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348 lines
5.2 KiB
HTML
348 lines
5.2 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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>Expressions</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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REL="PREVIOUS"
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TITLE="Local variables and memory segments"
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HREF="c329.html"><LINK
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REL="NEXT"
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TITLE="Advanced Memory Segments"
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HREF="c419.html"></HEAD
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><BODY
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CLASS="CHAPTER"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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ALINK="#0000FF"
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SUMMARY="Header navigation table"
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Programming with Ophis</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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ACCESSKEY="P"
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><<< Previous</A
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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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="AEN371"
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></A
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>Expressions</H1
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><P
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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 <TT
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CLASS="LITERAL"
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>print</TT
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> 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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</P
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><P
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> Here's our printing routine. It's fairly straightforward.
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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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>; 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</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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> However, now we are faced with the problem of what to do with
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the <TT
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CLASS="LITERAL"
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>print</TT
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> 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 <TT
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CLASS="LITERAL"
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><</TT
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> and <TT
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CLASS="LITERAL"
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>></TT
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> operators
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to take the low or high byte of a word, respectively.
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The <TT
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CLASS="LITERAL"
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>print</TT
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> macro becomes:
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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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>.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</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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> 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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</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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.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</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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> Note that we only have to name <TT
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CLASS="LITERAL"
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>cache</TT
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> once, but
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can use addition to refer to any offset from it.
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</P
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><P
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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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</P
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><DIV
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CLASS="TABLE"
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><A
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NAME="AEN388"
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></A
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><P
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><B
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>Table 1. Ophis Operators</B
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></P
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><TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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CELLSPACING="0"
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CELLPADDING="4"
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CLASS="CALSTABLE"
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><THEAD
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><TR
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><TH
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ALIGN="CENTER"
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>Operators</TH
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><TH
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ALIGN="CENTER"
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>Description</TH
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></TR
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></THEAD
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><TBODY
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><TR
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><TD
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><TT
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CLASS="LITERAL"
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>[ ]</TT
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></TD
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><TD
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>Parenthesized expressions</TD
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></TR
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><TR
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><TD
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><TT
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CLASS="LITERAL"
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>< ></TT
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></TD
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><TD
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>Byte selection (low, high)</TD
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></TR
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><TR
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><TD
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><TT
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CLASS="LITERAL"
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>* /</TT
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></TD
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><TD
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>Multiply, divide</TD
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></TR
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><TR
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><TD
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><TT
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CLASS="LITERAL"
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>+ -</TT
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></TD
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><TD
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>Add, subtract</TD
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></TR
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><TR
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><TD
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><TT
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CLASS="LITERAL"
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>| & ^</TT
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></TD
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><TD
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>Bitwise OR, AND, XOR</TD
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></TR
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></TBODY
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></TABLE
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></DIV
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><P
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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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</P
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><P
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> The code for this version of the code is
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in <A
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HREF="x493.html"
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><I
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><I
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>tutor6.oph</I
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></I
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></A
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>.
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</P
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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ALIGN="LEFT"
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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HREF="c329.html"
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ACCESSKEY="P"
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><<< Previous</A
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HREF="book1.html"
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>Home</A
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>Next >>></A
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><TR
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>Local variables and memory segments</TD
|
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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> </TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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>Advanced Memory Segments</TD
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></TR
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></BODY
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> |