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344 lines
6.1 KiB
HTML
344 lines
6.1 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<HTML
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><HEAD
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><TITLE
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>Character maps</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
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REL="HOME"
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TITLE="Programming with Ophis"
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HREF="book1.html"><LINK
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REL="PREVIOUS"
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TITLE="Example code"
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HREF="x287.html"><LINK
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REL="NEXT"
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TITLE="Local variables and memory segments"
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HREF="c329.html"></HEAD
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CLASS="CHAPTER"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Programming with Ophis</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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ACCESSKEY="P"
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><<< Previous</A
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="CHAPTER"
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><H1
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><A
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NAME="CH4-LINK"
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></A
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>Character maps</H1
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><P
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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 <TT
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CLASS="LITERAL"
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>delay</TT
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> that executes 2,560*(accumulator)
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<KBD
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CLASS="USERINPUT"
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>NOP</KBD
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>s. By the time the program is finished,
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we'll have executed 768,000 no-ops.
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</P
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><P
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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 <A
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HREF="c329.html"
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>the Chapter called <I
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>Local variables and memory segments</I
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></A
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>.
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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 <A
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HREF="x477.html"
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><I
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><I
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>tutor4a.oph</I
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></I
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></A
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>, 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 <TT
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CLASS="LITERAL"
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>greet</TT
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> macro.
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</P
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><P
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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 <TT
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CLASS="LITERAL"
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>upper'case</TT
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>
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and <TT
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CLASS="LITERAL"
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>lower'case</TT
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> aliases back
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in <A
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HREF="c236.html"
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>the Chapter called <I
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>Headers, Libraries, and Macros</I
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></A
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> as part of the
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standard <A
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HREF="x469.html"
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><I
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><I
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>kernal.oph</I
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></I
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></A
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>
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header, so we can add this before our invocations of
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the <TT
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CLASS="LITERAL"
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>greet</TT
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> macro:
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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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> lda #lower'case
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jsr chrout</PRE
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></TD
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></TR
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></TABLE
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><P
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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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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="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</PRE
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></TD
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></TR
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></TABLE
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><P
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> The code that does this is in <A
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HREF="x481.html"
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><I
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><I
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>tutor4b.oph</I
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></I
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></A
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>. 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 <SAMP
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CLASS="COMPUTEROUTPUT"
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>hELLO, sOLAR sYSTEM!</SAMP
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>. 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 the Apple II's
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character set, or targeting a game console that puts its letters
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in arbitrary locations. We need to remap how strings are turned
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into byte values. The <TT
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CLASS="LITERAL"
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>.charmap</TT
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>
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and <TT
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CLASS="LITERAL"
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>.charmapbin</TT
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> directives do what we need.
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</P
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><P
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> The <TT
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CLASS="LITERAL"
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>.charmap</TT
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> 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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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>.charmap 'A, "abcdefghijklmnopqrstuvwxyz"
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.charmap 'a, "ABCDEFGHIJKLMNOPQRSTUVWXYZ"</PRE
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></TD
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></TR
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></TABLE
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><P
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> Note that the <TT
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CLASS="LITERAL"
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>'a</TT
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> constant in the second
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directive refers to the <SPAN
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CLASS="QUOTE"
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>"a"</SPAN
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> character in the source,
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not in the current map.
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</P
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><P
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> The fixed code is in <A
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HREF="x485.html"
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><I
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><I
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>tutor4c.oph</I
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></I
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></A
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>, and will produce the expected results
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when run.
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</P
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><P
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> An alternative is to use a <TT
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CLASS="LITERAL"
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>.charmapbin</TT
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>
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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 <TT
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CLASS="FILENAME"
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>petscii.map</TT
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>. There are also three
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files, <TT
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CLASS="FILENAME"
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>a2normal.map</TT
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>, <TT
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CLASS="FILENAME"
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>a2inverse.map</TT
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>,
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and <TT
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CLASS="FILENAME"
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>a2blink.map</TT
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> that handle the Apple II's
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very nonstandard character encodings.
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</P
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></DIV
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><DIV
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><<< Previous</A
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>Next >>></A
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WIDTH="33%"
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>Example code</TD
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VALIGN="top"
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> </TD
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