CSC231 Analysis of the Towers of Hanoi

--D. Thiebaut 16:24, 25 November 2012 (EST)

---

Top Down Approach: Version 1

Source

;;; hanoi1.asm
;;; D.T.
;;; Towers of Hanoi, top down...


;;; assumptions:
;;; al = n   		Number of disk
;;; bl = Source 	character identifying the source
;;; cl = Dest		character identifying the destination
;;; dl = Temp		character identifying the extra peg

		section	.data
moveStr		db	"X --> X", 10
moveStrLen	equ	$-moveStr
source		equ	moveStr + 0
dest		equ	moveStr	+ 6	

	section	.text
	global	_start
;;; ------------------------------------------------------------
;;; M A I N
;;; ------------------------------------------------------------

_start: 	mov	al, 4	; move 4 disks
		mov	bl, 'A'	; from A
		mov	cl, 'B'	; to B
		mov	dl, 'C'	; using C as extra
	
		call	moveDisks

;;; return to OS
		mov	eax, 1
		mov	ebx, 0
		int	0x80

;;; ------------------------------------------------------------
;;; moveDisks:
;;; assumptions:
;;; al = n   		Number of disk
;;; bl = Source 	character identifying the source
;;; cl = Dest		character identifying the destination
;;; dl = Temp		character identifying the extra peg
;;; Does not return anything
;;; does not modify registers
;;; ------------------------------------------------------------
moveDisks:	push	ax
		push	bx
		push	cx
		push	dx

		mov	byte[source], bl
		mov	byte[dest], cl
		call	printMove
	
		pop	dx
		pop	cx
		pop	bx
		pop	ax
		ret

;;; ------------------------------------------------------------
;;; printMove
;;; Prints the string moveStr on the screen
;;; Does not alter any register
;;; ------------------------------------------------------------
printMove:	pushad
		mov	eax, 4
		mov	ebx, 1
		mov	ecx, moveStr
		mov	edx, moveStrLen
		int	0x80
		popad
		ret

Output

[231a@beowulf ~/handout]$ nasmld hanoi1
A --> B

Top Down Version 2

[231a@beowulf ~/handout]$ cat hanoi2.asm
;;; hanoi2.asm
;;; D.T.
;;; Towers of Hanoi, top down...


;;; assumptions:
;;; al = n   		Number of disk
;;; bl = Source 	character identifying the source
;;; cl = Dest		character identifying the destination
;;; dl = Temp		character identifying the extra peg

;;; %define		N	6	; number of disks to move
	
		section	.data
moveStr		db	"X X", 10
moveStrLen	equ	$-moveStr
source		equ	moveStr + 0
dest		equ	moveStr	+ 2	

	section	.text
	global	_start
;;; ------------------------------------------------------------
;;; M A I N
;;; ------------------------------------------------------------

_start: 	mov	al,  N	; move 4 disks
		mov	bl, 'A'	; from A
		mov	cl, 'B'	; to B
		mov	dl, 'C'	; using C as extra
	
		call	moveDisks

;;; return to OS
		mov	eax, 1
		mov	ebx, 0
		int	0x80

;;; ------------------------------------------------------------
;;; moveDisks:
;;; assumptions:
;;; al = n   		Number of disk
;;; bl = Source 	character identifying the source
;;; cl = Dest		character identifying the destination
;;; dl = Temp		character identifying the extra peg
;;; Does not return anything
;;; does not modify registers
;;; ------------------------------------------------------------
moveDisks:	push	ax
		push	bx
		push	cx
		push	dx

		cmp	al, 1
		ja	.recurse
	
;;; only 1 to move: move from source to dest!
		mov	byte[source], bl
		mov	byte[dest], cl
		call	printMove
		jmp	.return

.recurse:
;;; move n-1 disk from source to temp
		dec	al	; al <-- n-1
		xchg	cl, dl	; swap dest and temp
		call	moveDisks
		xchg	cl, dl	; swap back dest and temp
	
;;; move 1 from source to dest
		push	ax
		mov	al, 1
		mov	byte[source], bl
		mov	byte[dest], cl
		call	printMove
		pop	ax
	
;;; move n-1 disk from temp to dest
		xchg	bl, dl	; make temp the source
		call	moveDisks
	
.return:	pop	dx
		pop	cx
		pop	bx
		pop	ax
		ret

;;; ------------------------------------------------------------
;;; printMove
;;; Prints the string moveStr on the screen
;;; Does not alter any of the registers
;;; ------------------------------------------------------------
printMove:	pushad
		mov	eax, 4
		mov	ebx, 1
		mov	ecx, moveStr
		mov	edx, moveStrLen
;;; 		int	0x80
		popad
		ret