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https://github.com/michaelcmartin/Ophis.git
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209 lines
3.8 KiB
HTML
209 lines
3.8 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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>Call Stacks</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="A final reminder"
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HREF="x892.html"><LINK
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REL="NEXT"
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TITLE="Our Goals"
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HREF="x915.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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><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="x892.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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></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="x915.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="CHAPTER"
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><H1
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><A
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NAME="AEN900"
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></A
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>Call Stacks</H1
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><P
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> All our previous work has been assuming FORTRAN-style calling
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conventions. In this, all procedure-local variables are actually
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secretly globals. This means that a function that calls itself will
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end up stomping on its previous values, and everything will be
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hideously scrambled. Various workarounds for this are covered
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in <A
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HREF="c543.html"
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>the Chapter called <I
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>Structured Programming</I
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></A
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>. Here, we solve the problem fully.</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="AEN904"
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>Recursion</A
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></H1
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><P
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> A procedure in C or other similar languages declares a chunk of
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storage that's unique to that invocation. This chunk is just
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large enough to hold the return address and all the local
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variables, and is called the <I
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CLASS="EMPHASIS"
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>stack frame</I
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>.
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Stack frames are arranged on a <I
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CLASS="EMPHASIS"
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>call stack</I
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>;
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when a function is called, the stack grows with the new frame, and
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when that function returns, its frame is destroyed. Once the main
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function returns, the stack is empty.
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</P
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><P
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> Most modern architectures are designed to let you implement
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variable access like this directly, without touching the registers
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at all. The x86 architecture even dedicates a register to
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function explicitly as the <I
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CLASS="EMPHASIS"
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>stack pointer</I
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>, and
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then one could read, say, the fifth 16-bit variable into the
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register AX with the command <TT
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CLASS="LITERAL"
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>MOV AX, [SP+10]</TT
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>.
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</P
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><P
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> As we saw in <A
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HREF="c748.html"
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>the Chapter called <I
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>Pointers and Indirection</I
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></A
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>, the 6502 isn't nearly as
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convenient. We'd need to keep the stack pointer somewhere on the
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zero page, then load the Y register with 10, then load the
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accumulator with an indexed-indirect call. This is verbose, keeps
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trashing our registers, and it's very, very slow.
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</P
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><P
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> So, in the spirit of programmers everywhere, we'll cheat.
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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="x892.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="x915.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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>A final reminder</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
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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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>Our Goals</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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> |