Misc/abrash-black-book
Watch
1
0
Fork
You've already forked abrash-black-book
 0
Code Issues Pull requests Projects Releases Packages Wiki Activity Actions

Use backtick code blocks for syntax highlighting

Browse source
This commit is contained in:
James Gregory 2013-12-31 11:38:48 +11:00
commit f658bfbb40
203 changed files with 26546 additions and 25801 deletions
Download patch file Download diff file Expand all files Collapse all files

104
22-03.md
View file

@ -12,23 +12,25 @@ pages: 419-420
**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
```nasm
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
@ -44,22 +46,24 @@ 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
```nasm
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.
@ -103,22 +107,24 @@ With that problem dealt with, Listing 22.7 shows the Zenned version of
**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
```nasm
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