topic General multiplication in 8-bit Microcontrollers

General multiplication

c64 — Fri, 26 Jun 2009 17:55:10 GMT

Hi,

This is a general MCU question, and not really addressed to a particular architecture. Say you have an 8-bit MCU that can do 8-bit x 8-bit unsigned integer multiplication, with the result being 16 bits. Would it be possible to use that built-in arithmetic routine to perform a multiplication of M x N bit unsigned integers, where M and N are larger than 8. For instance, would it be possible to perform an 18 x 16 bit unsigned integer multiplication? Not directly of course, but by writing a procedure for it. If so, does anyone have a link to a website describing these algorithms?

Thanks!

Re: General multiplication

kef — Fri, 26 Jun 2009 19:59:33 GMT

Of course 8x8->16bits MUL instructtion can be used to multiply as many bits as you wish. Procedure? Sure you know how to multiply for example 3 decimal digits number by 2 decimal digits number. This algorithm is the same, just replace decimal digits with bytes.

03 FF FF

x FF FF

---------

FE 01 - first MUL

03 FF FF

x FF FF

---------

FE 01

+ FE 01 - second MUL

03 FF FF

x FF FF

---------

FE 01

+ FE 01

02 FD - 3rd MUL

... do another 3 MULs with higher order byte of 2nd multiplier and final picture will be this

03 FF FF

x FF FF

---------

FE 01

+ FE 01

02 FD - 3rd

FE 01

+ FE 01

02 FD - 6th

--------------------

03 FF FB 00 01 - 18+16 = 34bits product

Hope this helps

Re: General multiplication

tonyp — Fri, 26 Jun 2009 21:53:58 GMT

And here's a 9S08 assembly language re-entrant routine that multiplies any size (up to 127 bytes) numbers. For parameter passing simplicity both operands must be of the same size.

Re: General multiplication

bigmac — Sat, 27 Jun 2009 03:24:11 GMT

Hello,

The attached code contains functions for 16 x 16 multiply, and 32 x 32 multiply, plus some other division functions. It extensively makes use of inliine assembly for speed, so is restricted to HC908 and HCS08 devices.

Regards,

Mac

Re: General multiplication

c64 — Mon, 29 Jun 2009 14:22:20 GMT

Hey guys,

Thanks so much for the replies and sample codes! I really appreciate it!