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// ============================================================================

//        __

//   \\__/ o\    (C) 2020  Robert Finch, Waterloo

//    \  __ /    All rights reserved.

//     \/_//     robfinch@finitron.ca

//       ||

//

//      mult64x64.sv

//  - Karatsuba multiply

//  - 11 cycle latency

//

// BSD 3-Clause License

// Redistribution and use in source and binary forms, with or without

// modification, are permitted provided that the following conditions are met:

//

// 1. Redistributions of source code must retain the above copyright notice, this

//    list of conditions and the following disclaimer.

//

// 2. Redistributions in binary form must reproduce the above copyright notice,

//    this list of conditions and the following disclaimer in the documentation

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//

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//

// ============================================================================

module mult64x64(clk, ce, a, b, o);

input clk;

input ce;

input [63:0] a;

input [63:0] b;

output reg [127:0] o;

reg [31:0] a2, b2;

reg [32:0] a1, b1;

reg [63:0] z0, z2, z0a, z2a, z0b, z2b, z0c, z2c, z0d, z2d, p3, p4;

reg [64:0] z1;  // extra bit for carry

reg sgn2, sgn10;

wire sgn9;

always @(posedge clk)

        if (ce) a1 <= a[31: 0] - a[63:32];  // x0-x1

always @(posedge clk)

        if (ce) b1 <= b[63:32] - b[31: 0];  // y1-y0

always @(posedge clk)

        if (ce) a2 <= a1[32] ? -a1 : a1;

always @(posedge clk)

        if (ce) b2 <= b1[32] ? -b1 : b1;

always @(posedge clk)

  if (ce) sgn2 <= a1[32]^b1[32];

delay #(.WID(1), .DEP(7)) udl1 (.clk(clk), .ce(ce), .i(sgn2), .o(sgn9));

always @(posedge clk)

  if (ce) sgn10 <= sgn9;

// 6 cycle latency

mult32x32 u1 (

  .clk(clk),

  .ce(ce),

  .a(a[63:32]),

  .b(b[63:32]),

  .o(z2)          // z2 = x1 * y1

);

mult32x32 u2 (

  .clk(clk),

  .ce(ce),

  .a(a[31:0]),

  .b(b[31:0]),

  .o(z0)          // z0 = x0 * y0

);

mult32x32 u3 (

  .clk(clk),

  .ce(ce),

  .a(a2[31:0]),

  .b(b2[31:0]),

  .o(p3)        // p3 = abs(x0-x1) * abs(y1-y0)

);

always @(posedge clk)

        if (ce) p4 <= sgn9 ? -p3 : p3;

always @(posedge clk)

  if (ce) z2a <= z2;

always @(posedge clk)

  if (ce) z0a <= z0;

always @(posedge clk)

  if (ce) z2b <= z2a;

always @(posedge clk)

  if (ce) z0b <= z0a;

always @(posedge clk)

  if (ce) z2c <= z2b;

always @(posedge clk)

  if (ce) z0c <= z0b;

always @(posedge clk)

        if (ce) z1 <= {{64{sgn10}},p4} + z2c + z0c;

always @(posedge clk)

  if (ce) z2d <= z2c;

always @(posedge clk)

  if (ce) z0d <= z0c;

always @(posedge clk)

        if (ce) o <= {z2d,z0d} + {z1,32'd0};

endmodule