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LST14-15.ASM
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Assembly Source File
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1990-02-15
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2KB
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77 lines
;
; *** Listing 14-15 ***
;
; For comparison with the in-line-code-branched-to-via-a-
; jump-table approach of Listing 14-14, this is a loop-based
; string-search routine that searches at most the specified
; number of bytes of a zero-terminated string for the
; specified character.
;
jmp Skip
TestString label byte
db 'This is a string containing the letter '
db 'z but not containing capital q', 0
;
; Searches a zero-terminated string for a character.
; Searches until a match is found, the terminating zero
; is found, or the specified number of characters have been
; checked.
;
; Input:
; AL = character to search for
; BX = maximum # of characters to search
; DS:SI = string to search
;
; Output:
; SI = pointer to character, or 0 if character not
; found
;
; Registers altered: AX, CX, SI
;
; Direction flag cleared
;
; Note: Don't pass a string starting at offset 0, since a
; match there couldn't be distinguished from a failure
; to match.
;
SearchNBytes proc near
mov ah,al ;we'll need AL for LODSB
mov cx,bx ;for LOOP
SearchNBytesLoop:
lodsb
and al,al
jz NoMatch ;terminating 0, so no match
cmp ah,al
jz MatchFound ;match, so we're done
loop SearchNBytesLoop
;
; No match was found.
;
NoMatch:
sub si,si ;return no-match status
ret
;
; A match was found.
;
MatchFound:
dec si ;point back to matching
; location
ret
SearchNBytes endp
;
Skip:
call ZTimerOn
mov al,'Q'
mov bx,20 ;search up to the
mov si,offset TestString ; first 20 bytes of
call SearchNBytes ; TestString for 'Q'
mov al,'z'
mov bx,80 ;search up to the
mov si,offset TestString ; first 80 bytes of
call SearchNBytes ; TestString for 'z'
mov al,'a'
mov bx,10 ;search up to the
mov si,offset TestString ; first 10 bytes of
call SearchNBytes ; TestString for 'a'
call ZTimerOff