Category : Files from Magazines
Archive   : PTV1N4.ZIP
Filename : L2.ASM

; *** Listing 2 ***
;
; Program to illustrate searching through a buffer of a specified
; length until either a specified byte or a 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.
SampleString label byte
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 label word
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
Start endp

; 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
SearchMaxLength endp
end Start