132 lines
6.5 KiB
HTML
132 lines
6.5 KiB
HTML
<!DOCTYPE html>
|
|
|
|
<html xmlns="http://www.w3.org/1999/xhtml">
|
|
<head>
|
|
<meta name="vsisbn" content="1576101746" />
|
|
<meta name="vstitle" content="Michael Abrash's Graphics Programming Black Book, Special Edition" />
|
|
<meta name="vsauthor" content="Michael Abrash" />
|
|
<meta name="vspublisher" content="The Coriolis Group" />
|
|
<meta name="vspubdate" content="07/01/97" />
|
|
<meta name="vscategory" content="Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development" />
|
|
|
|
<title>Michael Abrash's Graphics Programming Black Book Special Edition: Pushing the 286 and 386</title>
|
|
<meta name="chapter" content="11" />
|
|
<meta name="pages" content="226-231" />
|
|
</head>
|
|
|
|
<body>
|
|
<center>
|
|
<table border="1">
|
|
<tr>
|
|
<td>
|
|
<a href="11-07.html">Previous</a>
|
|
</td>
|
|
|
|
<td>
|
|
<a href="index.html">Table of Contents</a>
|
|
</td>
|
|
|
|
<td>
|
|
<a href="12-01.html">Next</a>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
</center>
|
|
|
|
<p>Well, there’s only one instruction other than <b>POPF</b> that loads the FLAGS register directly from the stack, and that’s <b>IRET</b>, which loads the FLAGS register from the stack as it branches, as shown in Figure 11.5. iret has no known bugs of the sort that plague <b>POPF</b>, so it’s certainly a candidate to replace popf in non-interruptible applications. Unfortunately, <b>IRET</b> loads the FLAGS register with the <i>third</i> word down on the stack, not the word on top of the stack, as is the case with <b>POPF</b>; the far return address that <b>IRET</b> pops into CS:IP lies between the top of the stack and the word popped into the FLAGS register.</p>
|
|
|
|
<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>
|
|
|
|
<p><a id="Fig4"><img src="images/11-04.jpg" /><br />
|
|
<b>Figure 11.4</b></a> <i>The operation of POPF.</i></p>
|
|
|
|
<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>
|
|
<pre>
|
|
jmpshort popfskip
|
|
popfiret:
|
|
iret; branches to the instruction after the
|
|
; call, popping the word below the address
|
|
; pushed by CALL into the FLAGS register
|
|
popfskip:
|
|
call far ptr popfiret
|
|
;pushes the segment:offset of the next
|
|
; instruction on the stack just above
|
|
; the flags word, setting things up so
|
|
; that IRET will branch to the next
|
|
; instruction and pop the flags
|
|
; When execution reaches the instruction following this comment,
|
|
; the word that was on top of the stack when JMP SHORT POPFSKIP
|
|
; was reached has been popped into the FLAGS register, just as
|
|
; if a POPF instruction had been executed.
|
|
</pre>
|
|
|
|
<p><a id="Fig5"><img src="images/11-05.jpg" /><br />
|
|
<b>Figure 11.5</b></a> <i>The operation of IRET.</i></p>
|
|
|
|
<p>The operation of this code is illustrated in Figure 11.6.</p>
|
|
|
|
<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>
|
|
<pre>
|
|
EMULATE_POPF macro
|
|
local popfskip, popfiret
|
|
jmp short popfskip
|
|
popfiret:
|
|
iret
|
|
popfskip:
|
|
push cs
|
|
call popfiret
|
|
endm
|
|
</pre>
|
|
|
|
<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>
|
|
<pre>
|
|
EMULATE_POPF_TRASH_AX macro
|
|
push cs
|
|
mov ax,offset $+5
|
|
push ax
|
|
iret
|
|
endm
|
|
</pre>
|
|
|
|
<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>
|
|
<pre>
|
|
.286
|
|
:
|
|
EMULATE_POPFmacro
|
|
pushcs
|
|
pushoffset $+4
|
|
iret
|
|
endm
|
|
</pre>
|
|
|
|
<p><a id="Fig6"><img src="images/11-06.jpg" /><br />
|
|
<b>Figure 11.6</b></a> <i>Workaround code for the POPF bug.</i></p>
|
|
|
|
<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>
|
|
|
|
<p>And now you know the nature of and the workaround for the <b>POPF</b> bug. Whether you ever need the workaround or not, it’s a neatly packaged example of the tremendous flexibility of the x86 instruction set.</p>
|
|
|
|
<center>
|
|
<table border="1">
|
|
<tr>
|
|
<td>
|
|
<a href="11-07.html">Previous</a>
|
|
</td>
|
|
|
|
<td>
|
|
<a href="index.html">Table of Contents</a>
|
|
</td>
|
|
|
|
<td>
|
|
<a href="12-01.html">Next</a>
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
</center>
|
|
<hr width="90%" size="1" noshade="noshade" />
|
|
|
|
<div align="center">
|
|
Graphics Programming Black Book © 2001 Michael Abrash
|
|
</div>
|
|
</body>
|
|
</html>
|