bsearch.asm

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TITLE  Binary Search Procedure                  (Bsearch.asm)

; Binary Search procedure
; Last update: 1/18/02

INCLUDE Irvine32.inc

.code
;-------------------------------------------------------------
BinarySearch PROC uses ebx edx esi edi,
	pArray:PTR DWORD,		; pointer to array
	Count:DWORD,		; array size
	searchVal:DWORD		; search value
LOCAL first:DWORD,		; first position
	last:DWORD,		; last position
	mid:DWORD		; midpoint
;
; Search an array of signed integers for single value.
; Receives: Pointer to array, array size, search value.
; Returns: If a match is found, EAX = the array position of the
; matching element; otherwise, EAX = -1.
;-------------------------------------------------------------
	mov  first,0		; first = 0
	mov  eax,Count		; last = (count - 1)
	dec  eax
	mov  last,eax
	mov  edi,searchVal		; EDI = searchVal
	mov  ebx,pArray		; EBX points to the array

L1: ; while first <= last
	mov  eax,first
	cmp  eax,last
	jg   L5		; exit search

; mid = (last + first) / 2
	mov  eax,last
	add  eax,first
	shr  eax,1
	mov  mid,eax

; EDX = values[mid]
	mov  esi,mid
	shl  esi,2		; scale mid value by 4
	mov  edx,[ebx+esi]		; EDX = values[mid]

; if ( EDX < searchval(EDI) )
;   first = mid + 1;
	cmp  edx,edi
	jge  L2
	mov  eax,mid		; first = mid + 1
	inc  eax
	mov  first,eax
	jmp  L4

; else if( EDX > searchVal(EDI) )
;	last = mid - 1;
L2:	cmp  edx,edi		; (could be removed)
	jle  L3
	mov  eax,mid		; last = mid - 1
	dec  eax
	mov  last,eax
	jmp  L4

; else return mid
L3:	mov  eax,mid  		; value found
	jmp  L9		; return (mid)

L4:	jmp  L1		; continue the loop

L5:	mov  eax,-1		; search failed
L9:	ret
BinarySearch ENDP

END