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James Gregory 2013-12-30 12:25:34 +11:00
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@ -72,7 +72,7 @@ push word ptr [bx]
</P>
<P>Likewise, popping a memory location takes six cycles, but popping a register and writing it to memory takes only two cycles combined. The <I>i486 Microprocessor Programmer&#146;s Reference Manual</I> lists a 4-cycle execution time for popping a register, but pay that no mind; popping a register takes only 1 cycle.</P>
<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&#146;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/13-02i.jpg"><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%"><SMALL><I>Whenever possible, try to use the 486&#146;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&#146;s often possible to rearrange them for maximum pipeline efficiency, as is the case with Terje&#146;s optimization described earlier in this chapter.</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>Whenever possible, try to use the 486&#146;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&#146;s often possible to rearrange them for maximum pipeline efficiency, as is the case with Terje&#146;s optimization described earlier in this chapter.</I></SMALL>
</TABLE>
<H3><A NAME="Heading6"></A><FONT COLOR="#000077">Optimal 1-Bit Shifts and Rotates</FONT></H3>
<P>On a 486, the n-bit forms of the shift and rotate instructions&#151;as in <B>ROR AX,2</B> and <B>SHL BX,9</B>&#151;are 2-cycle instructions, but the 1-bit forms&#151;as in <B>ROR AX,1</B> and <B>SHL BX,1&#151;</B>are <I>3-cycle</I> instructions. Go figure.</P>