2012-06-09 03:21:33 -07:00

108 lines
3.7 KiB

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