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**LISTING 22.5 L22-5.ASM**
ClearS proc near
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
cmp word ptr [bp].BufOfs,0
je Bye
Start: cld ;make STOSW count up
mov ah,byte ptr [bp].Attrib[1];load AH with attribute
mov al,byte ptr [bp].Filler ;load AL with fill char
les di,dword ptr [bp].BufOfs ;load ES:DI with target buffer segment:offset
mov cx,[bp].BufSize ;load CX with buffer size
rep stosw ;fill the buffer
Bye:
pop bp ;restore caller’s BP
ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack
ClearS endp
(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.)
Another nifty instruction-rearrangement trick saves 6 more bytes.
**ClearS** 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.
**LISTING 22.6 L22-6.ASM**
ClearS proc near
push bp ;save caller’s BP
mov bp,sp ;point to stack frame
les di,dword ptr [bp].BufOfs ;load ES:DI with target buffer;segment:offset
mov ax,es ;put segment where we can test it
or ax,di ;is it a null pointer?
je Bye ;yes, so we’re done
Start: cld ;make STOSW count up
mov ah,byte ptr [bp].Attrib[1];load AH with attribute
mov al,byte ptr [bp].Filler ;load AL with fill char
mov cx,[bp].BufSize ;load CX with buffer size
rep stosw ;fill the buffer
Bye:
pop bp ;restore caller’s BP
ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack
ClearS endp
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?
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 **ClearS** do?
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 **ClearS**; 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.
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...*the stack*...THE STACK!
Ye gods! That’s easy—we can use the *stack pointer* to address the stack
rather than BP. While it’s true that the stack pointer can’t be used for
*mod-reg-rm* addressing, as BP can, it *can* be used to pop data off the
stack—and **POP** is a one-byte instruction. Instructions don’t get any
shorter than that.
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 **POP**.
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.
With that problem dealt with, Listing 22.7 shows the Zenned version of
**ClearS**.
**LISTING 22.7 L22-7.ASM**
ClearS procnear
pop dx ;get the return address
pop ax ;put fill char into AL
pop bx ;get the attribute
mov ah,bh ;put attribute into AH
pop cx ;get the buffer size
pop di ;get the offset of the buffer origin
pop es ;get the segment of the buffer origin
mov bx,es ;put the segment where we can test it
or bx,di ;null pointer?
je Bye ;yes, so we’re done
cld ;make STOSW count up
rep stosw ;do the string store
Bye:
jmp dx ;return to the calling code
ClearS endp
At long last, we’re down to the bare metal. This version of **ClearS**
is just 19 bytes long. That’s just 37 percent as long as the original
version, *without any change whatsoever in the functionality that
**ClearS** makes available to the calling code*. The code is bound to
run a bit faster too, given that there are far fewer instruction bytes
and fewer memory accesses.
All in all, the Zenned version of **ClearS** is a vast improvement over
the original. Probably not the best possible implementation—*never say
never!*—but an awfully good one.
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Graphics Programming Black Book © 2001 Michael Abrash