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page ,132 title strrev - reverse a string in place;***;strrev.asm - reverse a string in place;; Copyright (c) 1985-2001, Microsoft Corporation. All rights reserved.;;Purpose:; defines _strrev() - reverse a string in place (not including; '\0' character);;Revision History:; 10-26-83 RN initial version; 07-16-87 JCR Added check for empty string (fixes large model bug); 05-18-88 SJM Add model-independent (large model) ifdef; 08-04-88 SJM convert to cruntime/ add 32-bit support; 08-23-88 JCR 386 cleanup, minor alterations; 10-26-88 JCR General cleanup for 386-only code; 10-26-88 JCR Re-arrange regs to avoid push/pop ebx; 03-26-90 GJF Changed to _stdcall. Also, fixed the copyright.; 01-18-91 GJF ANSI naming.; 05-10-91 GJF Back to _cdecl, sigh...;;*******************************************************************************
.xlist include cruntime.inc .list
page;***;char *_strrev(string) - reverse a string in place;;Purpose:; Reverses the order of characters in the string. The terminating; null character remains in place.;; Algorithm:; char *; _strrev (string); char *string;; {; char *start = string;; char *left = string;; char ch;;; while (*string++); ;; string -= 2;; while (left < string); {; ch = *left;; *left++ = *string;; *string-- = ch;; }; return(start);; };; NOTE: There is a check for an empty string in the following code.; Normally, this would fall out of the "cmp si,di" instruction in the; loop portion of the routine. However, if the offset of the empty; string is 0 (as it could be in large model), then the cmp does not; catch the empty string and the routine essentially hangs (i.e., loops; moving bytes one at a time FFFFh times). An explicit empty string; check corrects this.;;Entry:; char *string - string to reverse;;Exit:; returns string - now with reversed characters;;Uses:;;Exceptions:;;*******************************************************************************
CODESEG
public _strrev_strrev proc \ uses edi esi, \ string:ptr byte
mov edi,[string] ; di = string mov edx,edi ; dx=pointer to string; save return value
mov esi,edi ; si=pointer to string xor eax,eax ; search value (null) or ecx,-1 ; cx = -1repne scasb ; find null cmp ecx,-2 ; is string empty? (if offset value is 0, the je short done ; cmp below will not catch it and we'll hang).
dec edi ; string is not empty, move di pointer back dec edi ; di points to last non-null byte
lupe: cmp esi,edi ; see if pointers have crossed yet jae short done ; exit when pointers meet (or cross)
mov ah,[esi] ; get front byte... mov al,[edi] ; and end byte mov [esi],al ; put end byte in front... mov [edi],ah ; and front byte at end inc esi ; front moves up... dec edi ; and end moves down jmp short lupe ; keep switching bytes
done: mov eax,edx ; return value: string addr
ifdef _STDCALL_ ret DPSIZE ; _stdcall returnelse ret ; _cdecl returnendif
_strrev endp end
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