/*

  Copyright (C) 2008 Martin Furter <mf@rola.ch>

  This file is part of wbavr.

  wbavr is free software/hardware; you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation; either version 2, or (at your option)
  any later version.

  wbavr is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.

  You should have received a copy of the GNU General Public License
  along with wbavr; see the file COPYING.  If not, write to
  the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.

==============================================================================*/
/*

wbavr_multiply:

8bit * 8bit -> 16bit plus carry supporting signed and float.

*/

module wbavr_multiply(
    a_i,            // input byte 'a'
    a_signed_i,     // 1 if 'a' is signed
    b_i,            // input byte 'b'
    b_signed_i,     // 1 if 'b' is signed
    float_i,        // 1 if float multiplication
    product_lo_o,   // output low byte
    product_hi_o,   // output high byte
    carry_o         // output carry bit
);

input   [7:0]   a_i;
input           a_signed_i;
input   [7:0]   b_i;
input           b_signed_i;
input           float_i;
output  [7:0]   product_lo_o;
output  [7:0]   product_hi_o;
output          carry_o;

wire [15:0] unsigned_result;
wire [7:0] a_sub1;
wire [7:0] b_sub1;
wire [7:0] a_sub2;
wire [7:0] b_sub2;
wire [7:0] sub;
wire [15:0] int_result;
wire [16:0] result;

// unsigned multiplication
assign unsigned_result = a_i * b_i;

// values to subtract if needed
assign a_sub1 = a_signed_i ? { 1'b0, a_i[6:0] } : { a_i[7:0] };
assign b_sub1 = b_signed_i ? { 1'b0, b_i[6:0] } : { b_i[7:0] };

// values to subtract or zero
assign a_sub2 = &{ b_signed_i, b_i[7] } ? a_sub1 : 8'b0000_0000;
assign b_sub2 = &{ a_signed_i, a_i[7] } ? b_sub1 : 8'b0000_0000;

// sum of the values to subtract
assign sub = a_sub2 + b_sub2;

// integer result, signed or unsigned
assign int_result = { unsigned_result[15:8] - sub, unsigned_result[7:0] };

// result, integer or float
assign result = float_i ? { int_result, 1'b0 } : { 0'b0, int_result };

// outputs
assign product_lo_o = result[7:0];
assign product_hi_o = result[15:8];
// carry is always the MSB of the integer result
assign carry_o = int_result[15];

endmodule