115 lines
3.7 KiB
Plaintext
115 lines
3.7 KiB
Plaintext
<chapter>
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<title>Labels and aliases</title>
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<para>
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Labels are an important part of your code. However, since each
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label must normally be unique, this can lead to <quote>namespace
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pollution,</quote> and you'll find yourself going through ever
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more contorted constructions to generate unique label names.
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Ophis offers two solutions to this: <emphasis>anonymous
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labels</emphasis> and <emphasis>temporary labels</emphasis>. This
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tutorial will cover both of these facilities, and also introduce
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the aliasing mechanism.
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</para>
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<section>
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<title>Temporary labels</title>
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<para>
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Temporary labels are the easiest to use. If a label begins with
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an underscore, it will only be reachable from inside the
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innermost enclosing scope. Scopes begin when
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a <literal>.scope</literal> statement is encountered. This
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produces a new, inner scope if there is another scope in use.
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The <literal>.scend</literal> command ends the innermost
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currently active scope.
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</para>
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<para>
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We can thus rewrite our header data using temporary labels, thus
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allowing the main program to have a label
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named <literal>next</literal> if it wants.
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</para>
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<programlisting>
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.word $0801
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.org $0801
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.scope
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.word _next, 10 ; Next line and current line number
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.byte $9e," 2064",0 ; SYS 2064
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_next: .word 0 ; End of program
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.scend
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.advance 2064
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</programlisting>
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<para>
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It's possible to have multiple temporary labels with the same
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name in different parts of the code. If you create a label map
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in those cases, you will have to look at the sourcefile location
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to distinguish them.
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</para>
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</section>
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<section>
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<title>Anonymous labels</title>
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<para>
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Anonymous labels are a way to handle short-ranged branches
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without having to come up with names for the then and else
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branches, for brief loops, and other such purposes. To define
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an anonymous label, use an asterisk. To refer to an anonymous
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label, use a series of <literal>+</literal>
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or <literal>-</literal> signs. <literal>+</literal> refers to
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the next anonymous label, <literal>++</literal> the label
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after that, etc. Likewise, <literal>-</literal> is the most
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recently defined label, <literal>--</literal> the one before
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that, and so on. The main body of the Hello World program
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with anonymous labels would be:
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</para>
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<programlisting>
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ldx #0
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* lda hello, x
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beq +
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jsr $ffd2
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inx
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bne -
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* rts
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</programlisting>
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<para>
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It is worth noting that anonymous labels are globally available.
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They are not temporary labels, and they ignore scoping
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restrictions.
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</para>
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</section>
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<section>
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<title>Aliasing</title>
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<para>
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Rather the reverse of anonymous labels, aliases are names
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given to specific memory locations. These make it easier to
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keep track of important constants or locations. The KERNAL
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routines are a good example of constants that deserve names.
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To assign the traditional name <literal>chrout</literal> to
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the routine at $FFD2, simply give the directive:
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</para>
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<programlisting>
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.alias chrout $ffd2
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</programlisting>
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<para>And change the <userinput>jsr</userinput> command
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to:</para>
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<programlisting>
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jsr chrout
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</programlisting>
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<para>
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The final version of the code is in <xref linkend="tutor2-src" endterm="tutor2-fname">. It should
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assemble to exactly the same program as <xref linkend="tutor1-src" endterm="tutor1-fname">.
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</para>
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</section>
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</chapter>
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