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This commit is contained in:
James Gregory 2013-12-30 18:34:31 +11:00
commit 18011c595a
352 changed files with 6040 additions and 6040 deletions
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22-02.md
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@ -5,7 +5,7 @@
**LISTING 22.2 L22-2.ASM**
ClearS proc near
push bp ;save caller’s BP
push bp ;save caller's BP
mov bp,sp ;point to stack frame
cmp word ptr [bp].BufSeg,0 ;skip the fill if a null
jne Start ; pointer is passed
@ -22,15 +22,15 @@
mov cx,[bp].BufSize ;load CX with buffer size
rep stosw ;fill the buffer
Bye:
pop bp ;restore caller’s BP
pop bp ;restore caller's BP
ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack
ClearS endp
(The **OnStack** structure definition doesn’t change in any of our
examples, so I’m not going clutter up this chapter by reproducing it for
(The **OnStack** structure definition doesn't change in any of our
examples, so I'm not going clutter up this chapter by reproducing it for
each new version of **ClearS**.)
Okay, loading ES and DI directly saves another four bytes. We’ve
Okay, loading ES and DI directly saves another four bytes. We've
squeezed a total of 6 bytes—about 11 percent—out of **ClearS**. What
next?
@ -40,7 +40,7 @@ loading ES and DI as shown in Listing 22.3.
**LISTING 22.3 L22-3.ASM**
ClearS proc near
push bp ;save caller’s BP
push bp ;save caller's BP
mov bp,sp ;point to stack frame
cmp word ptr [bp].BufSeg,0 ;skip the fill if a null
jne Start ; pointer is passed
@ -57,11 +57,11 @@ loading ES and DI as shown in Listing 22.3.
mov cx,[bp].BufSize ;load CX with buffer size
rep stosw ;fill the buffer
Bye:
pop bp ;restore caller’s BP
pop bp ;restore caller's BP
ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack
ClearS endp
That’s good for another three bytes. We’re down to 43 bytes, and
That's good for another three bytes. We're down to 43 bytes, and
counting.
We can save 3 more bytes by clearing the low and high bytes of AX and
@ -71,7 +71,7 @@ values as shown in Listing 22.4.
**LISTING 22.4 L22-4.ASM**
ClearS proc near
push bp ;save caller’s BP
push bp ;save caller's BP
mov bp,sp ;point to stack frame
cmp word ptr [bp].BufSeg,0 ;skip the fill if a null
jne Start ; pointer is passed
@ -88,13 +88,13 @@ values as shown in Listing 22.4.
mov cx,[bp].BufSize ;load CX with buffer size
rep stosw ;fill the buffer
Bye:
pop bp ;restore caller’s BP
pop bp ;restore caller's BP
ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack
ClearS endp
Now we’re down to 40 bytes—more than 20 percent smaller than the
original code. That’s pretty much it for simple instruction
optimizations. Now let’s look for instruction optimizations.
Now we're down to 40 bytes—more than 20 percent smaller than the
original code. That's pretty much it for simple instruction
optimizations. Now let's look for instruction optimizations.
It seems strange to load a word value into AX and then throw away AL.
Likewise, it seems strange to load a word value into BX and then throw
@ -102,10 +102,10 @@ away BH. However, those steps are necessary because the two modified
word values are ORed into a single character/attribute word value that
is then used to fill the target buffer.
Let’s step back and see what this code really *does*, though. All it
Let's step back and see what this code really *does*, though. All it
does in the end is load one byte addressed relative to BP into AH and
another byte addressed relative to BP into AL. Heck, we can just do that
directly! Presto—we’ve saved another 6 bytes, and turned two word-sized
directly! Presto—we've saved another 6 bytes, and turned two word-sized
memory accesses into byte-sized memory accesses as well. Listing 22.5
shows the new code.