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402 lines
7.2 KiB
HTML
402 lines
7.2 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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>Local variables and memory segments</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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TITLE="Character maps"
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TITLE="Expressions"
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HREF="c371.html"></HEAD
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><TH
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>Programming with Ophis</TH
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></TR
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><TD
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WIDTH="10%"
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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="CH5-LINK"
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></A
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>Local variables and memory segments</H1
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><P
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> As mentioned in <A
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HREF="c292.html"
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>the Chapter called <I
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>Character maps</I
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></A
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>, there are better ways
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to handle waiting than just executing vast numbers of NOPs. The
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Commodore 64 KERNAL library includes a <TT
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CLASS="LITERAL"
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>rdtim</TT
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>
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routine that returns the uptime of the machine, in
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60<SUP
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>th</SUP
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>s of a second, as a 24-bit integer.
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The Commodore 64 programmer's guide available online actually has
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a bug in it, reversing the significance of the A and Y registers.
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The accumulator holds the <I
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CLASS="EMPHASIS"
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>least</I
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> significant
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byte, not the most.
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</P
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><P
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> Here's a first shot at a better delay routine:
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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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>.scope
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; data used by the delay routine
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_tmp: .byte 0
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_target: .byte 0
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delay: sta _tmp ; save argument (rdtim destroys it)
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jsr rdtim
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clc
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adc _tmp ; add current time to get target
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sta _target
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* jsr rdtim
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cmp _target
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bmi - ; Buzz until target reached
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rts
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.scend</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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> This works, but it eats up two bytes of file space that don't
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really need to be specified. Also, it's modifying data inside a
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program text area, which isn't good if you're assembling to a ROM
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chip. (Since the Commodore 64 stores its programs in RAM, it's
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not an issue for us here.) A slightly better solution is to
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use <TT
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CLASS="LITERAL"
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>.alias</TT
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> to assign the names to chunks of RAM
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somewhere. There's a 4K chunk of RAM from $C000 through $CFFF
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between the BASIC ROM and the I/O ROM that should serve our
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purposes nicely. We can replace the definitions
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of <TT
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CLASS="LITERAL"
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>_tmp</TT
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> and <TT
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CLASS="LITERAL"
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>_target</TT
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> with:
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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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> ; data used by the delay routine
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.alias _tmp $C000
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.alias _target $C001</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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> This works better, but now we've just added a major bookkeeping
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burden upon ourselves—we must ensure that no routines step on
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each other. What we'd really like are two separate program
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counters—one for the program text, and one for our variable
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space.
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</P
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><P
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> Ophis lets us do this with the <TT
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CLASS="LITERAL"
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>.text</TT
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>
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and <TT
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CLASS="LITERAL"
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>.data</TT
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> commands.
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The <TT
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CLASS="LITERAL"
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>.text</TT
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> command switches to the program-text
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counter, and the <TT
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CLASS="LITERAL"
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>.data</TT
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> command switches to the
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variable-data counter. When Ophis first starts assembling a file,
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it starts in <TT
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CLASS="LITERAL"
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>.text</TT
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> mode.
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</P
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><P
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> To reserve space for a variable, use the .space command. This
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takes the form:
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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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>.space varname size</PRE
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></TD
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></TR
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></TABLE
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>
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which assigns the name <TT
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CLASS="LITERAL"
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>varname</TT
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> to the current
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program counter, then advances the program counter by the amount
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specified in <TT
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CLASS="LITERAL"
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>size</TT
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>. Nothing is output to the
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final binary as a result of the <TT
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CLASS="LITERAL"
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>.space</TT
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> command.
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</P
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><P
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> You may not put in any commands that produce output into
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a <TT
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CLASS="LITERAL"
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>.data</TT
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> segment. Generally, all you will be
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using are <TT
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CLASS="LITERAL"
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>.org</TT
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> and <TT
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CLASS="LITERAL"
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>.space</TT
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>
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commands. Ophis will not complain if you
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use <TT
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CLASS="LITERAL"
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>.space</TT
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> inside a <TT
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CLASS="LITERAL"
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>.text</TT
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>
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segment, but this is nearly always wrong.
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</P
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><P
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> The final version of <TT
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CLASS="LITERAL"
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>delay</TT
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> looks like this:
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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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>; DELAY routine. Takes values from the Accumulator and pauses
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; for that many jiffies (1/60th of a second).
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.scope
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.data
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.space _tmp 1
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.space _target 1
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.text
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delay: sta _tmp ; save argument (rdtim destroys it)
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jsr rdtim
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clc
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adc _tmp ; add current time to get target
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sta _target
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* jsr rdtim
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cmp _target
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bmi - ; Buzz until target reached
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rts
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.scend</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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> We're not quite done yet, however, because we have to tell the
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data segment where to begin. (If we don't, it starts at 0, which
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is usually wrong.) We add a very brief data segment to the top of
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our code:
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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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>.data
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.org $C000
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.text</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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> This will run. However, we also ought to make sure that we aren't
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overstepping any boundaries. Our program text shouldn't run into
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the BASIC chip at $A000, and our data shouldn't run into the I/O
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region at $D000. The <TT
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CLASS="LITERAL"
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>.checkpc</TT
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> command lets us
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assert that the program counter hasn't reached a specific point
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yet. We put, at the end of our code:
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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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>.checkpc $A000
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.data
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.checkpc $D000</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 final program is available as <A
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HREF="x489.html"
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><I
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><I
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>tutor5.oph</I
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></I
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></A
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>. Note that we based this on the
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all-uppercase version from the last section, not any of the
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charmapped versions.
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</P
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></DIV
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>Character maps</TD
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