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James Gregory 2013-12-30 13:57:02 +11:00
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@ -24,7 +24,7 @@
<!--CHAPTER=13//-->
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@ -53,7 +53,7 @@ looptop:
jnz looptop
</PRE>
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<H3><A NAME="Heading5"></A><FONT COLOR="#000077">Pushing and Popping Memory</FONT></H3>
<H3><A NAME="Heading5"></A>Pushing and Popping Memory</H3>
<P>Pushing or popping a memory location, as in <B>PUSH WORD PTR [BX]</B> or <B>POP [MemVar]</B>, is a compact, easy way to get a value onto or off of the stack, especially when pushing parameters for calling a C-compatible function. However, on a 486, these are unattractive instructions from a performance perspective. Pushing a memory location takes four cycles; by contrast, loading a memory location into a register takes only one cycle, and pushing a register takes just 1 more cycle, for a total of two cycles. Therefore,</P>
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<PRE>
@ -74,7 +74,7 @@ push word ptr [bx]
<P>Why is it that such a convenient operation as pushing or popping memory is so slow? The rule on the 486 is that simple operations, which can be executed in a single cycle by the 486&rsquo;s RISC core, are fast; whereas complex operations, which must be carried out in microcode just as they were on the 386, are almost all relatively slow. Slow, complex operations include all the string instructions except <B>REP MOVS,</B> as well as <B>XLAT, LOOP,</B> and, of course, <B>PUSH <I>mem</I></B> and <B>POP <I>mem.</I></B></P>
<TABLE WIDTH="100%"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%"><IMG SRC="images/i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>Whenever possible, try to use the 486&rsquo;s 1-cycle instructions, including <B>MOV, ADD, SUB, CMP, ADC, SBB, XOR, AND, OR, TEST, LEA</B>, and <B>PUSH reg</B> and <B>POP reg</B>. These instructions have an added benefit in that it&rsquo;s often possible to rearrange them for maximum pipeline efficiency, as is the case with Terje&rsquo;s optimization described earlier in this chapter.</I></SMALL>
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<H3><A NAME="Heading6"></A><FONT COLOR="#000077">Optimal 1-Bit Shifts and Rotates</FONT></H3>
<H3><A NAME="Heading6"></A>Optimal 1-Bit Shifts and Rotates</H3>
<P>On a 486, the n-bit forms of the shift and rotate instructions&mdash;as in <B>ROR AX,2</B> and <B>SHL BX,9</B>&mdash;are 2-cycle instructions, but the 1-bit forms&mdash;as in <B>ROR AX,1</B> and <B>SHL BX,1&mdash;</B>are <I>3-cycle</I> instructions. Go figure.</P>
<P>Assemblers default to the 1-bit instruction for 1-bit shifts and rotates. That&rsquo;s not unreasonable since the 1-bit form is a byte shorter and is just as fast as the n-bit forms on a 386 and faster on a 286, and the n-bit form doesn&rsquo;t even exist on an 8088. In a really critical loop, however, it might be worth hand-assembling the n-bit form of a single-bit shift or rotate in order to save that cycle. The easiest way to do this is to assemble a 2-bit form of the desired instruction, as in <B>SHL AX,2,</B> then look at the hex codes that the assembler generates and use <B>DB</B> to insert them in your program code, with the value two replaced with the value one. For example, you could determine that <B>SHL AX,2</B> assembles to the bytes 0C1H 0E0H 002H, either by looking at the disassembly in a debugger or by having the assembler generate a listing file. You could then insert the n-bit version of <B>SHL AX,1</B> in your code as follows:</P>
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@ -98,7 +98,7 @@ mov dx,ax
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<font face="Verdana,sans-serif" size="1">Graphics Programming Black Book &copy; 2001 Michael Abrash</font>
Graphics Programming Black Book &copy; 2001 Michael Abrash
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