I need help understanding a sample program (x86)

This was the sample program given to us.. and from this we are supposed to write a program that will prompt the user for a list of unsigned integers less than 101 (no error checking) (if user enters 101 that indicates the end of input), which will then compute and print out the count, the total, and the integer average.

This is due tomorrow and I'm completely stuck.. and I guess the only way i'll be able to write this is if i can at least understand the sample program given below  (though if you'd like to write it for me [by tonight] i'll paypal you-email me, i'm desperate)


SAMPLE CODE: takes a number from the user and does a countdown:  example output

Enter second to countdown? 3
T minus 3
T minus 2
T minus 1
blastoff

anyway ok so on to the questions where i'm having trouble understanding the sample code given..

EDIT: 1a) why is the first loop even there? it's only inputting one number from the user.... if it does loop.. i dont get this line here:

cmp al, cr
je endwl

i understand that when it compares cr to al it exits the loop if it does ssomething..but what is it comparing?  what's in al and what's in cr.. is 13 in cr?  if so that doesn't make sense at all..

1b) after the first loop.. it says that the number is stored in dx... for testing purposes.. how can i display to the screen what's stored in dx? or maybe do it through the assembler? (i'm using masm)

i know to display stuff is somethin similar to:

mov      dx, offset newline
mov      ah, 09h
int      21h

i still don't understand how to just output to the screen what's in the register dx though.  i'm just asking this for my general understanding i guess.. sigh i dont really know where to start.. right now i'm just trying to decipher this program..


2) where in the program is it outputting to the screen the decremented numbers?  i'm assuming it's the one under the comment ";;output character in dl"
mov ah, 02h
int 21h

but this is where i'm confused.. where is it getting the value to output from?  it says from dl... but it just has a '0' in it? or maybe from the stack that pushed dx?  i don't understand the while2 and while3 loop at all that's supposed to be outputting to the screen

3)  what's the purpose of these commands and what is it exactly doing

xor si, si
xor dx, dx

4)  why is si being incremented?

inc si

5?  what is "add dl, '0'" doing?


i'm thinking if i can just maybe understand what these lines are doing i can maybe understand how to make this program




As far as the program that i'm supposed to write is concerned... the sample program seems pretty straight forward.. but how the heck am i supposed to store all the data for the count, total, and average - are there enough registers for that?


ps - program assignment and sample *.asm files are in attached zip (rename *.txt to *.asm)

1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:
28:
29:
30:
31:
32:
33:
34:
35:
36:
37:
38:
39:
40:
41:
42:
43:
44:
45:
46:
47:
48:
49:
50:
51:
52:
53:
54:
55:
56:
57:
58:
59:
60:
61:
62:
63:
64:
65:
66:
67:
68:
69:
70:
71:
72:
73:
74:
75:
76:
77:
78:
79:
80:
81:
82:
83:
84:
85:
86:
87:
88:
89:
90:
91:
92:
93:
94:
95:
96:
97:
98:
99:
100:
101:

title	Count Down
	.model	small
	.stack	100h
 
	CR = 13
	LF = 10
	EOS = '$'
 
	.data
prompt	db	"Enter second to count down? ", EOS
tminus	db	"T minus ", EOS
blastof	db	"blast off", EOS
newline	db	CR, LF, EOS
count	dw	?
  
	.code
main	proc
	mov	ax, @data		; setup data segment reg
	mov	ds, ax
 
	mov	dx, offset prompt
	mov	ah, 9h			; display string function
	int	21h			; call os
	
;
;getud
; read a number from the stdin, no error checking
	mov	dx, 0	; running sum = sum * 10 + new digit
	mov	cx, 10	; base 10
while1:	mov	ah, 01h	; get a digit
	int	21h
	cmp	al, cr	; if cr, then done
	je	endw1
	sub	al, '0'	; turn ascii digit to binary
	mov	ah, 0	; wordsize digit
	mov	bx, ax	; save new digit
	mov	ax, dx	; sum * 10
	mul	cx	;
	mov	dx, bx	; get back new digit
	add	dx, ax	; now + new digit
	jmp	while1	; next digit...
endw1:
	;;;number is in dx
 
	mov	count, dx
 
	mov	dx, offset newline
	mov	ah, 09h
	int	21h
 
; main loop - output t minus <second> lef
 
while4:	cmp	count, 0
	jz	endw4
 
	mov	dx,offset tminus
	mov	ah, 09h
	int	21h
 
	mov	bx, count
 
; number to output is in bx
 
	mov	cx, 10		;
	xor	si, si		;
while2:	mov	ax, bx		;
	xor	dx, dx		;
	div	cx		;
	mov	bx, ax		;
	add	dl, '0'		;
	push	dx		;
	inc	si		;
	cmp	bx, 0		;
	jne	while2		;
 
while3:	pop	dx		;
 
	;;output character in dl
	mov	ah, 02h
	int	21h
 
	dec	si		;
	jne	while3		;
 
	mov	dx, offset newline
	mov	ah, 09h
	int	21h
 
	dec	count
	jmp	while4
endw4:	mov	dx,offset blastof
	mov	ah, 09h
	int	21h
 
	
	mov	al, 0			; exit code 0
	mov	ah, 4ch			; return to os
	int	21h			; call os	
 
main	endp
      	end	main			; execution entry point