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////////////////////////////////////////////////////////////////////////////////

//

// Filename:    div.v

//

// Project:     Zip CPU -- a small, lightweight, RISC CPU soft core

//

// Purpose:     Provide an Integer divide capability to the Zip CPU.  Provides

//              for both signed and unsigned divide.

//

// Steps:

//      i_rst   The DIVide unit starts in idle.  It can also be placed into an

//      idle by asserting the reset input.

//

//      i_wr    When i_rst is asserted, a divide begins.  On the next clock:

//

//        o_busy is set high so everyone else knows we are at work and they can

//              wait for us to complete.

//

//        pre_sign is set to true if we need to do a signed divide.  In this

//              case, we take a clock cycle to turn the divide into an unsigned

//              divide.

//

//        o_quotient, a place to store our result, is initialized to all zeros.

//

//        r_dividend is set to the numerator

//

//        r_divisor is set to 2^31 * the denominator (shift left by 31, or add

//              31 zeros to the right of the number.

//

//      pre_sign When true (clock cycle after i_wr), a clock cycle is used

//              to take the absolute value of the various arguments (r_dividend

//              and r_divisor), and to calculate what sign the output result

//              should be.

//

//

//      At this point, the divide is has started.  The divide works by walking

//      through every shift of the 

//

//                  DIVIDEND    over the

//              DIVISOR

//

//      If the DIVISOR is bigger than the dividend, the divisor is shifted

//      right, and nothing is done to the output quotient.

//

//                  DIVIDEND

//               DIVISOR

//

//      This repeats, until DIVISOR is less than or equal to the divident, as in

//

//              DIVIDEND

//              DIVISOR

//

//      At this point, if the DIVISOR is less than the dividend, the 

//      divisor is subtracted from the dividend, and the DIVISOR is again

//      shifted to the right.  Further, a '1' bit gets set in the output

//      quotient.

//

//      Once we've done this for 32 clocks, we've accumulated our answer into

//      the output quotient, and we can proceed to the next step.  If the

//      result will be signed, the next step negates the quotient, otherwise

//      it returns the result.

//

//      On the clock when we are done, o_busy is set to false, and o_valid set

//      to true.  (It is a violation of the ZipCPU internal protocol for both

//      busy and valid to ever be true on the same clock.  It is also a 

//      violation for busy to be false with valid true thereafter.)

//

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

////////////////////////////////////////////////////////////////////////////////

//

// Copyright (C) 2015-2017, Gisselquist Technology, LLC

//

// This program is free software (firmware): 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 3 of the License, or (at

// your option) any later version.

//

// This program is distributed in the hope that it will be useful, but WITHOUT

// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY 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 this program.  (It's in the $(ROOT)/doc directory.  Run make with no

// target there if the PDF file isn't present.)  If not, see

// <http://www.gnu.org/licenses/> for a copy.

//

// License:     GPL, v3, as defined and found on www.gnu.org,

//              http://www.gnu.org/licenses/gpl.html

//

//

////////////////////////////////////////////////////////////////////////////////

//

//

// `include "cpudefs.v"

//

module  div(i_clk, i_rst, i_wr, i_signed, i_numerator, i_denominator,

                o_busy, o_valid, o_err, o_quotient, o_flags);

        parameter               BW=32, LGBW = 5;

        input                   i_clk, i_rst;

        // Input parameters

        input                   i_wr, i_signed;

        input   [(BW-1):0]       i_numerator, i_denominator;

        // Output parameters

        output  reg             o_busy, o_valid, o_err;

        output  reg [(BW-1):0]   o_quotient;

        output  wire    [3:0]    o_flags;

        // r_busy is an internal busy register.  It will clear one clock

        // before we are valid, so it can't be o_busy ...

        //

        reg                     r_busy;

        reg     [(2*BW-2):0]     r_divisor;

        reg     [(BW-1):0]       r_dividend;

        wire    [(BW):0] diff; // , xdiff[(BW-1):0];

        assign  diff = r_dividend - r_divisor[(BW-1):0];

        // assign       xdiff= r_dividend - { 1'b0, r_divisor[(BW-1):1] };

        reg             r_sign, pre_sign, r_z, r_c, last_bit;

        reg     [(LGBW-1):0]     r_bit;

        reg     zero_divisor;

        initial zero_divisor = 1'b0;

        always @(posedge i_clk)

                zero_divisor <= (r_divisor == 0)&&(r_busy);

        initial r_busy = 1'b0;

        always @(posedge i_clk)

                if (i_rst)

                        r_busy <= 1'b0;

                else if (i_wr)

                        r_busy <= 1'b1;

                else if ((last_bit)||(zero_divisor))

                        r_busy <= 1'b0;

        initial o_busy = 1'b0;

        always @(posedge i_clk)

                if (i_rst)

                        o_busy <= 1'b0;

                else if (i_wr)

                        o_busy <= 1'b1;

                else if (((last_bit)&&(~r_sign))||(zero_divisor))

                        o_busy <= 1'b0;

                else if (~r_busy)

                        o_busy <= 1'b0;

        always @(posedge i_clk)

                if ((i_rst)||(i_wr))

                        o_valid <= 1'b0;

                else if (r_busy)

                begin

                        if ((last_bit)||(zero_divisor))

                                o_valid <= (zero_divisor)||(~r_sign);

                end else if (r_sign)

                begin

                        o_valid <= (~zero_divisor); // 1'b1;

                end else

                        o_valid <= 1'b0;

        initial o_err = 1'b0;

        always @(posedge i_clk)

                if((i_rst)||(o_valid))

                        o_err <= 1'b0;

                else if (((r_busy)||(r_sign))&&(zero_divisor))

                        o_err <= 1'b1;

                else

                        o_err <= 1'b0;

        initial last_bit = 1'b0;

        always @(posedge i_clk)

                if ((i_wr)||(pre_sign)||(i_rst))

                        last_bit <= 1'b0;

                else if (r_busy)

                        last_bit <= (r_bit == {{(LGBW-1){1'b0}},1'b1});

        always @(posedge i_clk)

                // if (i_rst) r_busy <= 1'b0;

                // else

                if (i_wr)

                begin

                        //

                        // Set our values upon an initial command.  Here's

                        // where we come in and start.

                        //

                        // r_busy <= 1'b1;

                        //

                        o_quotient <= 0;

                        r_bit <= {(LGBW){1'b1}};

                        r_divisor <= {  i_denominator, {(BW-1){1'b0}} };

                        r_dividend <=  i_numerator;

                        r_sign <= 1'b0;

                        pre_sign <= i_signed;

                        r_z <= 1'b1;

                end else if (pre_sign)

                begin

                        //

                        // Note that we only come in here, for one clock, if

                        // our initial value may have been signed.  If we are

                        // doing an unsigned divide, we then skip this step.

                        //

                        r_sign <= ((r_divisor[(2*BW-2)])^(r_dividend[(BW-1)]));

                        // Negate our dividend if necessary so that it becomes

                        // a magnitude only value

                        if (r_dividend[BW-1])

                                r_dividend <= -r_dividend;

                        // Do the same with the divisor--rendering it into

                        // a magnitude only.

                        if (r_divisor[(2*BW-2)])

                                r_divisor[(2*BW-2):(BW-1)] <= -r_divisor[(2*BW-2):(BW-1)];

                        //

                        // We only do this stage for a single clock, so go on

                        // with the rest of the divide otherwise.

                        pre_sign <= 1'b0;

                end else if (r_busy)

                begin

                        // While the divide is taking place, we examine each bit

                        // in turn here.

                        //

                        r_bit <= r_bit + {(LGBW){1'b1}}; // r_bit = r_bit - 1;

                        r_divisor <= { 1'b0, r_divisor[(2*BW-2):1] };

                        if (|r_divisor[(2*BW-2):(BW)])

                        begin

                        end else if (diff[BW])

                        begin

                                // 

                                // diff = r_dividend - r_divisor[(BW-1):0];

                                //

                                // If this value was negative, there wasn't

                                // enough value in the dividend to support

                                // pulling off a bit.  We'll move down a bit

                                // therefore and try again.

                                //

                        end else begin

                                //

                                // Put a '1' into our output accumulator.

                                // Subtract the divisor from the dividend,

                                // and then move on to the next bit

                                //

                                r_dividend <= diff[(BW-1):0];

                                o_quotient[r_bit[(LGBW-1):0]] <= 1'b1;

                                r_z <= 1'b0;

                        end

                        r_sign <= (r_sign)&&(~zero_divisor);

                end else if (r_sign)

                begin

                        r_sign <= 1'b0;

                        o_quotient <= -o_quotient;

                end

        // Set Carry on an exact divide

        wire    w_n;

        always @(posedge i_clk)

                r_c <= (r_busy)&&((diff == 0)||(r_dividend == 0));

        assign w_n = o_quotient[(BW-1)];

        assign o_flags = { 1'b0, w_n, r_c, r_z };

endmodule