Multiplication in assembly

jericotolentino,

Here's a good source of help:

http://heim.ifi.uio.no/~stanisls/helppc/

; Select 'normal' data/code architecture, i.e. max 64k of code + 64k of data.
.model small

.stack
.data

; Define names for some common values.
; lf = Line Feed code.
lf equ 0ah
; cr = Carriage Return code.
cr equ 0dh

; A couple of strings used in the code. '$' is used for string termination for many of the older DOS functions.
prompt db cr,lf,"Enter a number: ","$"
product db cr,lf,"The product is: ","$"

.code
main proc near
; Set up the DS register to point to our data segment.
; Note that you cannot directly load a segment register from memory.
mov ax,@data
mov ds,ax

; Get the address of the prompt string.
mov dx,offset prompt
; Select function 9. Print string to stdout. (See my 'helppc' link above).
mov ah,09h
; Perform the function.
int 21h

; Select function 1. Read one character from keyboard.
mov ah,01h
int 21h

; Set register BX to 10 decimal.
mov bx,10d
; Multiply AX by BX (10).
mul bx

; Display the prompt again.
mov dx,offset prompt
mov ah,09h
int 21h

; Pull the result of the multiply into lower half of BX register.
: NOTE: This may be a fault because int 21h often trashes some registers.
mov bl,al
; Select function 2 (Print one character)
mov ah,02h
int 21h

; Exit the program.
mov ax,4ch
int 21h

main endp
end main


Paul

Thanks for the quick reply and the link. It's a mine of info.

I understand the code better now. I was thinking that the mul and div syntax were similar to add and sub.

I still get an error though. It says

C:\Windows\System32\cmd.exe - tlink temp.obj,temp.exe
The NTVDM CPU has encountered an illegal instruction. CS:01b4 IP:263d OP:ff ff ff ff ff Choose 'Close' to terminate the application.

So I have no choice but to close it. I don't know if I did something wrong. The assembler didn't give any errors.

Anyway, one of your explanations intrigue me. Why does

mov ah,01h
int 21h

read only one character? I was hoping I could get it to read more characters, maybe something like 1.5 or 2.

Oops. I think I already found my answer in your reply. Anyway, is there a workaround for it?

jericotolentino,

RE: Reading more than one character, try something like:

http://heim.ifi.uio.no/~stanisls/helppc/int_21-a.html

For an easier list to search, try:

http://heim.ifi.uio.no/~stanisls/helppc/int_21.html

You will also need to convert the entered number (decimal string of characters) into a number in a register. That can be fun! Post here when you're at that point and we'll help you through it, or post another Q.


Paul

Ok. I'll do what you just told me and get back with the results soon. Thanks.

Hi, I'm back. I've made some progress with reading more than one character using 0Ah instead of 01h. It won't print on the screen though.

After reading some more articles on the internet, conversion of the characters seems a little blurry right now so I think I'll just stick first with printing the output on screen.

I'm looking at the link below for conversions. I think I can understand it in a day or two. So I'm trying to understand the algorithms outlined and how it is implemented. Some of the commands seem unfamiliar to me at this point though.

http://goforit.unk.edu/asm/asm_4.htm

Anyway, right now it just prints "The product is $". I don't know why it prints "$" with the string. Based on what I've read so far "$" is supposed to be a terminating character or something. Is my assumption correct?

Thanks!

The '$' was used back in the early days as a string terminator so you may see it used in some of the older calls. Most ot the newer ones use a zero code for termination. You'll have to check the call description for which one to use.

The link you gave points to code that isnt x86. You might find it more confusing than helpful.

Paul

I see. Thanks for clearing that up. I'm not sure though what you meant by this:

"You will also need to convert the entered number (decimal string of characters) into a number in a register."

When someone enters a number at a prompt, you, as the program, receive it as a sequence of characters (hopefully digits). To turn that into a number in a register ready for multiplying it you have to examine each digit in turn and build up its actual value by successive multiplication by 10 and addition.

To convert a number, say "1234", you will need to evaluate (((((((0+1)*10)+2)*10)+3)*10)+4). Remembering that a digit is an ascii code ('0' = 0x30, '1' = 0x31 ...) so you must subtract '0' from each digit as you go.

Try it, and post your code as a new question and we can help get it right.

Paul