223cc6685e
git-svn-id: svn://svn.compuextreme.de/Viitor/V963/Viitor_cc65@5933 504e572c-2e33-0410-9681-be2bf7408885
59 lines
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59 lines
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<TITLE>cc65 Users Guide: Register variables</TITLE>
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<H2><A NAME="regvars"></A> <A NAME="s8">8.</A> <A HREF="cc65.html#toc8">Register variables</A></H2>
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<P>The runtime for all supported platforms has 6 bytes of zero page space
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available for register variables (this could be increased, but I think it's a
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good value). So you can declare register variables up to a total size of 6 per
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function. The compiler will allocate register space on a "first come, first
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served" base and convert any <CODE>register</CODE> declarations that exceed the
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available register space silently to <CODE>auto</CODE>. Parameters can also be
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declared as <CODE>register</CODE>, this will in fact give slightly shorter code than
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using a register variable.</P>
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<P>Since a function must save the current values of the registers on entry and
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restore them on exit, there is an overhead associated with register variables,
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and this overhead is quite high (about 20 bytes per variable). This means that
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just declaring anything as <CODE>register</CODE> is not a good idea.</P>
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<P>The best use for register variables are pointers, especially those that point
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to structures. The magic number here is about 3 uses of a struct field: If the
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function contains this number or even more, the generated code will be usually
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shorter and faster when using a register variable for the struct pointer. The
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reason for this is that the register variable can in many cases be used as a
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pointer directly. Having a pointer in an auto variable means that this pointer
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must first be copied into a zero page location, before it can be dereferenced.</P>
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<P>Second best use for register variables are counters. However, there is not
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much difference in the code generated for counters, so you will need at least
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100 operations on this variable (for example in a loop) to make it worth the
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trouble. The only savings you get here are by the use of a zero page variable
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instead of one on the stack or in the data segment.</P>
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<P>Register variables must be explicitly enabled by using <CODE>-Or</CODE> or <CODE>-r</CODE> on
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the command line. Register variables are only accepted on function top level,
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register variables declared in interior blocks are silently converted to
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<CODE>auto</CODE>. With register variables disabled, all variables declared as
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<CODE>register</CODE> are actually auto variables.</P>
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<P>Please take care when using register variables: While they are helpful and can
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lead to a tremendous speedup when used correctly, improper usage will cause
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bloated code and a slowdown.</P>
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