mirror of
https://github.com/michaelcmartin/Ophis.git
synced 2024-09-10 01:54:25 +00:00
336 lines
6.2 KiB
HTML
336 lines
6.2 KiB
HTML
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<!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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>Our Goals</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="Call Stacks"
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HREF="c900.html"><LINK
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REL="PREVIOUS"
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TITLE="Call Stacks"
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HREF="c900.html"><LINK
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REL="NEXT"
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TITLE="Example: Fibonnacci Numbers"
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HREF="x967.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="c900.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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>Call Stacks</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="x967.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="AEN915"
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>Our Goals</A
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></H1
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><P
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> The system we develop should have all of the following
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characteristics.
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</P
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><P
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></P
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><UL
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><LI
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><P
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>It should be <I
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CLASS="EMPHASIS"
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>intuitive to program for</I
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>. The procedure bodies should be easily readable and writable by humans, even in assembler form.</P
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></LI
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><LI
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><P
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>It should be <I
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CLASS="EMPHASIS"
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>efficient</I
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>. Variable accesses are very common, so procedures shouldn't cost much to run.</P
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></LI
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><LI
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><P
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>It should allow <I
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CLASS="EMPHASIS"
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>multiple arity</I
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> in both arguments and return values. We won't require that an unlimited amount of information be passable, but it should allow more than the three bytes the registers give us.</P
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></LI
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><LI
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><P
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>It should permit <I
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CLASS="EMPHASIS"
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>tail call elimination</I
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>, an optimization that will allow certain forms of recursion to actually not grow the stack.</P
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></LI
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></UL
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><P
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> Here is a system that meets all these properties.
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</P
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><P
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></P
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><UL
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><LI
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><P
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>Reserve two bytes of the zero page for a stack pointer. At the beginning of the program, set it to the top of memory.</P
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></LI
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><LI
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><P
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>Divide the remainder of Zero Page into two parts:
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<P
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></P
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><UL
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><LI
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><P
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>The <I
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CLASS="EMPHASIS"
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>scratch space</I
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>, which is where arguments and return values go, and which may be scrambled by any function call, and</P
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></LI
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><LI
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><P
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>The <I
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CLASS="EMPHASIS"
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>local area</I
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>, which all functions must restore to their initial state once finished.</P
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></LI
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></UL
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>
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</P
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></LI
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><LI
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><P
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>Assign to each procedure a <I
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CLASS="EMPHASIS"
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>frame size</I
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> S, which is a maximum size on the amount of the local area the procedure can use. The procedure's variables will sit in the first S bytes of the local area.</P
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></LI
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><LI
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><P
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>Upon entering the procedure, push the first S bytes of the local area onto the stack; upon exit, pop hose S bytes back on top of the local area.</P
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></LI
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><LI
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><P
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>While the procedure is running, only touch the local area and the scratch space.</P
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></LI
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></UL
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><P
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>This meets our design criteria neatly:</P
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><P
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></P
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><UL
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><LI
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><P
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>It's as intuitive as such a system will get. You have to call <TT
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CLASS="LITERAL"
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>init'stack</TT
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> at the beginning, and you need to ensure that <TT
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CLASS="LITERAL"
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>save'stack</TT
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> and <TT
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CLASS="LITERAL"
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>restore'stack</TT
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> are called right. The procedure's program text can pretend that it's just referring to its own variables, just like with the old style. If a procedure doesn't call <I
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CLASS="EMPHASIS"
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>anyone</I
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>, then it can just do all its work in the scratch space.</P
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></LI
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><LI
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><P
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>It's efficient; the inside of the procedure is likely to be faster and smaller than its FORTRAN-style counterpart, because all variable references are on the Zero Page.</P
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></LI
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><LI
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><P
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>Both arguments and return values can be as large as the scratch space. It's not infinite, but it's probably good enough.</P
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></LI
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><LI
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><P
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>Tail call elimination is possible; just restore the stack before making the JMP to the tail call target.</P
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></LI
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></UL
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><P
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> The necessary support code is pretty straightforward. The stack
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modification routines take the size of the frame in the
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accumulator, and while saving the local area, it copies over the
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corresponding values from the scratch space. (This is because
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most functions will be wanting to keep their arguments around
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across calls.)
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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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>.scope
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; Stack routines
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.data zp
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.space _sp $02
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.space _counter $01
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.space fun'args $10
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.space fun'vars $40
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.text
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init'stack:
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lda #$00
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sta _sp
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lda #$A0
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sta _sp+1
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rts
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save'stack:
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sta _counter
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sec
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lda _sp
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sbc _counter
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sta _sp
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lda _sp+1
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sbc #$00
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sta _sp+1
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ldy #$00
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* lda fun'vars, y
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sta (_sp), y
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lda fun'args, y
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sta fun'vars, y
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iny
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dec _counter
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bne -
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rts
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restore'stack:
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pha
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sta _counter
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ldy #$00
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* lda (_sp), y
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sta fun'vars, y
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iny
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dec _counter
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bne -
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pla
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clc
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adc _sp
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sta _sp
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lda _sp+1
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adc #$00
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sta _sp+1
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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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></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="c900.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="x967.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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>Call Stacks</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="c900.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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|
>Example: Fibonnacci Numbers</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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>
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