Remove colour attributes from body and strip most of the font tags out
This commit is contained in:
James Gregory
parent
7e957c0bb4
commit
c1f88ddb41
362 changed files with 1632 additions and 1706 deletions
14
06-02.html
14
06-02.html
|
|
@ -24,7 +24,7 @@
|
|||
<!--CHAPTER=06//-->
|
||||
<!--PAGES=129-133//-->
|
||||
<!--UNASSIGNED1//-->
|
||||
<!--UNASSIGNED2//--></HEAD><BODY LINK=#0000FF ALINK=#000099 VLINK=#0000FF BGCOLOR=#FFFFFF>
|
||||
<!--UNASSIGNED2//--></HEAD><body>
|
||||
|
||||
<CENTER>
|
||||
<TABLE BORDER>
|
||||
|
|
@ -49,7 +49,7 @@ LoopTop:
|
|||
<P>Here, <B>MOV AL,[BX]</B> is two cycles faster than <B>MOV AL,[BX+SI]</B>.</P>
|
||||
<P>On a 286 or 386, however, the balance shifts. <B>MOV AL,[BX+SI]</B> takes no longer than <B>MOV AL,[BX]</B> on these processors because effective address calculations generally take no extra time at all. (According to the MASM manual, one extra clock is required if three memory addressing components, as in <B>MOV AL,[BX+SI+1]</B>, are used. I have not been able to confirm this from Intel publications, but then I haven’t looked all that hard.) If you’re optimizing for the 286 or 386, then, you can take advantage of the processor’s ability to perform arithmetic as part of memory address calculations without taking a performance hit.</P>
|
||||
<P>The 486 is an odd case, in which the use of an index register or the use of a base register that’s the destination of the previous instruction may slow things down, so it is generally but not always better to perform the addition outside the loop on the 486. All memory addressing calculations are free on the Pentium, however. I’ll discuss 486 performance issues in Chapters 12 and 13, and the Pentium in Chapters 19 through 21.</P>
|
||||
<H3><A NAME="Heading4"></A><FONT COLOR="#000077">Math via Memory Addressing</FONT></H3>
|
||||
<H3><A NAME="Heading4"></A>Math via Memory Addressing</H3>
|
||||
<P>You’re probably not particularly wowed to hear that you can use addressing modes to perform memory addressing arithmetic that would otherwise have to be performed with separate arithmetic instructions. You may, however, be a tad more interested to hear that you can also use addressing modes to perform arithmetic that has nothing to do with memory addressing, and with a couple of advantages over arithmetic instructions, at that.
|
||||
</P>
|
||||
<P>How?</P>
|
||||
|
|
@ -87,13 +87,13 @@ lea di,[si+2]
|
|||
<!-- END CODE SNIP //-->
|
||||
<P>Mind you, the only components <B>LEA</B> can add are BX or BP, SI or DI, and a constant displacement, so it’s not going to replace <B>ADD</B> most of the time. Also, <B>LEA</B> is considerably slower than <B>ADD</B> on an 8088, although it is just as fast as <B>ADD</B> on a 286 or 386 when fewer than three memory addressing components are used. <B>LEA</B> is 1 cycle slower than <B>ADD</B> on a 486 if the sum of two registers is used to point to memory, but no slower than <B>ADD</B> on a Pentium. On both a 486 and Pentium, <B>LEA</B> can also be slowed down by addressing interlocks.</P>
|
||||
<P><A NAME="Fig1"><!-- </A><A HREF="javascript:displayWindow('images/06-01.jpg',418,276 )"> --><IMG SRC="images/06-01.jpg"><BR><!-- </A>
|
||||
<BR><A HREF="javascript:displayWindow('images/06-01.jpg',418,276)"> --><FONT COLOR="#000077"><B>Figure 6.1</B></FONT></A> <I>Operation of ADD Reg,Reg vs. LEA Reg,{Addr}.</I>
|
||||
<BR><A HREF="javascript:displayWindow('images/06-01.jpg',418,276)"> --><B>Figure 6.1</B></A> <I>Operation of ADD Reg,Reg vs. LEA Reg,{Addr}.</I>
|
||||
</P>
|
||||
<H4 ALIGN="LEFT"><A NAME="Heading5"></A><FONT COLOR="#000077">The Wonders of LEA on the 386</FONT></H4>
|
||||
<H4 ALIGN="LEFT"><A NAME="Heading5"></A>The Wonders of LEA on the 386</H4>
|
||||
<P><B>LEA</B> really comes into its own as a “super-ADD” instruction on the 386, 486, and Pentium, where it can take advantage of the enhanced memory addressing modes of those processors. (The 486 and Pentium offer the same modes as the 386, so I’ll refer only to the 386 from now on.) The 386 can do two very interesting things: It can use <I>any</I> 32-bit register (EAX, EBX, and so on) as the memory addressing base register and/or the memory addressing index register, and it can multiply any 32-bit register used as an index by two, four, or eight in the process of calculating a memory address, as shown in Figure 6.2. Let’s see what that’s good for.</P>
|
||||
<P>Well, the obvious advantage is that any two 32-bit registers, or any 32-bit register and any constant, or any two 32-bit registers and any constant, can be added together, with the result stored in any register. This makes the 32-bit <B>LEA</B> much more generally useful than the standard 16-bit <B>LEA</B> in the role of an <B>ADD</B> with an independent destination.</P>
|
||||
<P><A NAME="Fig2"><!-- </A><A HREF="javascript:displayWindow('images/06-02.jpg',421,197 )"> --><IMG SRC="images/06-02.jpg"><BR><!-- </A>
|
||||
<BR><A HREF="javascript:displayWindow('images/06-02.jpg',421,197)"> --><FONT COLOR="#000077"><B>Figure 6.2</B></FONT></A> <I>Operation of the 32-bit LEA reg,[Addr].</I>
|
||||
<BR><A HREF="javascript:displayWindow('images/06-02.jpg',421,197)"> --><B>Figure 6.2</B></A> <I>Operation of the 32-bit LEA reg,[Addr].</I>
|
||||
</P>
|
||||
<P>But what else can <B>LEA</B> do on a 386, besides add?</P>
|
||||
<P>It can multiply any register used as an index. <B>LEA</B> can multiply only by the power-of-two values 2, 4, or 8, but that’s useful more often than you might imagine, especially when dealing with pointers into tables. Besides, multiplying by 2, 4, or 8 amounts to a left shift of 1, 2, or 3 bits, so we can now add up to two 32-bit registers and a constant, <I>and</I> shift (or multiply) one of the registers to some extent—all with a single instruction. For example,</P>
|
||||
|
|
@ -114,7 +114,7 @@ add edi,offset TableBase
|
|||
<!-- END CODE SNIP //-->
|
||||
<P>when pointing to an entry in a doubly indexed table.
|
||||
</P>
|
||||
<H3><A NAME="Heading6"></A><FONT COLOR="#000077">Multiplication with LEA Using Non-Powers of Two</FONT></H3>
|
||||
<H3><A NAME="Heading6"></A>Multiplication with LEA Using Non-Powers of Two</H3>
|
||||
<P>Are you impressed yet with all that <B>LEA</B> can do on the 386? Believe it or not, one more feature still awaits us. <B>LEA</B> can actually perform a fast multiply of a 32-bit register by some values <I>other</I> than powers of two. You see, the same 32-bit register can be both base and index on the 386, and can be scaled as the index while being used unchanged as the base. That means that you can, for example, multiply EBX by 5 with:</P>
|
||||
<!-- CODE SNIP //-->
|
||||
<PRE>
|
||||
|
|
@ -147,7 +147,7 @@ add ebx,edx
|
|||
|
||||
<hr width="90%" size="1" noshade>
|
||||
<div align="center">
|
||||
<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book © 2001 Michael Abrash</font>
|
||||
Graphics Programming Black Book © 2001 Michael Abrash
|
||||
</div>
|
||||
<!-- all of the reference materials (books) have the footer and subfoot reveresed -->
|
||||
<!-- reference_subfoot = footer -->
|
||||
|
|
|
|||
Loading…
Reference in a new issue