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Altera_Forum's avatar
Altera_Forum
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16 years ago

signed/unsigned multipliers

I can't seem to find a way to instantiate a multiplier that will dynamically select between signed and unsigned multiplication. This is on a Cyclone II, but applies to other parts as well. Read...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
16 years ago

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Hi tricky,

I am afraid they are not. not only MSB meaning is different, addition/subtraction are similar(taking into account carry bit difference).

multiplication/div are different.

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Yes they are.....

take the following example:

0111 x 1011

Lets assume it is unsigned (7 x 11). Add 0's forward

00000111 x 00001011

result = 


 00000111
+    111
+                  
+  111
----------------
 01001101   (77)
Now do the same but assume its signed (7x-5)

sign extend both inputs to 8 bits

00000111 x 11111011 


         00000111
       +     111
       +
       +   111
       +  111
       + 111
       .... etc
--------------------
= (..111)11011101 (= -35)
now if you had treated this as unsigned (251 x 7) the result would be

11011011101 (1757) see the resemblence yet?

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I still don't understand why signa and signb aren't available though.

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That is because the LPM library applies accross ALL technology, and some may not have specific signa/signb inputs. You will have to instantiate the specific multiplier for the technology you are using to get full access to internal signals.