140 lines
8.1 KiB
HTML
140 lines
8.1 KiB
HTML
<HTML>
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<HEAD>
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<META name=vsisbn content="1576101746">
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<META name=vstitle content="Michael Abrash's Graphics Programming Black Book, Special Edition">
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<META name=vsauthor content="Michael Abrash">
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<META name=vspublisher content="The Coriolis Group">
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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: Zenning and the Flexible Mind</TITLE>
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<!-- 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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<!--CHAPTER=22//-->
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<!--PAGES=419-420//-->
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<!--UNASSIGNED1//-->
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<!--UNASSIGNED2//--></HEAD><body>
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<CENTER>
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<TABLE BORDER>
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<TR>
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<TD><A HREF="22-02.html">Previous</A></TD>
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<TD><A HREF="index.html">Table of Contents</A></TD>
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<TD><A HREF="23-01.html">Next</A></TD>
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</TR>
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</CENTER>
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<P><BR></P>
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<P><B>LISTING 22.5 L22-5.ASM</B></P>
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<!-- CODE //-->
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<PRE>
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ClearS proc near
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push bp ;save caller’s BP
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mov bp,sp ;point to stack frame
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cmp word ptr [bp].BufSeg,0 ;skip the fill if a null
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jne Start ; pointer is passed
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cmp word ptr [bp].BufOfs,0
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je Bye
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Start: cld ;make STOSW count up
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mov ah,byte ptr [bp].Attrib[1];load AH with attribute
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mov al,byte ptr [bp].Filler ;load AL with fill char
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les di,dword ptr [bp].BufOfs ;load ES:DI with target buffer segment:offset
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mov cx,[bp].BufSize ;load CX with buffer size
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rep stosw ;fill the buffer
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Bye:
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pop bp ;restore caller’s BP
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ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack
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ClearS endp
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</PRE>
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<!-- END CODE //-->
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<P>(We could get rid of yet another instruction by having the calling code pack both the attribute and the fill value into the same word, but that’s not part of the specification for this particular routine.)
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</P>
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<P>Another nifty instruction-rearrangement trick saves 6 more bytes. <B>ClearS</B> checks to see whether the far pointer is null (zero) at the start of the routine...then loads and uses that same far pointer later on. Let’s get that pointer into registers and keep it there; that way we can check to see whether it’s null with a single comparison, and can use it later without having to reload it from memory. This technique is shown in Listing 22.6.</P>
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<P><B>LISTING 22.6 L22-6.ASM</B></P>
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<!-- CODE //-->
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<PRE>
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ClearS proc near
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push bp ;save caller’s BP
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mov bp,sp ;point to stack frame
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les di,dword ptr [bp].BufOfs ;load ES:DI with target buffer;segment:offset
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mov ax,es ;put segment where we can test it
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or ax,di ;is it a null pointer?
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je Bye ;yes, so we’re done
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Start: cld ;make STOSW count up
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mov ah,byte ptr [bp].Attrib[1];load AH with attribute
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mov al,byte ptr [bp].Filler ;load AL with fill char
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mov cx,[bp].BufSize ;load CX with buffer size
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rep stosw ;fill the buffer
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Bye:
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pop bp ;restore caller’s BP
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ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack
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ClearS endp
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</PRE>
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<!-- END CODE //-->
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<P>Well. Now we’re down to 28 bytes, having reduced the size of this subroutine by nearly 50 percent. Only 13 instructions remain. Realistically, how much smaller can we make this code?
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</P>
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<P>About one-third smaller yet, as it turns out—but in order to do that, we must stretch our minds and use the 8088’s instructions in unusual ways. Let me ask you this: What do most of the instructions in the current version of <B>ClearS</B> do?</P>
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<P>They either load parameters from the stack frame or set up the registers so that the parameters can be accessed. Mind you, there’s nothing wrong with the stack-frame-oriented instructions used in <B>ClearS</B>; those instructions access the stack frame in a highly efficient way, exactly as the designers of the 8088 intended, and just as the code generated by a high-level language would. That means that we aren’t going to be able to improve the code if we don’t bend the rules a bit.</P>
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<P>Let’s think...the parameters are sitting on the stack, and most of our instruction bytes are being used to read bytes off the stack with BP-based addressing...we need a more efficient way to address the stack...<I>the stack</I>...THE STACK!</P>
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<P>Ye gods! That’s easy—we can use the <I>stack pointer</I> to address the stack rather than BP. While it’s true that the stack pointer can’t be used for <I>mod-reg-rm</I> addressing, as BP can, it <I>can</I> be used to pop data off the stack—and <B>POP</B> is a one-byte instruction. Instructions don’t get any shorter than that.</P>
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<P>There is one detail to be taken care of before we can put our plan into action: The return address—the address of the calling code—is on top of the stack, so the parameters we want can’t be reached with <B>POP</B>. That’s easily solved, however—we’ll just pop the return address into an unused register, then branch through that register when we’re done, as we learned to do in Chapter 14. As we pop the parameters, we’ll also be removing them from the stack, thereby neatly avoiding the need to discard them when it’s time to return.</P>
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<P>With that problem dealt with, Listing 22.7 shows the Zenned version of <B>ClearS</B>.</P>
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<P><B>LISTING 22.7 L22-7.ASM</B></P>
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<!-- CODE //-->
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<PRE>
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ClearS procnear
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pop dx ;get the return address
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pop ax ;put fill char into AL
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pop bx ;get the attribute
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mov ah,bh ;put attribute into AH
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pop cx ;get the buffer size
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pop di ;get the offset of the buffer origin
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pop es ;get the segment of the buffer origin
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mov bx,es ;put the segment where we can test it
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or bx,di ;null pointer?
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je Bye ;yes, so we’re done
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cld ;make STOSW count up
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rep stosw ;do the string store
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Bye:
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jmp dx ;return to the calling code
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ClearS endp
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</PRE>
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<!-- END CODE //-->
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<P>At long last, we’re down to the bare metal. This version of <B>ClearS</B> is just 19 bytes long. That’s just 37 percent as long as the original version, <I>without any change whatsoever in the functionality that <B>ClearS</B> makes available to the calling code</I>. The code is bound to run a bit faster too, given that there are far fewer instruction bytes and fewer memory accesses.</P>
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<P>All in all, the Zenned version of <B>ClearS</B> is a vast improvement over the original. Probably not the best possible implementation—<I>never say never!</I>—but an awfully good one.</P><P><BR></P>
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<TR>
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<TD><A HREF="22-02.html">Previous</A></TD>
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<TD><A HREF="index.html">Table of Contents</A></TD>
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<TD><A HREF="23-01.html">Next</A></TD>
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</TR>
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<hr width="90%" size="1" noshade>
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<div align="center">
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Graphics Programming Black Book © 2001 Michael Abrash
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</div>
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