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https://github.com/michaelcmartin/Ophis.git
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193 lines
4.2 KiB
HTML
193 lines
4.2 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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>Comparison with the other indexed forms</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="Pointers and Indirection"
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HREF="c748.html"><LINK
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REL="PREVIOUS"
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TITLE="What about Indexed Indirect?"
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HREF="x798.html"><LINK
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REL="NEXT"
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TITLE="Conclusion"
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HREF="x813.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="x798.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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>Pointers and Indirection</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="x813.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="AEN805"
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>Comparison with the other indexed forms</A
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></H1
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><P
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> Pointers are slow. It sounds odd saying this, when C is the
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fastest language around on modern machines precisely because of
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its powerful and extensive use of pointers. However, modern
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architectures are designed to be optimized for C-style code (as an
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example, the x86 architecture allows statements like <TT
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CLASS="LITERAL"
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>mov
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eax, [bs+bx+4*di]</TT
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> as a single instruction), while the
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6502 is not. An (Indirect, Y) operation can take up to 6 cycles
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to complete just on its own, while the preparation of that command
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costs additional time <I
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CLASS="EMPHASIS"
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>and</I
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> scribbles over a
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bunch of registers, meaning memory operations to save the values
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and yet more time spent. The simple code given at the beginning
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of this essay—loading <TT
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CLASS="LITERAL"
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>*b</TT
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> into the
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accumulator—takes 7 cycles, not counting the 6 it takes to
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load b with the appropriate value to begin with. If b is known to
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contain a specific value, we can write a single Absolute mode
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instruction to load its value, which takes only 4 cycles and also
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preserves the value in the Y register. Clearly, Absolute mode
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should be used whenever possible.
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</P
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><P
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> One might be tempted to use self-modifying code to solve this
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problem. This actually doesn't pay off near enough for the hassle
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it generates; for self-modifying code, the address must be
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generated, then stored in the instruction, and then the data must
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be loaded. Cost: 16 cycles for 2 immediate loads, 2 absolute
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stores, and 1 absolute load. For the straight pointer
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dereference, we generate the address, store it in the pointer,
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clear the index, then dereference that. Cost: 17 cycles for 3
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immediate loads, 2 zero page stores, and 1 indexed indirect load.
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Furthermore, unlike in the self-modifying case, loops where simple
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arithmetic is being continuously performed only require repeating
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the final load instruction, which allows for much greater time
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savings over an equivalent self-modifying loop.
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</P
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><P
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> (This point is also completely moot for NES programmers or anyone
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else whose programs are sitting in ROM, because programs stored on
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a ROM cannot modify themselves.)
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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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><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="x798.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="x813.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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>What about Indexed Indirect?</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="c748.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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>Conclusion</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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> |