108 lines
3.7 KiB
Plaintext
108 lines
3.7 KiB
Plaintext
<chapter id="ch4-link">
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<title>Character maps</title>
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<para>
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Now we will close the gap between the Commodore's
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version of ASCII and the real one. We'll also add a time-delay
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routine to slow down the output. This routine isn't really of
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interest to us right now, so we'll add a subroutine
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called <literal>delay</literal> that executes 2,560*(accumulator)
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<userinput>NOP</userinput>s. By the time the program is finished,
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we'll have executed 768,000 no-ops.
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</para>
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<para>
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There actually are better ways of getting a time-delay on the
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Commodore 64; we'll deal with those in <xref linkend="ch5-link">.
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As a result, there isn't really a lot to discuss here. The later
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tutorials will be building off of <xref linkend="tutor4a-src"
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endterm="tutor4a-fname">, so you may want to get familiar with
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that. Note also the change to the body of
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the <literal>greet</literal> macro.
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</para>
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<para>
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On to the topic at hand. Let's change the code to use mixed case.
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We defined the <literal>upper'case</literal>
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and <literal>lower'case</literal> aliases back
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in <xref linkend="ch3-link"> as part of the
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standard <xref linkend="kernal-src" endterm="kernal-fname">
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header, so we can add this before our invocations of
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the <literal>greet</literal> macro:
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</para>
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<programlisting>
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lda #lower'case
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jsr chrout
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</programlisting>
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<para>
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And that will put us into mixed case mode. So, now we just need
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to redefine the data so that it uses the mixed-case:
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</para>
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<programlisting>
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hello1: .byte "Hello, ",0
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hello2: .byte "!", 13, 0
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target1: .byte "programmer", 0
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target2: .byte "room", 0
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target3: .byte "building", 0
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target4: .byte "neighborhood", 0
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target5: .byte "city", 0
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target6: .byte "nation", 0
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target7: .byte "world", 0
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target8: .byte "Solar System", 0
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target9: .byte "Galaxy", 0
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target10: .byte "Universe", 0
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</programlisting>
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<para>
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The code that does this is in <xref linkend="tutor4b-src"
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endterm="tutor4b-fname">. If you assemble and run it, you will
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notice that the output is not what we want. In particular, upper
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and lowercase are reversed, so we have messages
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like <computeroutput>hELLO, sOLAR sYSTEM!</computeroutput>. For
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the specific case of PETSCII, we can just fix our strings, but
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that's less of an option if we're writing for a game console that
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puts its letters in arbitrary locations. We need to remap how
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strings are turned into byte values.
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The <literal>.charmap</literal> and <literal>.charmapbin</literal>
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directives do what we need.
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</para>
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<para>
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The <literal>.charmap</literal> directive usually takes two
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arguments; a byte (usually in character form) indicating the ASCII
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value to start remapping from, and then a string giving the new
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values. To do our case-swapping, we write two directives before
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defining any string constants:
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</para>
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<programlisting>
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.charmap 'A, "abcdefghijklmnopqrstuvwxyz"
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.charmap 'a, "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
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</programlisting>
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<para>
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Note that the <literal>'a</literal> constant in the second
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directive refers to the <quote>a</quote> character in the source,
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not in the current map.
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</para>
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<para>
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The fixed code is in <xref linkend="tutor4c-src"
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endterm="tutor4c-fname">, and will produce the expected results
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when run.
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</para>
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<para>
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An alternative is to use a <literal>.charmapbin</literal>
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directive to replace the entire character map directly. This
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specifies an external file, 256 bytes long, that is loaded in at
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that point. A binary character map for the Commodore 64 is
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provided with the sample programs
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as <filename>petscii.map</filename>.
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</para>
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</chapter>
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