML
134 lines
3.8 KiB
HTML
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<TITLE>ca65 Users Guide: Structs and unions</TITLE>
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<LINK HREF="ca65-15.html" REL=next>
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<LINK HREF="ca65-13.html" REL=previous>
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<LINK HREF="ca65.html#toc14" REL=contents>
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<A HREF="ca65-15.html">Next</A>
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<A HREF="ca65.html#toc14">Contents</A>
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<H2><A NAME="structs"></A> <A NAME="s14">14.</A> <A HREF="ca65.html#toc14">Structs and unions</A></H2>
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<H2><A NAME="ss14.1">14.1</A> <A HREF="ca65.html#toc14.1">Overview</A>
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</H2>
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<P>Structs and unions are special forms of
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<A HREF="ca65-6.html#scopes">scopes</A>. They
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are to some degree comparable to their C counterparts. Both have a list of
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members. Each member allocates storage and may optionally have a name, which,
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in case of a struct, is the offset from the beginning and, in case of a union,
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is always zero.</P>
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<H2><A NAME="ss14.2">14.2</A> <A HREF="ca65.html#toc14.2">Declaration</A>
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</H2>
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<P>Here is an example for a very simple struct with two members and a total size
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of 4 bytes:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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.struct Point
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xcoord .word
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ycoord .word
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.endstruct
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>A union shares the total space between all its members, its size is the same
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as that of the largest member.</P>
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<P>A struct or union must not necessarily have a name. If it is anonymous, no
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local scope is opened, the identifiers used to name the members are placed
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into the current scope instead.</P>
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<P>A struct may contain unnamed members and definitions of local structs. The
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storage allocators may contain a multiplier, as in the example below:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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.struct Circle
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.struct Point
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.word 2 ; Allocate two words
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.endstruct
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Radius .word
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.endstruct
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<H2><A NAME="ss14.3">14.3</A> <A HREF="ca65.html#toc14.3">The <CODE>.TAG</CODE> keyword</A>
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</H2>
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<P>Using the
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<A HREF="ca65-10.html#.TAG">.TAG</A> keyword, it is possible to reserve space
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for an already defined struct or unions within another struct:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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.struct Point
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xcoord .word
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ycoord .word
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.endstruct
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.struct Circle
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Origin .tag Point
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Radius .byte
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.endstruct
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Space for a struct or union may be allocated using the
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<A HREF="ca65-10.html#.TAG">.TAG</A> directive.</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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C: .tag Circle
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>Currently, members are just offsets from the start of the struct or union. To
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access a field of a struct, the member offset has to be added to the address
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of the struct itself:</P>
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<P>
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<BLOCKQUOTE><CODE>
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<PRE>
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lda C+Circle::Radius ; Load circle radius into A
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</PRE>
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</CODE></BLOCKQUOTE>
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</P>
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<P>This may change in a future version of the assembler.</P>
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<H2><A NAME="ss14.4">14.4</A> <A HREF="ca65.html#toc14.4">Limitations</A>
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</H2>
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<P>Structs and unions are currently implemented as nested symbol tables (in fact,
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they were a by-product of the improved scoping rules). Currently, the
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assembler has no idea of types. This means that the
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<A HREF="ca65-10.html#.TAG">.TAG</A> keyword will only allocate space. You won't be able to initialize
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variables declared with
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<A HREF="ca65-10.html#.TAG">.TAG</A>, and adding an embedded
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structure to another structure with
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<A HREF="ca65-10.html#.TAG">.TAG</A> will not make
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this structure accessible by using the '::' operator.</P>
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<HR>
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<A HREF="ca65-15.html">Next</A>
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