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James Gregory
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9e52de9586
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353 changed files with 5091 additions and 5091 deletions
30
09-06.html
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09-06.html
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@ -69,23 +69,23 @@ parmsends
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.code
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public _Div
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_Divprocnear
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push bp ;preserve caller’s stack frame
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push bp ;preserve caller’s stack frame
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mov bp,sp ;point to our stack frame
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push si ;preserve caller’s register variables
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push si ;preserve caller’s register variables
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push di
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std ;we’re working from msb to lsb
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std ;we’re working from msb to lsb
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mov ax,ds
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mov es,ax ;for STOS
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mov cx,[bp+DividendLength]
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mov cx,[bp+DividendLength]
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sub cx,2
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mov si,[bp+Dividend]
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mov si,[bp+Dividend]
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add si,cx ;point to the last word of the dividend
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; (the most significant word)
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mov di,[bp+Quotient]
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mov di,[bp+Quotient]
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add di,cx ;point to the last word of the quotient
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; buffer (the most significant word)
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mov bx,[bp+Divisor]
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mov bx,[bp+Divisor]
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shr cx,1
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inc cx ;# of words to process
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sub dx,dx ;convert initial divisor word to a 32-bit
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@ -99,9 +99,9 @@ DivLoop:
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loop DivLoop
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mov ax,dx ;return the remainder
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cld ;restore default Direction flag setting
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pop di ;restore caller’s register variables
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pop di ;restore caller’s register variables
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pop si
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pop bp ;restore caller’s stack frame
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pop bp ;restore caller’s stack frame
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ret
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_Divendp
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end
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@ -111,7 +111,7 @@ _Divendp
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<!-- CODE //-->
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<PRE>
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/* Sample use of Div function to perform division when the result
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doesn’t fit in 16 bits */
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doesn’t fit in 16 bits */
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#include <stdio.h>
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@ -124,17 +124,17 @@ main() {
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unsigned int k, j = 0x10;
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k = Div((unsigned int *)&i, sizeof(i), j, (unsigned int *)&m);
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printf(“%lu / %u = %lu r %u\n”, i, j, m, k);
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printf(“%lu / %u = %lu r %u\n”, i, j, m, k);
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}
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</PRE>
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<!-- END CODE //-->
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<H4 ALIGN="LEFT"><A NAME="Heading9"></A><FONT COLOR="#000077">Sweet Spot Revisited</FONT></H4>
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<P>Way back in Volume 1, Number 1 of <I>PC TECHNIQUES</I>, (April/May 1990) I wrote the very first of that magazine’s HAX (#1), which extolled the virtues of placing your most commonly-used automatic (stack-based) variables within the stack’s “sweet spot,” the area between +127 to -128 bytes away from BP, the stack frame pointer. The reason was that the 8088 can store addressing displacements that fall within that range in a single byte; larger displacements require a full word of storage, increasing code size by a byte per instruction, and thereby slowing down performance due to increased instruction fetching time.</P>
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<P>Way back in Volume 1, Number 1 of <I>PC TECHNIQUES</I>, (April/May 1990) I wrote the very first of that magazine’s HAX (#1), which extolled the virtues of placing your most commonly-used automatic (stack-based) variables within the stack’s “sweet spot,” the area between +127 to -128 bytes away from BP, the stack frame pointer. The reason was that the 8088 can store addressing displacements that fall within that range in a single byte; larger displacements require a full word of storage, increasing code size by a byte per instruction, and thereby slowing down performance due to increased instruction fetching time.</P>
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<P>This takes on new prominence in 386 native mode, where straying from the sweet spot costs not one, but two or three bytes. Where the 8088 had two possible displacement sizes, either byte or word, on the 386 there are three possible sizes: byte, word, or dword. In native mode (32-bit protected mode), however, a prefix byte is needed in order to use a word-sized displacement, so a variable located outside the sweet spot requires either two extra bytes (an extra displacement byte plus a prefix byte) or three extra bytes (a dword displacement rather than a byte displacement). Either way, instructions grow alarmingly.</P>
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<P>Performance may or may not suffer from missing the sweet spot, depending on the processor, the memory architecture, and the code mix. On a 486, prefix bytes often cost a cycle; on a 386SX, increased code size often slows performance because instructions must be fetched through the half-pint 16-bit bus; on a 386, the effect depends on the instruction mix and whether there’s a cache.</P>
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<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP" ALIGN="LEFT"><IMG SRC="images/i.jpg"><TD WIDTH="95%" VALIGN="TOP" ALIGN="LEFT"><SMALL><I>On balance, though, it’s as important to keep your most-used variables in the stack’s sweet spot in 386 native mode as it was on the 8088.</I></SMALL>
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<P>Performance may or may not suffer from missing the sweet spot, depending on the processor, the memory architecture, and the code mix. On a 486, prefix bytes often cost a cycle; on a 386SX, increased code size often slows performance because instructions must be fetched through the half-pint 16-bit bus; on a 386, the effect depends on the instruction mix and whether there’s a cache.</P>
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<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP" ALIGN="LEFT"><IMG SRC="images/i.jpg"><TD WIDTH="95%" VALIGN="TOP" ALIGN="LEFT"><SMALL><I>On balance, though, it’s as important to keep your most-used variables in the stack’s sweet spot in 386 native mode as it was on the 8088.</I></SMALL>
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</TABLE>
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<P>In assembly, it’s easy to control the organization of your stack frame. In C, however, you’ll have to figure out the allocation scheme your compiler uses to allocate automatic variables, and declare automatics appropriately to produce the desired effect. It can be done: I did it in Turbo C some years back, and trimmed the size of a program (admittedly, a large one) by several K—not bad, when you consider that the “sweet spot” optimization is essentially free, with no code reorganization, change in logic, or heavy thinking involved.
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<P>In assembly, it’s easy to control the organization of your stack frame. In C, however, you’ll have to figure out the allocation scheme your compiler uses to allocate automatic variables, and declare automatics appropriately to produce the desired effect. It can be done: I did it in Turbo C some years back, and trimmed the size of a program (admittedly, a large one) by several K—not bad, when you consider that the “sweet spot” optimization is essentially free, with no code reorganization, change in logic, or heavy thinking involved.
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</P><P><BR></P>
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<CENTER>
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<TABLE BORDER>
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