mirror of
https://github.com/michaelcmartin/Ophis.git
synced 2024-11-05 17:07:33 +00:00
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">
|
|
<HTML
|
|
><HEAD
|
|
><TITLE
|
|
>Comparison with the other indexed forms</TITLE
|
|
><META
|
|
NAME="GENERATOR"
|
|
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
|
|
REL="HOME"
|
|
TITLE="Programming with Ophis"
|
|
HREF="book1.html"><LINK
|
|
REL="UP"
|
|
TITLE="Pointers and Indirection"
|
|
HREF="c885.html"><LINK
|
|
REL="PREVIOUS"
|
|
TITLE="What about Indexed Indirect?"
|
|
HREF="x935.html"><LINK
|
|
REL="NEXT"
|
|
TITLE="Conclusion"
|
|
HREF="x950.html"></HEAD
|
|
><BODY
|
|
CLASS="SECTION"
|
|
BGCOLOR="#FFFFFF"
|
|
TEXT="#000000"
|
|
LINK="#0000FF"
|
|
VLINK="#840084"
|
|
ALINK="#0000FF"
|
|
><DIV
|
|
CLASS="NAVHEADER"
|
|
><TABLE
|
|
SUMMARY="Header navigation table"
|
|
WIDTH="100%"
|
|
BORDER="0"
|
|
CELLPADDING="0"
|
|
CELLSPACING="0"
|
|
><TR
|
|
><TH
|
|
COLSPAN="3"
|
|
ALIGN="center"
|
|
>Programming with Ophis</TH
|
|
></TR
|
|
><TR
|
|
><TD
|
|
WIDTH="10%"
|
|
ALIGN="left"
|
|
VALIGN="bottom"
|
|
><A
|
|
HREF="x935.html"
|
|
ACCESSKEY="P"
|
|
><<< Previous</A
|
|
></TD
|
|
><TD
|
|
WIDTH="80%"
|
|
ALIGN="center"
|
|
VALIGN="bottom"
|
|
>Pointers and Indirection</TD
|
|
><TD
|
|
WIDTH="10%"
|
|
ALIGN="right"
|
|
VALIGN="bottom"
|
|
><A
|
|
HREF="x950.html"
|
|
ACCESSKEY="N"
|
|
>Next >>></A
|
|
></TD
|
|
></TR
|
|
></TABLE
|
|
><HR
|
|
ALIGN="LEFT"
|
|
WIDTH="100%"></DIV
|
|
><DIV
|
|
CLASS="SECTION"
|
|
><H1
|
|
CLASS="SECTION"
|
|
><A
|
|
NAME="AEN942"
|
|
>Comparison with the other indexed forms</A
|
|
></H1
|
|
><P
|
|
> Pointers are slow. It sounds odd saying this, when C is the
|
|
fastest language around on modern machines precisely because of
|
|
its powerful and extensive use of pointers. However, modern
|
|
architectures are designed to be optimized for C-style code (as an
|
|
example, the x86 architecture allows statements like <TT
|
|
CLASS="LITERAL"
|
|
>mov
|
|
eax, [bs+bx+4*di]</TT
|
|
> as a single instruction), while the
|
|
6502 is not. An (Indirect, Y) operation can take up to 6 cycles
|
|
to complete just on its own, while the preparation of that command
|
|
costs additional time <I
|
|
CLASS="EMPHASIS"
|
|
>and</I
|
|
> scribbles over a
|
|
bunch of registers, meaning memory operations to save the values
|
|
and yet more time spent. The simple code given at the beginning
|
|
of this essay—loading <TT
|
|
CLASS="LITERAL"
|
|
>*b</TT
|
|
> into the
|
|
accumulator—takes 7 cycles, not counting the 6 it takes to
|
|
load b with the appropriate value to begin with. If b is known to
|
|
contain a specific value, we can write a single Absolute mode
|
|
instruction to load its value, which takes only 4 cycles and also
|
|
preserves the value in the Y register. Clearly, Absolute mode
|
|
should be used whenever possible.
|
|
</P
|
|
><P
|
|
> One might be tempted to use self-modifying code to solve this
|
|
problem. This actually doesn't pay off near enough for the hassle
|
|
it generates; for self-modifying code, the address must be
|
|
generated, then stored in the instruction, and then the data must
|
|
be loaded. Cost: 16 cycles for 2 immediate loads, 2 absolute
|
|
stores, and 1 absolute load. For the straight pointer
|
|
dereference, we generate the address, store it in the pointer,
|
|
clear the index, then dereference that. Cost: 17 cycles for 3
|
|
immediate loads, 2 zero page stores, and 1 indexed indirect load.
|
|
Furthermore, unlike in the self-modifying case, loops where simple
|
|
arithmetic is being continuously performed only require repeating
|
|
the final load instruction, which allows for much greater time
|
|
savings over an equivalent self-modifying loop.
|
|
</P
|
|
><P
|
|
> (This point is also completely moot for NES programmers or anyone
|
|
else whose programs are sitting in ROM, because programs stored on
|
|
a ROM cannot modify themselves.)
|
|
</P
|
|
></DIV
|
|
><DIV
|
|
CLASS="NAVFOOTER"
|
|
><HR
|
|
ALIGN="LEFT"
|
|
WIDTH="100%"><TABLE
|
|
SUMMARY="Footer navigation table"
|
|
WIDTH="100%"
|
|
BORDER="0"
|
|
CELLPADDING="0"
|
|
CELLSPACING="0"
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="x935.html"
|
|
ACCESSKEY="P"
|
|
><<< Previous</A
|
|
></TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="book1.html"
|
|
ACCESSKEY="H"
|
|
>Home</A
|
|
></TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="x950.html"
|
|
ACCESSKEY="N"
|
|
>Next >>></A
|
|
></TD
|
|
></TR
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
>What about Indexed Indirect?</TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="c885.html"
|
|
ACCESSKEY="U"
|
|
>Up</A
|
|
></TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
>Conclusion</TD
|
|
></TR
|
|
></TABLE
|
|
></DIV
|
|
></BODY
|
|
></HTML
|
|
> |