TITLE Fast Multiplication                           (fastMultiplication.asm)

Comment !
Description: Write a procedure named FastMultiply that multiplies any
unsigned 32-bit integer by EAX, using only shifting and addition. Pass
the integer to the procedure in the EBX register, and return the product
in the EAX register. Write a short test program that calls the procedure
and displays the product. (We will assume that the product is never
larger than 32 bits.)

Last update: 11/20/02  (uses new ReadDec procedure)
!
INCLUDE Irvine32.inc
INCLUDE Macros.inc

.data
str1 BYTE "Fast Multiplication Program - ** DEMO **",
	       0dh,0ah,0dh,0ah,0

.code
main PROC
	call Clrscr
	mov  edx,OFFSET str1
	call WriteString

L1:
	mWrite "Enter an unsigned integer (0 to quit): "
	call ReadDec
	.IF CARRY?
	  mWrite <"Error: Integer is out of range",0dh,0ah>
	  jmp L1
	.ELSE
	mov  ebx,eax
	.ENDIF
	cmp eax,0
	je  quit


L2:
	mWrite "Enter an unsigned integer: "
	call ReadDec
	.IF CARRY?
	  mWrite <"Error: Integer is out of range",0dh,0ah>
	  jmp L2
	.ENDIF

	mWrite <"--------------------------------",0dh,0ah>

	; EBX = multiplicand, EAX = multiplier
	call FastMultiply	; EAX = product
	jc   L1	; CF=1 indicates error

	mWrite "The product is:       "
	call WriteDec	; display product
	call Crlf
	call Crlf
	jmp  L1

quit:
	exit
main ENDP

;-----------------------------------------------------------------
FastMultiply PROC uses ecx edx esi

COMMENT !
 Multiplies any unsigned 32-bit integer by EAX, using only
 shifting and addition.
 Receives: EBX = multiplier, EAX = multiplicand
 Returns: If CF=0, EAX = product; otherwise, CF=1 and
          EAX equals 0.

 The algorithm used here is more elegant than the one discussed
 on page 237.

 	FOR count = 1 to 32
 	  if lowbit(multiplier) == 1
 	    product += multiplicand;		; check Carry flag
 	  multiplier SHR 1;
 	  multiplicand SHL 1;		; check Carry flag
 	NEXT count

This was obtained from the web site of Dr. Wang Jian-Sheng at
the National University of Singapore (http://www.cz3.nus.edu.sg/~wangjs/)
----------------------------------------------------------------!

	mov  edx,0	; clear product to zero
	mov  ecx,32	; loop counter

L1:	test ebx,1	; LSB set?
	jz   L2	; no: skip next statement
	add  edx,eax	; yes: add multiplicand to product
	jc   L3	; display error if CF=1

L2:
	shr  ebx,1	; shift multiplier right
	shl  eax,1	; shift multiplicand left
	jc   L3	; display error if CF=1
	loop L1	; repeat for 32 bits

L3:	mov  eax,edx	; return the product

	ret
FastMultiply ENDP

END main