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220 lines
3.7 KiB
HTML
220 lines
3.7 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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>Functionals</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="To HLL and Back"
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HREF="p481.html"><LINK
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REL="PREVIOUS"
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TITLE="Conclusion"
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HREF="x813.html"><LINK
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REL="NEXT"
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TITLE="A quick digression on how subroutines work"
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HREF="x836.html"></HEAD
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><BODY
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CLASS="CHAPTER"
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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="x813.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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></TD
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><TD
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WIDTH="10%"
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VALIGN="bottom"
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></TD
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></TR
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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="AEN816"
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></A
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>Functionals</H1
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><P
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> This essay deals with indirect calls. These are the core of an
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enormous number of high level languages: LISP's closures, C's
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function pointers, C++ and Java's virtual method calls, and some
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implementations of the <TT
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CLASS="LITERAL"
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>switch</TT
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> statement.</P
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><P
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> These techniques vary in complexity, and most will not be
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appropriate for large-scale assembler projects. Of them, however,
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the Data-Directed approach is the most likely to lead to organized
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and maintainable code.</P
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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="AEN821"
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>Function Pointers</A
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></H1
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><P
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> Because assembly language is totally untyped, function pointers
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are the same as any other sixteen-bit integer. This makes
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representing them really quite easy; most assemblers should permit
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routines to be declared simply by naming the routine as
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a <TT
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CLASS="LITERAL"
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>.word</TT
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> directly.
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</P
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><P
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> To actually invoke these methods, copy them to some sixteen-bit
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location (say, <TT
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CLASS="LITERAL"
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>target</TT
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>) and then invoking the
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method is a simple matter of the using an indirect jump:
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the <TT
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CLASS="LITERAL"
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>JMP<EFBFBD>(target)</TT
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> instruction.
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</P
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><P
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> There's really only one subtlety here, and it's that the indirect
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jump is an indirect <I
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CLASS="EMPHASIS"
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>jump</I
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>, not an
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indirect <I
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CLASS="EMPHASIS"
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>function call</I
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>. Thus, if some
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function <TT
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CLASS="LITERAL"
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>A</TT
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> makes in indirect jump to some
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routine, when that routine returns, it returns to whoever
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called <TT
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CLASS="LITERAL"
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>A</TT
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>, not <TT
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CLASS="LITERAL"
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>A</TT
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>
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itself.
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</P
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><P
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> There are several ways of dealing with this, but only one correct
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way, which is to structure your procedures so that any call
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to <TT
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CLASS="LITERAL"
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>JMP<EFBFBD>(xxxx)</TT
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> occurs at the very
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end.
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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="x813.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="x836.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"
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>Conclusion</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="p481.html"
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ACCESSKEY="U"
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>Up</A
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></TD
|
||
><TD
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||
WIDTH="33%"
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||
ALIGN="right"
|
||
VALIGN="top"
|
||
>A quick digression on how subroutines work</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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> |