hardware multiply/divide functionality in CPUs (6809) - test-drb@ccmp.vtda.org

12 Mar 2011

...
   the bottom n
bits. Thus the 6809's MUL instruction produces the correct
 answer to A = -20 * B = -3 as it will store -60 in the B register.  
 -20*-3=-60 ?
 (((-20) * (-3)) .EQ. (-60))  ?

 if (-20 * -3 == -60) printf("always knew there was something
 fundamentally wrong with the universe"); 
Ooops! If A=-20 and B=-3, then MUL stores 60 in the B register (the full 
answer being 0xe93c) ;-)

If A=-20 and B=3, then MUL stores -60 in B (the full answer being 0x2c4)!

MUL does in fact perform 8-bit x 8-bit => 8-bit signed multiplication.

However I don't always type correctly!

-cheers from julz @P