Strip comments and inline ids onto h*
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James Gregory
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11-08.html
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11-08.html
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@ -10,16 +10,9 @@
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<meta name="vspubdate" content="07/01/97" />
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<meta name="vscategory" content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development" />
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<title>Michael Abrash's Graphics Programming Black Book Special Edition: Pushing the 286 and 386</title><!-- HEADER -->
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<!-- Empty Reference Subhead -->
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<!--ISBN=1576101746//-->
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<!--TITLE=Michael Abrash's Graphics Programming Black Book Special Edition//-->
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<!--AUTHOR=Michael Abrash//-->
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<!--PUBLISHER=The Coriolis Group, Inc.//-->
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<title>Michael Abrash's Graphics Programming Black Book Special Edition: Pushing the 286 and 386</title>
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<meta name="chapter" content="11" />
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<meta name="pages" content="226-231" />
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<!--UNASSIGNED1//-->
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<!--UNASSIGNED2//-->
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</head>
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<body>
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@ -41,11 +34,10 @@
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<p>Obviously, the segment:offset that <b>IRET</b> expects to find on the stack above the pushed flags isn’t present when the stack is set up for <b>POPF</b>, so we’ll have to adjust the stack a bit before we can substitute <b>IRET</b> for <b>POPF</b>. What we’ll have to do is push the segment:offset of the instruction after our workaround code onto the stack right above the pushed flags. <b>IRET</b> will then branch to that address and pop the flags, ending up at the instruction after the workaround code with the flags popped. That’s just the result that would have occurred had we executed <b>POPF</b>—WITH the bonus that no interrupts can accidentally occur when the Interrupt flag is 0 both before and after the pop.</p>
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<p><a id="Fig4"><!-- </A><A HREF="javascript:displayWindow('images/11-04.jpg',412,383 )"> --><img src="images/11-04.jpg" /><br />
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<!-- </A>
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<BR><A HREF="javascript:displayWindow('images/11-04.jpg',412,383)"> --><b>Figure 11.4</b></a> <i>The operation of POPF.</i></p>
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<p><a id="Fig4"><img src="images/11-04.jpg" /><br />
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<b>Figure 11.4</b></a> <i>The operation of POPF.</i></p>
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<p>How can we push the segment:offset of the next instruction? Well, finding the offset of the next instruction by performing a near call to that instruction is a tried-and-true trick. We can do something similar here, but in this case we need a far call, since <b>IRET</b> requires both a segment and an offset. We’ll also branch backward so that the address pushed on the stack will point to the instruction we want to continue with. The code works out like this:</p><!-- CODE //-->
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<p>How can we push the segment:offset of the next instruction? Well, finding the offset of the next instruction by performing a near call to that instruction is a tried-and-true trick. We can do something similar here, but in this case we need a far call, since <b>IRET</b> requires both a segment and an offset. We’ll also branch backward so that the address pushed on the stack will point to the instruction we want to continue with. The code works out like this:</p>
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<pre>
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jmpshort popfskip
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popfiret:
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@ -63,15 +55,14 @@ popfskip:
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; the word that was on top of the stack when JMP SHORT POPFSKIP
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; was reached has been popped into the FLAGS register, just as
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; if a POPF instruction had been executed.
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</pre><!-- END CODE //-->
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</pre>
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<p><a id="Fig5"><!-- </A><A HREF="javascript:displayWindow('images/11-05.jpg',410,520 )"> --><img src="images/11-05.jpg" /><br />
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<!-- </A>
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<BR><A HREF="javascript:displayWindow('images/11-05.jpg',410,520)"> --><b>Figure 11.5</b></a> <i>The operation of IRET.</i></p>
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<p><a id="Fig5"><img src="images/11-05.jpg" /><br />
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<b>Figure 11.5</b></a> <i>The operation of IRET.</i></p>
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<p>The operation of this code is illustrated in Figure 11.6.</p>
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<p>The <b>POPF</b> workaround can best be implemented as a macro; we can also emulate a far call by pushing CS and performing a near call, thereby shrinking the workaround code by 1 byte:</p><!-- CODE //-->
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<p>The <b>POPF</b> workaround can best be implemented as a macro; we can also emulate a far call by pushing CS and performing a near call, thereby shrinking the workaround code by 1 byte:</p>
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<pre>
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EMULATE_POPF macro
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local popfskip, popfiret
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@ -82,9 +73,9 @@ popfskip:
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push cs
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call popfiret
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endm
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</pre><!-- END CODE //-->
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</pre>
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<p>By the way, the flags can be popped much more quickly if you’re willing to alter a register in the process. For example, the following macro emulates <b>POPF</b> with just one branch, but wipes out AX:</p><!-- CODE SNIP //-->
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<p>By the way, the flags can be popped much more quickly if you’re willing to alter a register in the process. For example, the following macro emulates <b>POPF</b> with just one branch, but wipes out AX:</p>
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<pre>
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EMULATE_POPF_TRASH_AX macro
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push cs
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@ -92,9 +83,9 @@ EMULATE_POPF_TRASH_AX macro
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push ax
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iret
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endm
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</pre><!-- END CODE SNIP //-->
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</pre>
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<p>It’s not a perfect substitute for <b>POPF</b>, since <b>POPF</b> doesn’t alter any registers, but it’s faster and shorter than <b>EMULATE_POPF</b> when you can spare the register. If you’re using 286-specific instructions, you can use which is shorter still, alters no registers, and branches just once. (Of course, this version of <b>EMULATE_POPF</b> won’t work on an 8088.)</p><!-- CODE SNIP //-->
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<p>It’s not a perfect substitute for <b>POPF</b>, since <b>POPF</b> doesn’t alter any registers, but it’s faster and shorter than <b>EMULATE_POPF</b> when you can spare the register. If you’re using 286-specific instructions, you can use which is shorter still, alters no registers, and branches just once. (Of course, this version of <b>EMULATE_POPF</b> won’t work on an 8088.)</p>
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<pre>
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.286
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:
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@ -103,11 +94,10 @@ EMULATE_POPFmacro
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pushoffset $+4
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iret
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endm
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</pre><!-- END CODE SNIP //-->
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</pre>
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<p><a id="Fig6"><!-- </A><A HREF="javascript:displayWindow('images/11-06.jpg',409,447 )"> --><img src="images/11-06.jpg" /><br />
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<!-- </A>
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<BR><A HREF="javascript:displayWindow('images/11-06.jpg',409,447)"> --><b>Figure 11.6</b></a> <i>Workaround code for the POPF bug.</i></p>
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<p><a id="Fig6"><img src="images/11-06.jpg" /><br />
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<b>Figure 11.6</b></a> <i>Workaround code for the POPF bug.</i></p>
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<p>The standard version of <b>EMULATE_POPF</b> is 6 bytes longer than <b>POPF</b> and much slower, as you’d expect given that it involves three branches. Anyone in his/her right mind would prefer <b>POPF</b> to a larger, slower, three-branch macro—given a choice. In noncode, however, there’s no choice here; the safer—if slower—approach is the best. (Having people associate your programs with crashed computers is <i>not</i> a desirable situation, no matter how unfair the circumstances under which it occurs.)</p>
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<div align="center">
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Graphics Programming Black Book © 2001 Michael Abrash
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</div><!-- all of the reference materials (books) have the footer and subfoot reveresed -->
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<!-- reference_subfoot = footer -->
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<!-- reference_footer = subfoot -->
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<!-- BEGIN SUB FOOTER -->
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<!-- END FOOTER -->
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</div>
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</body>
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</html>
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