Assembly

Word Multiplication Problem

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Join Date: Mar 2009

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Word Multiplication Problem

Apr 23rd, 2009

Hello,

I need a little help for word multiplication, according to my notes, to multiply a word, we should put the multiplicand in AX, the multiplier in a register or memory variable and the result should appears in AX(low) and DX(high).

The problem is how to get the product ? It is found in two seperate registers, how to merge them together.

This is a simple example:

 Help with Code Tags 
 Assembly Syntax (Toggle Plain Text) 
                        call GetDec
		        mov bx, ax
                        dec bx
			cwd
			imul bx
                        call GetDec
		        mov bx, ax
                        dec bx
			cwd
			imul bx

From the example above, the product is in AX and DX but how to access the product and save it in a memory variable?

Thanks in advance for your answer.

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Join Date: Apr 2009

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sysop_fb

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Re: Word Multiplication Problem

Apr 24th, 2009

If you have a 32 bit system you could just do something like

Either checking if it overflowed into dx or just zeroing everything out before hand

 Help with Code Tags 
 Assembly Syntax (Toggle Plain Text) 
mov cx, dx
shl ecx, 16
mov cx, ax
mov cx, dx
shl ecx, 16
mov cx, ax

Or you could set aside space on the stack something along the lines of

 Help with Code Tags 
 Assembly Syntax (Toggle Plain Text) 
enter 0,4
....code goes here...
....multiplication code.....
mov [ebp - 2], dx
mov [ebp - 4], ax
enter 0,4
....code goes here...
....multiplication code.....
mov [ebp - 2], dx
mov [ebp - 4], ax

And now your number resides at [ebp - 4]

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Join Date: Mar 2009

Posts: 123

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neutralfox neutralfox is offline

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Re: Word Multiplication Problem

Apr 24th, 2009

Hello, thanks for the answer, I have not tested your solution, because I got the solution by myself.

Solution:
Move 0 in eax
push ax
mov ax, dx
Make 16 left shift in eax, to push ax to the end
pop ax
move ax, in eax

Voila.

Anyway, thanks for the answer dude.