multiplication prob

Discussion in 'VHDL' started by Nisheeth, Mar 31, 2005.

  1. Nisheeth

    Nisheeth Guest

    hello
    my project demands to do
    (84 bit no * 42 bit no)
    42 BIT NO * 42 BIT no)
    comparing 126 bit nos..
    ip core supports maximum 64 bit no * 64 bit no.
    i dont understand how to go about it....any library that
    can help me out ?


    nisheeth
     
    Nisheeth, Mar 31, 2005
    #1
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  2. On 31 Mar 2005 06:01:41 -0800, (Nisheeth) wrote:

    > my project demands to do
    > (84 bit no * 42 bit no)
    > 42 BIT NO * 42 BIT no)
    > comparing 126 bit nos..
    > i dont understand how to go about it....


    Imagine a constant F = 2^42. Then your 84-bit number
    can be represented as (F*A)+B, where A is the upper 42
    bits and B is the lower 42 bits. If the other 42-bit
    number is C, then you want

    result = ((F*A)+B) * C
    = F*A*C + B*C

    In other words:
    - get B*C, an 84-bit result
    - get A*C, an 84-bit result
    - add the upper 42 bits of B*C to A*C to get the top 84
    bits of result
    - use the lower 42 bits of B*C as the bottom 42 bits of
    result

    It's just long multiplication, same as we learnt at school.

    > ip core supports maximum 64 bit no * 64 bit no.


    So - don't use the IP core. Hell, it's only a multiplier.
    What about your synthesis tool? Try feeding the calculation
    directly to it. The tool should infer a suitable collection
    of multipliers and adders for you. For extra credit,
    think about pipelining.
    --
    Jonathan Bromley, Consultant

    DOULOS - Developing Design Know-how
    VHDL, Verilog, SystemC, Perl, Tcl/Tk, Verification, Project Services

    Doulos Ltd. Church Hatch, 22 Market Place, Ringwood, BH24 1AW, UK
    Tel: +44 (0)1425 471223 mail:
    Fax: +44 (0)1425 471573 Web: http://www.doulos.com

    The contents of this message may contain personal views which
    are not the views of Doulos Ltd., unless specifically stated.
     
    Jonathan Bromley, Mar 31, 2005
    #2
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