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

//

// Filename:    cpuops.v

//

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

//

// Purpose:     This supports the instruction set reordering of operations

//              created by the second generation instruction set, as well as

//      the new operations of POPC (population count) and BREV (bit reversal).

//

//

// 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  cpuops(i_clk,i_rst, i_ce, i_op, i_a, i_b, o_c, o_f, o_valid,

                        o_busy);

        parameter       IMPLEMENT_MPY = `OPT_MULTIPLY;

        input           i_clk, i_rst, i_ce;

        input           [3:0]    i_op;

        input           [31:0]   i_a, i_b;

        output  reg     [31:0]   o_c;

        output  wire    [3:0]    o_f;

        output  reg             o_valid;

        output  wire            o_busy;

        // Shift register pre-logic

        wire    [32:0]           w_lsr_result, w_asr_result, w_lsl_result;

        wire    signed  [32:0]   w_pre_asr_input, w_pre_asr_shifted;

        assign  w_pre_asr_input = { i_a, 1'b0 };

        assign  w_pre_asr_shifted = w_pre_asr_input >>> i_b[4:0];

        assign  w_asr_result = (|i_b[31:5])? {(33){i_a[31]}}

                                : w_pre_asr_shifted;// ASR

        assign  w_lsr_result = ((|i_b[31:6])||(i_b[5]&&(i_b[4:0]!=0)))? 33'h00

                                :((i_b[5])?{32'h0,i_a[31]}

                                : ( { i_a, 1'b0 } >> (i_b[4:0]) ));// LSR

        assign  w_lsl_result = ((|i_b[31:6])||(i_b[5]&&(i_b[4:0]!=0)))? 33'h00

                                :((i_b[5])?{i_a[0], 32'h0}

                                : ({1'b0, i_a } << i_b[4:0]));   // LSL

        // Bit reversal pre-logic

        wire    [31:0]   w_brev_result;

        genvar  k;

        generate

        for(k=0; k<32; k=k+1)

        begin : bit_reversal_cpuop

                assign w_brev_result[k] = i_b[31-k];

        end endgenerate

        // Prelogic for our flags registers

        wire    z, n, v;

        reg     c, pre_sign, set_ovfl, keep_sgn_on_ovfl;

        always @(posedge i_clk)

                if (i_ce) // 1 LUT

                        set_ovfl<=(((i_op==4'h0)&&(i_a[31] != i_b[31]))//SUB&CMP

                                ||((i_op==4'h2)&&(i_a[31] == i_b[31])) // ADD

                                ||(i_op == 4'h6) // LSL

                                ||(i_op == 4'h5)); // LSR

        always @(posedge i_clk)

                if (i_ce) // 1 LUT

                        keep_sgn_on_ovfl<=

                                (((i_op==4'h0)&&(i_a[31] != i_b[31]))//SUB&CMP

                                ||((i_op==4'h2)&&(i_a[31] == i_b[31]))); // ADD

        wire    [63:0]   mpy_result; // Where we dump the multiply result

        reg     mpyhi;          // Return the high half of the multiply

        wire    mpybusy;        // The multiply is busy if true

        wire    mpydone;        // True if we'll be valid on the next clock;

        // A 4-way multiplexer can be done in one 6-LUT.

        // A 16-way multiplexer can therefore be done in 4x 6-LUT's with

        //      the Xilinx multiplexer fabric that follows. 

        // Given that we wish to apply this multiplexer approach to 33-bits,

        // this will cost a minimum of 132 6-LUTs.

        wire    this_is_a_multiply_op;

        assign  this_is_a_multiply_op = (i_ce)&&((i_op[3:1]==3'h5)||(i_op[3:0]==4'hc));

        generate

        if (IMPLEMENT_MPY == 0)

        begin // No multiply support.

                assign  mpy_result = 63'h00;

        end else if (IMPLEMENT_MPY == 1)

        begin // Our single clock option (no extra clocks)

                wire    signed  [63:0]   w_mpy_a_input, w_mpy_b_input;

                assign  w_mpy_a_input = {{(32){(i_a[31])&(i_op[0])}},i_a[31:0]};

                assign  w_mpy_b_input = {{(32){(i_b[31])&(i_op[0])}},i_b[31:0]};

                assign  mpy_result = w_mpy_a_input * w_mpy_b_input;

                assign  mpybusy = 1'b0;

                assign  mpydone = 1'b0;

                always @(*) mpyhi = 1'b0; // Not needed

        end else if (IMPLEMENT_MPY == 2)

        begin // Our two clock option (ALU must pause for 1 clock)

                reg     signed  [63:0]   r_mpy_a_input, r_mpy_b_input;

                always @(posedge i_clk)

                begin

                        r_mpy_a_input <={{(32){(i_a[31])&(i_op[0])}},i_a[31:0]};

                        r_mpy_b_input <={{(32){(i_b[31])&(i_op[0])}},i_b[31:0]};

                end

                assign  mpy_result = r_mpy_a_input * r_mpy_b_input;

                assign  mpybusy = 1'b0;

                initial mpypipe = 1'b0;

                reg     mpypipe;

                always @(posedge i_clk)

                        if (i_rst)

                                mpypipe <= 1'b0;

                        else

                                mpypipe <= (this_is_a_multiply_op);

                assign  mpydone = mpypipe; // this_is_a_multiply_op;

                always @(posedge i_clk)

                        if (this_is_a_multiply_op)

                                mpyhi  = i_op[1];

        end else if (IMPLEMENT_MPY == 3)

        begin // Our three clock option (ALU pauses for 2 clocks)

                reg     signed  [63:0]   r_smpy_result;

                reg             [63:0]   r_umpy_result;

                reg     signed  [31:0]   r_mpy_a_input, r_mpy_b_input;

                reg             [1:0]    mpypipe;

                reg             [1:0]    r_sgn;

                initial mpypipe = 2'b0;

                always @(posedge i_clk)

                        if (i_rst)

                                mpypipe <= 2'b0;

                        else

                        mpypipe <= { mpypipe[0], this_is_a_multiply_op };

                // First clock

                always @(posedge i_clk)

                begin

                        r_mpy_a_input <= i_a[31:0];

                        r_mpy_b_input <= i_b[31:0];

                        r_sgn <= { r_sgn[0], i_op[0] };

                end

                // Second clock

`ifdef  VERILATOR

                wire    signed  [63:0]   s_mpy_a_input, s_mpy_b_input;

                wire            [63:0]   u_mpy_a_input, u_mpy_b_input;

                assign  s_mpy_a_input = {{(32){r_mpy_a_input[31]}},r_mpy_a_input};

                assign  s_mpy_b_input = {{(32){r_mpy_b_input[31]}},r_mpy_b_input};

                assign  u_mpy_a_input = {32'h00,r_mpy_a_input};

                assign  u_mpy_b_input = {32'h00,r_mpy_b_input};

                always @(posedge i_clk)

                        r_smpy_result = s_mpy_a_input * s_mpy_b_input;

                always @(posedge i_clk)

                        r_umpy_result = u_mpy_a_input * u_mpy_b_input;

`else

                wire            [31:0]   u_mpy_a_input, u_mpy_b_input;

                assign  u_mpy_a_input = r_mpy_a_input;

                assign  u_mpy_b_input = r_mpy_b_input;

                always @(posedge i_clk)

                        r_smpy_result = r_mpy_a_input * r_mpy_b_input;

                always @(posedge i_clk)

                        r_umpy_result = u_mpy_a_input * u_mpy_b_input;

`endif

                always @(posedge i_clk)

                        if (this_is_a_multiply_op)

                                mpyhi  = i_op[1];

                assign  mpybusy = mpypipe[0];

                assign  mpy_result = (r_sgn[1])?r_smpy_result:r_umpy_result;

                assign  mpydone = mpypipe[1];

                // Results are then set on the third clock

        end else // if (IMPLEMENT_MPY <= 4)

        begin // The three clock option

                reg     [63:0]   r_mpy_result;

                reg     [31:0]   r_mpy_a_input, r_mpy_b_input;

                reg             r_mpy_signed;

                reg     [2:0]    mpypipe;

                // First clock, latch in the inputs

                always @(posedge i_clk)

                begin

                        // mpypipe indicates we have a multiply in the

                        // pipeline.  In this case, the multiply

                        // pipeline is a two stage pipeline, so we need 

                        // two bits in the pipe.

                        if (i_rst)

                                mpypipe <= 3'h0;

                        else begin

                                mpypipe[0] <= this_is_a_multiply_op;

                                mpypipe[1] <= mpypipe[0];

                                mpypipe[2] <= mpypipe[1];

                        end

                        if (i_op[0]) // i.e. if signed multiply

                        begin

                                r_mpy_a_input <= {(~i_a[31]),i_a[30:0]};

                                r_mpy_b_input <= {(~i_b[31]),i_b[30:0]};

                        end else begin

                                r_mpy_a_input <= i_a[31:0];

                                r_mpy_b_input <= i_b[31:0];

                        end

                        // The signed bit really only matters in the

                        // case of 64 bit multiply.  We'll keep track

                        // of it, though, and pretend in all other

                        // cases.

                        r_mpy_signed  <= i_op[0];

                        if (this_is_a_multiply_op)

                                mpyhi  = i_op[1];

                end

                assign  mpybusy = |mpypipe[1:0];

                assign  mpydone = mpypipe[2];

                // Second clock, do the multiplies, get the "partial

                // products".  Here, we break our input up into two

                // halves, 

                //

                //   A  = (2^16 ah + al)

                //   B  = (2^16 bh + bl)

                //

                // and use these to compute partial products.

                //

                //   AB = (2^32 ah*bh + 2^16 (ah*bl + al*bh) + (al*bl)

                //

                // Since we're following the FOIL algorithm to get here,

                // we'll name these partial products according to FOIL.

                //

                // The trick is what happens if A or B is signed.  In

                // those cases, the real value of A will not be given by

                //      A = (2^16 ah + al)

                // but rather

                //      A = (2^16 ah[31^] + al) - 2^31

                //  (where we have flipped the sign bit of A)

                // and so ...

                //

                // AB= (2^16 ah + al - 2^31) * (2^16 bh + bl - 2^31)

                //      = 2^32(ah*bh)

                //              +2^16 (ah*bl+al*bh)

                //              +(al*bl)

                //              - 2^31 (2^16 bh+bl + 2^16 ah+al)

                //              - 2^62

                //      = 2^32(ah*bh)

                //              +2^16 (ah*bl+al*bh)

                //              +(al*bl)

                //              - 2^31 (2^16 bh+bl + 2^16 ah+al + 2^31)

                //

                reg     [31:0]   pp_f, pp_l; // F and L from FOIL

                reg     [32:0]   pp_oi; // The O and I from FOIL

                reg     [32:0]   pp_s;

                always @(posedge i_clk)

                begin

                        pp_f<=r_mpy_a_input[31:16]*r_mpy_b_input[31:16];

                        pp_oi<=r_mpy_a_input[31:16]*r_mpy_b_input[15: 0]

                                + r_mpy_a_input[15: 0]*r_mpy_b_input[31:16];

                        pp_l<=r_mpy_a_input[15: 0]*r_mpy_b_input[15: 0];

                        // And a special one for the sign

                        if (r_mpy_signed)

                                pp_s <= 32'h8000_0000-(

                                        r_mpy_a_input[31:0]

                                        + r_mpy_b_input[31:0]);

                        else

                                pp_s <= 33'h0;

                end

                // Third clock, add the results and produce a product

                always @(posedge i_clk)

                begin

                        r_mpy_result[15:0] <= pp_l[15:0];

                        r_mpy_result[63:16] <=

                                { 32'h00, pp_l[31:16] }

                                + { 15'h00, pp_oi }

                                + { pp_s, 15'h00 }

                                + { pp_f, 16'h00 };

                end

                assign  mpy_result = r_mpy_result;

                // Fourth clock -- results are clocked into writeback

        end

        endgenerate // All possible multiply results have been determined

        //

        // The master ALU case statement

        //

        always @(posedge i_clk)

        if (i_ce)

        begin

                pre_sign <= (i_a[31]);

                c <= 1'b0;

                casez(i_op)

                4'b0000:{c,o_c } <= {1'b0,i_a}-{1'b0,i_b};// CMP/SUB

                4'b0001:   o_c   <= i_a & i_b;          // BTST/And

                4'b0010:{c,o_c } <= i_a + i_b;          // Add

                4'b0011:   o_c   <= i_a | i_b;          // Or

                4'b0100:   o_c   <= i_a ^ i_b;          // Xor

                4'b0101:{o_c,c } <= w_lsr_result[32:0];  // LSR

                4'b0110:{c,o_c } <= w_lsl_result[32:0]; // LSL

                4'b0111:{o_c,c } <= w_asr_result[32:0];  // ASR

                4'b1000:   o_c   <= w_brev_result;      // BREV

                4'b1001:   o_c   <= { i_a[31:16], i_b[15:0] }; // LODILO

                4'b1010:   o_c   <= mpy_result[63:32];  // MPYHU

                4'b1011:   o_c   <= mpy_result[63:32];  // MPYHS

                4'b1100:   o_c   <= mpy_result[31:0];    // MPY

                default:   o_c   <= i_b;                // MOV, LDI

                endcase

        end else // if (mpydone)

                o_c <= (mpyhi)?mpy_result[63:32]:mpy_result[31:0];

        reg     r_busy;

        initial r_busy = 1'b0;

        always @(posedge i_clk)

                if (i_rst)

                        r_busy <= 1'b0;

                else

                        r_busy <= ((IMPLEMENT_MPY > 1)

                                        &&(this_is_a_multiply_op))||mpybusy;

        assign  o_busy = (r_busy); // ||((IMPLEMENT_MPY>1)&&(this_is_a_multiply_op));

        assign  z = (o_c == 32'h0000);

        assign  n = (o_c[31]);

        assign  v = (set_ovfl)&&(pre_sign != o_c[31]);

        wire    vx = (keep_sgn_on_ovfl)&&(pre_sign != o_c[31]);

        assign  o_f = { v, n^vx, c, z };

        initial o_valid = 1'b0;

        always @(posedge i_clk)

                if (i_rst)

                        o_valid <= 1'b0;

                else if (IMPLEMENT_MPY <= 1)

                        o_valid <= (i_ce);

                else

                        o_valid <=((i_ce)&&(!this_is_a_multiply_op))||(mpydone);

endmodule