Category : Assembly Language Source Code
Archive   : ZENASM.ZIP
Filename : LST11-27.ASM

;
; *** Listing 11-27 ***
;
; Determines whether two zero-terminated strings differ
; ignoring case-only differences, and if so where, using
; LODS, with an XLAT-based table look-up to convert to
; uppercase.
;
jmp Skip
;
TestString1 label byte
db 'THIS IS A TEST STRING THAT IS '
db 'Z'
db 'TERMINATED WITH A ZERO BYTE...',0
TestString2 label byte
db 'This is a test string that is '
db 'a'
db 'terminated with a zero byte...',0
;
; Table of conversions between characters and their
; uppercase equivalents. (Could be just 128 bytes long if
; only 7-bit ASCII characters are used.)
;
ToUpperTable label word
CHAR=0
rept 256
if (CHAR lt 'a') or (CHAR gt 'z')
db CHAR ;not a lowercase character
else
db CHAR and not 20h
;convert in the range 'a'-'z' to
; uppercase
endif
CHAR=CHAR+1
endm
;
; Compares two zero-terminated strings, ignoring differences
; that are only uppercase/lowercase differences.
;
; Input:
; DS:SI = first zero-terminated string
; ES:DI = second zero-terminated string
;
; Output:
; DS:SI = pointer to first case-insensitive differing
; location in first string, or 0 if the byte
; wasn't found
; ES:DI = pointer to first case-insensitive differing
; location in second string, or 0 if the byte
; wasn't found
;
; Registers altered: AX, BX, DX, SI, DI
;
; Direction flag cleared
;
; Note: Does not handle strings that are longer than 64K
; bytes or cross segment boundaries.
;
CompareStringsNoCase:
cld
mov bx,offset ToUpperTable
CompareStringsLoop:
lodsw ;get the next 2 bytes
mov dx,es:[di] ; from each string
inc di ;point to the next word in the
inc di ; second string
xlat ;convert the first byte in the
; first string to uppercase
xchg dl,al ;set aside the first byte &
xlat ; convert the first byte in the
; second string to uppercase
cmp al,dl ;do the first bytes match?
jnz CompareStringsDifferent1 ;the strings differ
and al,al ;is this the terminating zero?
jz CompareStringsSame
;yes, we're done, with a match
mov al,ah
xlat ;convert the second byte from the
; first string to uppercase
xchg dh,al ;set aside the second byte &
xlat ; convert the second byte from the
; second string to uppercase
cmp al,dh ;do the second bytes match?
jnz CompareStringsDifferent ;the strings differ
and ah,ah ;is this the terminating zero?
jnz CompareStringsLoop
;no, do the next 2 bytes
CompareStringsSame:
sub si,si ;return 0 pointers indicating that
mov di,si ; the strings are identical
ret
CompareStringsDifferent1:
dec si ;point back to the second byte of
dec di ; the word we just compared
CompareStringsDifferent:
dec si ;point back to the first byte of the
dec di ; word we just compared
ret
;
Skip:
call ZTimerOn
mov si,offset TestString1 ;point to one string
mov di,seg TestString2
mov es,di
mov di,offset TestString2 ;point to other string
call CompareStringsNoCase ;and compare the
; strings without
; regard for case
call ZTimerOff