Fast MUL in ASM?

Nick Mueller

Hi!

Is there a fast multiply in ASM, or do I have to write my own?
It is a 16bit by 8 bits multiply. Or, if you want, 32 bit by 8 bit without overflow.

Any code snipplets welcome!


Nick

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Never use force, just go for a bigger hammer!

The DIY Digital-Readout for mills, lathes etc.:
YADRO

Ale

There is a document by Parallax with code for mul. If you unroll it, it is quite fast

' 
' Multiply x[noparse][[/noparse]15..0] by y[noparse][[/noparse]15..0] (y[noparse][[/noparse]31..16] must be 0) 
' on exit, product in y[noparse][[/noparse]31..0] 
' 
multiply        shl     x,#16           'get multiplicand into x[noparse][[/noparse]31..16] 
                mov     t,#16           'ready for 16 multiplier bits 
                shr     y,#1    wc      'get initial multiplier bit into c 
 
:loop   if_c    add     y,x     wc      'if c set, add multiplicand into product 
                rcr     y,#1    wc      'get next multiplier bit into c, shift product 
                djnz    t,#:loop        'loop until done 
 
multiply_ret    ret                     'return with product in y[noparse][[/noparse]31..0] 

just unrool and enjoy

It is available in "Propeller Guts"

Direct Digital Labs

Now if we could get a 16x16 signed multiply example......


-Dan

Post Edited (Direct Digital Labs) : 12/14/2007 6:40:30 PM GMT

Mike Green

It's not hard to do yourself. Use ABS to fetch the X and Y values and do the multiply. Then XOR the original values and shift the sign bit into the carry flag. NEGC will negate the result if the carry flag is true. There you are.

deSilva

As the MSB will be never set, you can also reduce the loopcount...