Ophis/doc/tutor5.sgm
2012-06-08 23:41:16 -07:00

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<chapter id="ch5-link">
<title>Local variables and memory segments</title>
<para>
As mentioned in <xref linkend="ch4-link">, there are better ways
to handle waiting than just executing vast numbers of NOPs. The
Commodore 64 KERNAL library includes a <literal>rdtim</literal>
routine that returns the uptime of the machine, in
60<superscript>th</superscript>s of a second, as a 24-bit integer.
The Commodore 64 programmer's guide available online actually has
a bug in it, reversing the significance of the A and Y registers.
The accumulator holds the <emphasis>least</emphasis> significant
byte, not the most.
</para>
<para>
Here's a first shot at a better delay routine:
</para>
<programlisting>
.scope
; data used by the delay routine
_tmp: .byte 0
_target: .byte 0
delay: sta _tmp ; save argument (rdtim destroys it)
jsr rdtim
clc
adc _tmp ; add current time to get target
sta _target
* jsr rdtim
cmp _target
bmi - ; Buzz until target reached
rts
.scend
</programlisting>
<para>
This works, but it eats up two bytes of file space that don't
really need to be specified. Also, it's modifying data inside a
program text area, which isn't good if you're assembling to a ROM
chip. (Since the Commodore 64 stores its programs in RAM, it's
not an issue for us here.) A slightly better solution is to
use <literal>.alias</literal> to assign the names to chunks of RAM
somewhere. There's a 4K chunk of RAM from $C000 through $CFFF
between the BASIC ROM and the I/O ROM that should serve our
purposes nicely. We can replace the definitions
of <literal>_tmp</literal> and <literal>_target</literal> with:
</para>
<programlisting>
; data used by the delay routine
.alias _tmp $C000
.alias _target $C001
</programlisting>
<para>
This works better, but now we've just added a major bookkeeping
burden upon ourselves&mdash;we must ensure that no routines step on
each other. What we'd really like are two separate program
counters&mdash;one for the program text, and one for our variable
space.
</para>
<para>
Ophis lets us do this with the <literal>.text</literal>
and <literal>.data</literal> commands.
The <literal>.text</literal> command switches to the program-text
counter, and the <literal>.data</literal> command switches to the
variable-data counter. When Ophis first starts assembling a file,
it starts in <literal>.text</literal> mode.
</para>
<para>
To reserve space for a variable, use the .space command. This
takes the form:
<programlisting>
.space varname size
</programlisting>
which assigns the name <literal>varname</literal> to the current
program counter, then advances the program counter by the amount
specified in <literal>size</literal>. Nothing is output to the
final binary as a result of the <literal>.space</literal> command.
</para>
<para>
You may not put in any commands that produce output into
a <literal>.data</literal> segment. Generally, all you will be
using are <literal>.org</literal> and <literal>.space</literal>
commands. Ophis will not complain if you
use <literal>.space</literal> inside a <literal>.text</literal>
segment, but this is nearly always wrong.
</para>
<para>
The final version of <literal>delay</literal> looks like this:
</para>
<programlisting>
; DELAY routine. Takes values from the Accumulator and pauses
; for that many jiffies (1/60th of a second).
.scope
.data
.space _tmp 1
.space _target 1
.text
delay: sta _tmp ; save argument (rdtim destroys it)
jsr rdtim
clc
adc _tmp ; add current time to get target
sta _target
* jsr rdtim
cmp _target
bmi - ; Buzz until target reached
rts
.scend
</programlisting>
<para>
We're not quite done yet, however, because we have to tell the
data segment where to begin. (If we don't, it starts at 0, which
is usually wrong.) We add a very brief data segment to the top of
our code:
</para>
<programlisting>
.data
.org $C000
.text
</programlisting>
<para>
This will run. However, we also ought to make sure that we aren't
overstepping any boundaries. Our program text shouldn't run into
the BASIC chip at $A000, and our data shouldn't run into the I/O
region at $D000. The <literal>.checkpc</literal> command lets us
assert that the program counter hasn't reached a specific point
yet. We put, at the end of our code:
</para>
<programlisting>
.checkpc $A000
.data
.checkpc $D000
</programlisting>
<para>
The final program is available as <xref linkend="tutor5-src"
endterm="tutor5-fname">. Note that we based this on the
all-uppercase version from the last section, not any of the
charmapped versions.
</para>
</chapter>