358 lines
6.4 KiB
HTML
358 lines
6.4 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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>Macros</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="UP"
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TITLE="Ophis Command Reference"
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HREF="a505.html"><LINK
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REL="PREVIOUS"
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TITLE="Memory Model"
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HREF="x647.html"><LINK
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REL="NEXT"
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TITLE="Assembler directives"
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HREF="x732.html"></HEAD
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><BODY
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CLASS="SECTION"
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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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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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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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HREF="x647.html"
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ACCESSKEY="P"
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><<< Previous</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Ophis Command Reference</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="x732.html"
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ACCESSKEY="N"
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>Next >>></A
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></TD
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></TR
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></TABLE
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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="SECTION"
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><H1
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CLASS="SECTION"
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><A
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NAME="AEN692"
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>Macros</A
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></H1
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><P
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> Assembly language is a powerful tool—however, there are
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many tasks that need to be done repeatedly, and with
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mind-numbing minor modifications. Ophis includes a facility
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for <I
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CLASS="EMPHASIS"
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>macros</I
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> to allow this. Ophis macros
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are very similar in form to function calls in higher level
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languages.
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</P
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><DIV
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CLASS="SECTION"
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><H2
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CLASS="SECTION"
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><A
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NAME="AEN696"
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>Defining Macros</A
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></H2
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><P
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> Macros are defined with the <TT
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CLASS="LITERAL"
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>.macro</TT
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>
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and <TT
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CLASS="LITERAL"
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>.macend</TT
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> commands. Here's a
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simple one that will clear the screen on a Commodore
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64:
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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 clr'screen
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lda #147
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jsr $FFD2
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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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></DIV
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><DIV
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CLASS="SECTION"
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><H2
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CLASS="SECTION"
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><A
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NAME="AEN702"
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>Invoking Macros</A
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></H2
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><P
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> To invoke a macro, either use
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the <TT
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CLASS="LITERAL"
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>.invoke</TT
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> command or backquote the
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name of the routine. The previous macro may be expanded
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out in either of two ways, at any point in the
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source:
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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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>.invoke clr'screen</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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>or</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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>`clr'screen</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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>will work equally well.</P
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></DIV
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><DIV
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CLASS="SECTION"
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><H2
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CLASS="SECTION"
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><A
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NAME="AEN710"
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>Passing Arguments to Macros</A
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></H2
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><P
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> Macros may take arguments. The arguments to a macro are
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all of the <SPAN
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CLASS="QUOTE"
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>"word"</SPAN
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> type, though byte values may
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be passed and used as bytes as well. The first argument in
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an invocation is bound to the label
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<TT
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CLASS="LITERAL"
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>_1</TT
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>, the second
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to <TT
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CLASS="LITERAL"
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>_2</TT
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>, and so on. Here's a macro
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for storing a 16-bit value into a word pointer:
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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 store16 ; `store16 dest, src
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lda #<_2
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sta _1
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lda #>_2
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sta _1+1
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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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> Macro arguments behave, for the most part, as if they were
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defined by <TT
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CLASS="LITERAL"
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>.alias</TT
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>
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commands <I
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CLASS="EMPHASIS"
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>in the calling context</I
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>.
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(They differ in that they will not produce duplicate-label
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errors if those names already exist in the calling scope,
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and in that they disappear after the call is
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completed.)
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</P
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></DIV
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><DIV
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CLASS="SECTION"
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><H2
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CLASS="SECTION"
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><A
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NAME="AEN720"
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>Features and Restrictions of the Ophis Macro Model</A
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></H2
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><P
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> Unlike most macro systems (which do textual replacement),
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Ophis macros evaluate their arguments and bind them into the
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symbol table as temporary labels. This produces some
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benefits, but it also puts some restrictions on what kinds of
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macros may be defined.
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</P
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><P
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> The primary benefit of this <SPAN
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CLASS="QUOTE"
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>"expand-via-binding"</SPAN
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>
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discipline is that there are no surprises in the semantics.
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The expression <TT
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CLASS="LITERAL"
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>_1+1</TT
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> in the macro above
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will always evaluate to one more than the value that was
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passed as the first argument, even if that first argument is
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some immensely complex expression that an
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expand-via-substitution method may accidentally
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mangle.
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</P
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><P
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> The primary disadvantage of the expand-via-binding
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discipline is that only fixed numbers of words and bytes
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may be passed. A substitution-based system could define a
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macro including the line <TT
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CLASS="LITERAL"
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>LDA _1</TT
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> and
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accept as arguments both <TT
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CLASS="LITERAL"
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>$C000</TT
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>
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(which would put the value of memory location $C000 into
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the accumulator) and <TT
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CLASS="LITERAL"
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>#$40</TT
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> (which
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would put the immediate value $40 into the accumulator).
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If you <I
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CLASS="EMPHASIS"
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>really</I
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> need this kind of
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behavior, a run a C preprocessor over your Ophis source,
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and use <TT
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CLASS="LITERAL"
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>#define</TT
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> to your heart's
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content.
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</P
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></DIV
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="x647.html"
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ACCESSKEY="P"
|
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><<< Previous</A
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></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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><A
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HREF="book1.html"
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ACCESSKEY="H"
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>Home</A
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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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VALIGN="top"
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><A
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HREF="x732.html"
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ACCESSKEY="N"
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>Next >>></A
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></TD
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></TR
|
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><TR
|
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><TD
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WIDTH="33%"
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|
ALIGN="left"
|
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VALIGN="top"
|
|
>Memory Model</TD
|
|
><TD
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|
WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
|
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><A
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HREF="a505.html"
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ACCESSKEY="U"
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>Up</A
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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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VALIGN="top"
|
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>Assembler directives</TD
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></TR
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></TABLE
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></DIV
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></BODY
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></HTML
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> |