page ,132
title strrev - reverse a string in place
;***
;strrev.asm - reverse a string in place
;
; Copyright (c) 1985-1991, 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 = -1
repne 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 return
else
ret ; _cdecl return
endif
_strrev endp
end
