**LISTING 7.2 L7-2.ASM** ; Program to illustrate searching through a buffer of a specified ; length until a specified zero byte is encountered. ; A loop unrolled four times and terminated with LOOP is used. .model small .stack 100h .data ; Sample string to search through. SampleStringlabelbyte db ‘This is a sample string of a long enough length ' db ‘so that raw searching speed can outweigh any ' db ‘extra set-up time that may be required.',0 SAMPLE_STRING_LENGTH equ $-SampleString ; User prompt. Prompt db ‘Enter character to search for:$' ; Result status messages. ByteFoundMsg db 0dh,0ah db ‘Specified byte found.',0dh,0ah,‘$' ZeroByteFoundMsg db 0dh,0ah db ‘Zero byte encountered.', 0dh, 0ah, ‘$' NoByteFoundMsg db 0dh,0ah db ‘Buffer exhausted with no match.', 0dh, 0ah, ‘$' ; Table of initial, possibly partial loop entry points for ; SearchMaxLength. SearchMaxLengthEntryTable labelword dw SearchMaxLengthEntry4 dw SearchMaxLengthEntry1 dw SearchMaxLengthEntry2 dw SearchMaxLengthEntry3 .code Start proc near mov ax,@data ;point to standard data segment mov ds,ax mov dx,offset Prompt mov ah,9 ;DOS print string function int 21h ;prompt the user mov ah,1 ;DOS get key function int 21h ;get the key to search for mov ah,al ;put character to search for in AH mov cx,SAMPLE_STRING_LENGTH ;# of bytes to search mov si,offset SampleString ;point to buffer to search call SearchMaxLength ;search the buffer mov dx,offset ByteFoundMsg ;assume we found the byte jc PrintStatus ;we did find the byte ;we didn't find the byte, figure out ;whether we found a zero byte or ;ran out of buffer mov dx,offset NoByteFoundMsg ;assume we didn't find a zero byte jcxz PrintStatus ;we didn't find a zero byte mov dx,offset ZeroByteFoundMsg ;we found a zero byte PrintStatus: mov ah,9 ;DOS print string function int 21h ;report status mov ah,4ch ;return to DOS int 21h Startendp ; Function to search a buffer of a specified length until either a ; specified byte or a zero byte is encountered. ; Input: ; AH = character to search for ; CX = maximum length to be searched (must be > 0) ; DS:SI = pointer to buffer to be searched ; Output: ; CX = 0 if and only if we ran out of bytes without finding ; either the desired byte or a zero byte ; DS:SI = pointer to searched-for byte if found, otherwise byte ; after zero byte if found, otherwise byte after last ; byte checked if neither searched-for byte nor zero ; byte is found ; Carry Flag = set if searched-for byte found, reset otherwise SearchMaxLength proc near cld mov bx,cx add cx,3 ;calculate the maximum # of passes shr cx,1 ;through the loop, which is shr cx,1 ;unrolled 4 times and bx,3 ;calculate the index into the entry ;point table for the first, ;possibly partial loop shl bx,1 ;prepare for a word-sized look-up jmp SearchMaxLengthEntryTable[bx] ;branch into the unrolled loop to do ;the first, possibly partial loop SearchMaxLengthLoop: SearchMaxLengthEntry4: lodsb ;get the next byte cmp al,ah ;is this the byte we want? jz ByteFound ;yes, we're done with success and al,al ;is this the terminating 0 byte? jz ByteNotFound ;yes, we're done with failure SearchMaxLengthEntry3: lodsb ;get the next byte cmp al,ah ;is this the byte we want? jz ByteFound ;yes, we're done with success and al,al ;is this the terminating 0 byte? jz ByteNotFound ;yes, we're done with failure SearchMaxLengthEntry2: lodsb ;get the next byte cmp al,ah ;is this the byte we want? jz ByteFound ;yes, we're done with success and al,al ;is this the terminating 0 byte? jz ByteNotFound ;yes, we're done with failure SearchMaxLengthEntry1: lodsb ;get the next byte cmp al,ah ;is this the byte we want? jz ByteFound ;yes, we're done with success and al,al ;is this the terminating 0 byte? jz ByteNotFound ;yes, we're done with failure loop SearchMaxLengthLoop ;it's neither, so check the next ; four bytes, if any ByteNotFound: clc ;return "not found" status ret ByteFound: dec si ;point back to the location at which ; we found the searched-for byte stc ;return "found" status ret SearchMaxLengthendp end Start How much difference? Listing 7.2 runs in 121 µs—40 percent faster than Listing 7.1, even though Listing 7.2 still uses **LOOP** rather than **DEC CX/JNZ.** (The loop in Listing 7.2 could be unrolled further, too; it's just a question of how much more memory you want to trade for ever-decreasing performance benefits.) That's typical of local optimization; it won't often yield the order-of-magnitude improvements that algorithmic improvements can produce, but it can get you a critical 50 percent or 100 percent improvement when you've exhausted all other avenues. ------------------- ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ![](images/i.jpg) *The point is simply this: You can gain far more by stepping back a bit and thinking of the fastest overall way for the CPU to perform a task than you can by saving a cycle here or there using different instructions. Try to think at the level of sequences of instructions rather than individual instructions, and learn to treat x86 instructions as building blocks with unique characteristics rather than as instructions dedicated to specific tasks.* ------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------