Remove colour attributes from body and strip most of the font tags out
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James Gregory
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7e957c0bb4
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362 changed files with 1632 additions and 1706 deletions
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07-05.html
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07-05.html
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@ -24,7 +24,7 @@
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<!--CHAPTER=07//-->
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<!--PAGES=145-148//-->
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<!--UNASSIGNED1//-->
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<!--UNASSIGNED2//--></HEAD><BODY LINK=#0000FF ALINK=#000099 VLINK=#0000FF BGCOLOR=#FFFFFF>
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<!--UNASSIGNED2//--></HEAD><body>
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<CENTER>
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<TABLE BORDER>
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@ -36,7 +36,7 @@
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</TABLE>
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</CENTER>
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<P><BR></P>
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<H4 ALIGN="LEFT"><A NAME="Heading8"></A><FONT COLOR="#000077">Rotating and Shifting with Tables</FONT></H4>
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<H4 ALIGN="LEFT"><A NAME="Heading8"></A>Rotating and Shifting with Tables</H4>
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<P>As another example of local optimization, consider the matter of rotating or shifting a mask into position. First, let’s look at the simple task of setting bit N of AX to 1.
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</P>
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<P>The obvious way to do this is to place N in CL, rotate the bit into position, and OR it with AX, as follows:</P>
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@ -81,13 +81,13 @@ BIT_PATTERN=BIT_PATTERN SHL 1
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<TABLE WIDTH="100%">
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<TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>Besides illustrating the advantages of local optimization, this example also shows that it generally pays to precalculate results; this is often done at or before assembly time, but precalculated tables can also be built at run time. This is merely one aspect of a fundamental optimization rule: Move as much work as possible out of your critical code by whatever means necessary.</I></SMALL>
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</TABLE>
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<H4 ALIGN="LEFT"><A NAME="Heading9"></A><FONT COLOR="#000077">NOT Flips Bits—Not Flags</FONT></H4>
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<H4 ALIGN="LEFT"><A NAME="Heading9"></A>NOT Flips Bits—Not Flags</H4>
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<P>The <B>NOT</B> instruction flips all the bits in the operand, from 0 to 1 or from 1 to 0. That’s as simple as could be, but <B>NOT</B> nonetheless has a minor but interesting talent: It doesn’t affect the flags. That can be irritating; I once spent a good hour tracking down a bug caused by my unconscious assumption that <B>NOT</B> does set the flags. After all, every other arithmetic and logical instruction sets the flags; why not <B>NOT</B>? Probably because <B>NOT</B> isn’t considered to be an arithmetic or logical instruction at all; rather, it’s a data manipulation instruction, like <B>MOV</B> and the various rotates. (These are <B>RCR, RCL, ROR,</B> and <B>ROL,</B> which affect only the Carry and Overflow flags.) NOT is often used for tasks, such as flipping masks, where there’s no reason to test the state of the result, and in that context it can be handy to keep the flags unmodified for later testing.</P>
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<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>Besides, if you want to <B>NOT</B> an operand and set the flags in the process, you can just <B>XOR</B> it with -1. Put another way, the only functional difference between <B>NOT AX</B> and <B>XOR AX,0FFFFH</B> is that <B>XOR</B> modifies the flags and <B>NOT</B> doesn’t.</I></SMALL>
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</TABLE>
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<P>The x86 instruction set offers many ways to accomplish almost any task. Understanding the subtle distinctions between the instructions—whether and which flags are set, for example—can be critical when you’re trying to optimize a code sequence and you’re running out of registers, or when you’re trying to minimize branching.
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</P>
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<H4 ALIGN="LEFT"><A NAME="Heading10"></A><FONT COLOR="#000077">Incrementing with and without Carry</FONT></H4>
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<H4 ALIGN="LEFT"><A NAME="Heading10"></A>Incrementing with and without Carry</H4>
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<P>Another case in which there are two slightly different ways to perform a task involves adding 1 to an operand. You can do this with <B>INC,</B> as in <B>INC AX,</B> or you can do it with <B>ADD,</B> as in <B>ADD AX,1.</B> What’s the difference? The obvious difference is that <B>INC</B> is usually a byte or two shorter (the exception being <B>ADD AL,1,</B> which at two bytes is the same length as <B>INC AL</B>), and is faster on some processors. Less obvious, but no less important, is that <B>ADD</B> sets the Carry flag while <B>INC</B> leaves the Carry flag untouched.</P>
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<P>Why is that important? Because it allows <B>INC</B> to function as a data pointer manipulation instruction for multi-word arithmetic. You can use <B>INC</B> to advance the pointers in code like that shown in Listing 7.5 without having to do any work to preserve the Carry status from one addition to the next.</P>
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<P><B>LISTING 7.5 L7-5.ASM</B></P>
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@ -153,7 +153,7 @@ ADC DX,0
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<hr width="90%" size="1" noshade>
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<div align="center">
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<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book © 2001 Michael Abrash</font>
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Graphics Programming Black Book © 2001 Michael Abrash
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</div>
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<!-- all of the reference materials (books) have the footer and subfoot reveresed -->
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<!-- reference_subfoot = footer -->
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