Multiplying in assembly

[panda1234]

Hi,
I want to know how to multiply two 64bit number in part of 32 bit?suppose A has two parts AH&AL and B has two parts BH&BL
How Multiply these numbers?

 

[esp1]

hi,
The on line PIC list site has a number of maths subs

http://piclist.com/techref/microchip/math/basic.htm

 

[KlausST]

Hi,

How do you multiply 123 x 456 in decimal?

either

123 x 400 = 49200
123 x 50  =  6150  
123 x 6   =   738
-----------------------
            56088

or 
4----, 1 x 4, shift 4 times left
-8---, 2 x 4, shift 3 times left
-12--, 3 x 4, shift 2 times left
-5---, 1 x 5, shift 3 times left
-10--, 2 x 5, shift 2 times left
--15-, 3 x 5, shift 1 times left
--6--, 1 x 6, shift 2 times left
--12-, 2 x 6, shift 1 times left
---18, 3 x 6, no shift
--------
56088

 

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Here is the math involved for doing the 64x64-bit multiplication on a 32-bit machine. How you write software to do this is not my forte, I'm a hardware guy.

For 64-bit multiply with 32-bit inputs resulting in 128-bit result:
AHAL x BHBL

(AH x BH) << 64
(AH x BL) << 32
(AL x BH) << 32
(AL x BL) << 0

A simple testcase 8-bit by 8-bit multiplication for 4 bit values of AH=1001, AL=1100, BH=0010, and BL=0101.
(AH x BH) << 8 = 1001 x 0010 << 8 = 0001001000000000
(AH x BL) << 4 = 1001 x 0101 << 4 = 0000001011010000
(AL x BH) << 4 = 1100 x 0010 << 4 = 0000000110000000
(AL x BL) << 0 = 1100 x 0101 << 0 = 0000000000111100
----------------------------------------------------
                                    0001011010001100