122 lines
3.4 KiB
Plaintext
122 lines
3.4 KiB
Plaintext
<chapter>
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<title>Expressions</title>
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<para>
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Ophis permits a reasonably rich set of arithmetic operations to be
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done at assemble time. So far, all of our arguments and values
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have either been constants or label names. In this chapter, we
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will modify the <literal>print</literal> macro so that it calls a
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subroutine to do the actual printing. This will shrink the final
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code size a fair bit.
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</para>
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<para>
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Here's our printing routine. It's fairly straightforward.
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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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printstr:
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sta $10
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stx $11
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ldy #$00
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_lp: lda ($10), 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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However, now we are faced with the problem of what to do with
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the <literal>print</literal> macro. We need to take a 16-bit
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value and store it in two 8-bit registers. We can use
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the <literal><</literal> and <literal>></literal> operators
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to take the low or high byte of a word, respectively.
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The <literal>print</literal> macro becomes:
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</para>
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<programlisting>
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.macro print
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lda #<_1
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ldx #>_1
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jsr printstr
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.macend
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</programlisting>
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<para>
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Also, since BASIC uses the locations $10 and $11, we should really
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cache them at the start of the program and restore them at the
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end:
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</para>
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<programlisting>
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.data
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.org $C000
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.space cache 2
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.text
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; Save the zero page locations that printstr uses.
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lda $10
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sta cache
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lda $11
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sta cache+1
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; ... main program goes here ...
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; Restore the zero page values printstr uses.
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lda cache
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sta $10
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lda cache+1
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sta $11
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</programlisting>
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<para>
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Note that we only have to name <literal>cache</literal> once, but
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can use addition to refer to any offset from it.<footnote><para>We
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could spare ourselves some trouble here and use $fb instead of
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$10, which BASIC does <emphasis>not</emphasis> use, but the
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example is more thorough this way.</para></footnote>
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</para>
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<para>
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Ophis supports following operations, with the following precedence
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levels (higher entries bind more tightly):
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</para>
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<table frame="all">
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<title>Ophis Operators</title>
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<tgroup cols='2'>
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<thead>
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<row>
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<entry align="center">Operators</entry>
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<entry align="center">Description</entry>
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</row>
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</thead>
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<tbody>
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<row><entry><literal>[ ]</literal></entry><entry>Parenthesized expressions</entry></row>
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<row><entry><literal>< ></literal></entry><entry>Byte selection (low, high)</entry></row>
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<row><entry><literal>* /</literal></entry><entry>Multiply, divide</entry></row>
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<row><entry><literal>+ -</literal></entry><entry>Add, subtract</entry></row>
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<row><entry><literal>| & ^</literal></entry><entry>Bitwise OR, AND, XOR</entry></row>
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</tbody>
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</tgroup>
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</table>
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<para>
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Note that brackets, not parentheses, are used to group arithmetic
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operations. This is because parentheses are used for the indirect
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addressing modes, and it makes parsing much easier.
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</para>
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<para>
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The code for this version of the code is
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in <xref linkend="tutor6-src" endterm="tutor6-fname">.
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</para>
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</chapter>
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