Re: gEDA-user: Icarus multiplier lpm

[Stephen Williams <steve@xxxxxxxxxx>]

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Tomasz Motylewski wrote:
|>However, it turns out that the IEEE1364-1995 standard was overruled
|>by the -2001 standard which changes the rule to the width of a
|>multiply being figured the same way the widths of a sum are figured.
|>The older behavior makes much more mathematical sense, but engineers
|>are not always logical, and practical engineers even less so:-)
|
|
| What? For me it is clear that when I multiply two 8 bit numbers I will get 16
| bit number.
|
| If the default has changed to only give upper 8 bit of the result, how do I
| still force my code to generate 16 significant bits? Should I do:

Upper 8 bits? You wish:-) No, it's worse then that, you get the *lower*
8 bits.

| input [7:0] a;
| input [7:0] b;
|
| {8'b0,a}*{8'b0,b}
|
| to get full a*b ?

Yes. Or assign the result to a 16bit variable, i.e.:

~ wire [15:0] foo = a * b;

In this case, the width is *not* self determined, but determined by
the destination location, and everything will be expanded out to the
desired 16 bits. Really, things only get weird when the multiply is
inside a concatenation operator.

| And what do some FPGA vendors mean by "embedded 18 bit multiplier" ?

They mean "embedded in their FPGA fabric."

- --
Steve Williams                "The woods are lovely, dark and deep.
steve at icarus.com           But I have promises to keep,
http://www.icarus.com         and lines to code before I sleep,
http://www.picturel.com       And lines to code before I sleep."
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