Deduped images

07-05.html

@@ -79,11 +79,11 @@ BIT_PATTERN=BIT_PATTERN SHL 1
 </PRE>
 <!-- END CODE SNIP //-->
 <TABLE WIDTH="100%">
-<TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/07-06i.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>
+<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>
 </TABLE>
 <H4 ALIGN="LEFT"><A NAME="Heading9"></A><FONT COLOR="#000077">NOT Flips Bits&#151;Not Flags</FONT></H4>
 <P>The <B>NOT</B> instruction flips all the bits in the operand, from 0 to 1 or from 1 to 0. That&#146;s as simple as could be, but <B>NOT</B> nonetheless has a minor but interesting talent: It doesn&#146;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&#146;t considered to be an arithmetic or logical instruction at all; rather, it&#146;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&#146;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>
-<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/07-07i.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&#146;t.</I></SMALL>
+<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&#146;t.</I></SMALL>
 </TABLE>
 <P>The x86 instruction set offers many ways to accomplish almost any task. Understanding the subtle distinctions between the instructions&#151;whether and which flags are set, for example&#151;can be critical when you&#146;re trying to optimize a code sequence and you&#146;re running out of registers, or when you&#146;re trying to minimize branching.
 </P>
@@ -121,7 +121,7 @@ LOOP_TOP:
 <!-- END CODE //-->
 <P>It&#146;s not that the Listing 7.6 approach is necessarily better or worse; that depends on the processor and the situation. The Listing 7.6 approach is <I>different,</I> and if you understand the differences, you&#146;ll be able to choose the best approach for whatever code you happen to write. (<B>DEC</B> has the same property of preserving the Carry flag, by the way.)</P>
 <P>There are a couple of interesting aspects to the last example. First, note that <B>LOOP</B> doesn&#146;t affect any flags at all; this allows the Carry flag to remain unchanged from one addition to the next. Not altering the arithmetic flags is a common characteristic of program control instructions (as opposed to arithmetic and logical instructions like <B>SUB</B> and <B>AND,</B> which do alter the flags).</P>
-<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/07-08i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>The rule is not that the arithmetic flags change whenever the CPU performs a calculation; rather, the flags change whenever you execute an arithmetic, logical, or flag control (such as <B>CLC</B> to clear the Carry flag) instruction.</I></SMALL>
+<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>The rule is not that the arithmetic flags change whenever the CPU performs a calculation; rather, the flags change whenever you execute an arithmetic, logical, or flag control (such as <B>CLC</B> to clear the Carry flag) instruction.</I></SMALL>
 </TABLE>
 <P>Not only do <B>LOOP</B> and <B>JCXZ</B> not alter the flags, but <B>REP MOVS</B>, which counts down CX to 0, doesn&#146;t affect the flags either.</P>
 <P>The other interesting point about the last example is the use of <B>LAHF</B> and <B>SAHF,</B> which transfer the low byte of the FLAGS register to and from AH, respectively. These instructions were created to help provide compatibility with the 8080&#146;s (that&#146;s <I>8080</I>, not <I>8088</I>) <B>PUSH</B> <B>PSW</B> and <B>POP PSW</B> instructions, but turn out to be compact (one byte) instructions for saving and restoring the arithmetic flags. A word of caution, however: <B>SAHF</B> restores the Carry, Zero, Sign, Auxiliary Carry, and Parity flags&#151;but <I>not</I> the Overflow flag, which resides in the high byte of the FLAGS register. Also, be aware that <B>LAHF</B> and <B>SAHF</B> provide a fast way to preserve the flags on an 8088 but are relatively slow instructions on the 486 and Pentium.</P>