142 lines
3.7 KiB
Plaintext
142 lines
3.7 KiB
Plaintext
<chapter>
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<title>Advanced Memory Segments</title>
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<para>
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By now we've covered the basics of every command in the assembler;
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in this final installment we show the full capabilities of
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the <literal>.text</literal> and <literal>.data</literal> commands
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as we produce a more sophisticated set of Commodore 64 header
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files.
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</para>
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<section>
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<title>The Problem</title>
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<para>
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Our <literal>print'str</literal> routine
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in <xref linkend="tutor6-src" endterm="tutor6-fname"> accesses
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memory locations $10 and $11 directly. We'd prefer to have
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symbolic names for them. This reprises our concerns back in
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<xref linkend="ch5-link"> when we concluded that we wanted two
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separate program counters. Now we realize that we really need
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three; one for the text, one for the data, and one for the zero
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page data. And if we're going to allow three, we really should
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allow any number.
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</para>
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</section>
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<section>
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<title>The Solution</title>
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<para>
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The <literal>.data</literal> and <literal>.text</literal>
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commands can take a label name after them—this names a new
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segment. We'll define a new segment
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called <literal>zp</literal> (for <quote>zero page</quote>) and
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have our zero-page variables be placed there. We can't actually
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use the default origin of $0000 here either, though, because the
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Commodore 64 reserves memory locations 0 and 1 to control its
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memory mappers:
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</para>
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<programlisting>
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.data zp
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.org $0002
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</programlisting>
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<para>
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Now, actually, the rest of the zero page is reserved too:
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locations $02-$8F are used by the BASIC interpreter, and
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locations $90-$FF are used by the KERNAL. We don't need the
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BASIC interpreter, though, so we can back up all of $02-$8F at
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the start of our program and restore it all when we're done.
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</para>
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<para>
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In fact, since we're disablng BASIC, we can actually also swap
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out its ROM entirely and get a contiguous block of RAM from
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$0002 to $CFFF:
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</para>
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<programlisting>
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.scope
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; Cache BASIC zero page at top of available RAM
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ldx #$8e
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* lda $01, x
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sta $cf81, x
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dex
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bne -
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; Swap out the BASIC ROM for RAM
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lda $01
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and #$fe
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ora #$06
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sta $01
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; Run the real program
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jsr _main
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; Restore BASIC ROM
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lda $01
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ora #$07
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sta $01
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; Restore BASIC zero page
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ldx #$8e
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* lda $cf81, x
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sta $01, x
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dex
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bne -
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; Back to BASIC
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rts
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_main:
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; _main points at the start of the real program,
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; which is actually outside of this scope
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.scend
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</programlisting>
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<para>
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Our <literal>print'str</literal> routine is then rewritten to
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declare and use a zero-page variable, like so:
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</para>
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<programlisting>
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; PRINTSTR routine. Accumulator stores the low byte of the address,
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; X register stores the high byte. Destroys the values of $10 and
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; $11.
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.scope
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.data zp
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.space _ptr 2
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.text
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printstr:
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sta _ptr
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stx _ptr+1
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ldy #$00
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_lp: lda (_ptr),y
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beq _done
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jsr chrout
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iny
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bne _lp
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_done: rts
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.scend
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</programlisting>
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<para>
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Also, we ought to put in an extra check to make sure our
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zero-page allocations don't overflow, either:
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</para>
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<programlisting>
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.data zp
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.checkpc $80
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</programlisting>
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<para>
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The final source file is <xref linkend="tutor7-src"
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endterm="tutor7-fname">.
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</para>
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</section>
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</chapter>
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