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

////                                                              ////

////  OR1200's Instruction decode                                 ////

////                                                              ////

////  This file is part of the OpenRISC 1200 project              ////

////  http://www.opencores.org/cores/or1k/                        ////

////                                                              ////

////  Description                                                 ////

////  Majority of instruction decoding is performed here.         ////

////                                                              ////

////  To Do:                                                      ////

////   - make it smaller and faster                               ////

////                                                              ////

////  Author(s):                                                  ////

////      - Damjan Lampret, lampret@opencores.org                 ////

////                                                              ////

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

////                                                              ////

//// Copyright (C) 2000 Authors and OPENCORES.ORG                 ////

////                                                              ////

//// This source file may be used and distributed without         ////

//// restriction provided that this copyright statement is not    ////

//// removed from the file and that any derivative work contains  ////

//// the original copyright notice and the associated disclaimer. ////

////                                                              ////

//// This source file is free software; you can redistribute it   ////

//// and/or modify it under the terms of the GNU Lesser General   ////

//// Public License as published by the Free Software Foundation; ////

//// either version 2.1 of the License, or (at your option) any   ////

//// later version.                                               ////

////                                                              ////

//// This source 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 Lesser General Public License for more ////

//// details.                                                     ////

////                                                              ////

//// You should have received a copy of the GNU Lesser General    ////

//// Public License along with this source; if not, download it   ////

//// from http://www.opencores.org/lgpl.shtml                     ////

////                                                              ////

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

//

// CVS Revision History

//

// $Log: not supported by cvs2svn $

// Revision 1.12  2005/01/07 09:31:07  andreje

// sign/zero extension for l.sfxxi instructions corrected

//

// Revision 1.11  2004/06/08 18:17:36  lampret

// Non-functional changes. Coding style fixes.

//

// Revision 1.10  2004/05/09 19:49:04  lampret

// Added some l.cust5 custom instructions as example

//

// Revision 1.9  2004/04/05 08:29:57  lampret

// Merged branch_qmem into main tree.

//

// Revision 1.8.4.1  2004/02/11 01:40:11  lampret

// preliminary HW breakpoints support in debug unit (by default disabled). To enable define OR1200_DU_HWBKPTS.

//

// Revision 1.8  2003/04/24 00:16:07  lampret

// No functional changes. Added defines to disable implementation of multiplier/MAC

//

// Revision 1.7  2002/09/07 05:42:02  lampret

// Added optional SR[CY]. Added define to enable additional (compare) flag modifiers. Defines are OR1200_IMPL_ADDC and OR1200_ADDITIONAL_FLAG_MODIFIERS.

//

// Revision 1.6  2002/03/29 15:16:54  lampret

// Some of the warnings fixed.

//

// Revision 1.5  2002/02/01 19:56:54  lampret

// Fixed combinational loops.

//

// Revision 1.4  2002/01/28 01:15:59  lampret

// Changed 'void' nop-ops instead of insn[0] to use insn[16]. Debug unit stalls the tick timer. Prepared new flag generation for add and and insns. Blocked DC/IC while they are turned off. Fixed I/D MMU SPRs layout except WAYs. TODO: smart IC invalidate, l.j 2 and TLB ways.

//

// Revision 1.3  2002/01/18 14:21:43  lampret

// Fixed 'the NPC single-step fix'.

//

// Revision 1.2  2002/01/14 06:18:22  lampret

// Fixed mem2reg bug in FAST implementation. Updated debug unit to work with new genpc/if.

//

// Revision 1.1  2002/01/03 08:16:15  lampret

// New prefixes for RTL files, prefixed module names. Updated cache controllers and MMUs.

//

// Revision 1.14  2001/11/30 18:59:17  simons

// force_dslot_fetch does not work -  allways zero.

//

// Revision 1.13  2001/11/20 18:46:15  simons

// Break point bug fixed

//

// Revision 1.12  2001/11/18 08:36:28  lampret

// For GDB changed single stepping and disabled trap exception.

//

// Revision 1.11  2001/11/13 10:02:21  lampret

// Added 'setpc'. Renamed some signals (except_flushpipe into flushpipe etc)

//

// Revision 1.10  2001/11/12 01:45:40  lampret

// Moved flag bit into SR. Changed RF enable from constant enable to dynamic enable for read ports.

//

// Revision 1.9  2001/11/10 03:43:57  lampret

// Fixed exceptions.

//

// Revision 1.8  2001/10/21 17:57:16  lampret

// Removed params from generic_XX.v. Added translate_off/on in sprs.v and id.v. Removed spr_addr from dc.v and ic.v. Fixed CR+LF.

//

// Revision 1.7  2001/10/14 13:12:09  lampret

// MP3 version.

//

// Revision 1.1.1.1  2001/10/06 10:18:36  igorm

// no message

//

// Revision 1.2  2001/08/13 03:36:20  lampret

// Added cfg regs. Moved all defines into one defines.v file. More cleanup.

//

// Revision 1.1  2001/08/09 13:39:33  lampret

// Major clean-up.

//

//

// synopsys translate_off

`include "timescale.v"

// synopsys translate_on

`include "or1200_defines.v"

module or1200_ctrl_cm2(

                clk_i_cml_1,

        // Clock and reset

        clk, rst,

        // Internal i/f

        id_freeze, ex_freeze, wb_freeze, flushpipe, if_insn, ex_insn, branch_op, branch_taken,

        rf_addra, rf_addrb, rf_rda, rf_rdb, alu_op, mac_op, shrot_op, comp_op, rf_addrw, rfwb_op,

        wb_insn, simm, branch_addrofs, lsu_addrofs, sel_a, sel_b, lsu_op,

        cust5_op, cust5_limm,

        multicycle, spr_addrimm, wbforw_valid, du_hwbkpt, sig_syscall, sig_trap,

        force_dslot_fetch, no_more_dslot, ex_void, id_macrc_op, ex_macrc_op, rfe, except_illegal

);

input clk_i_cml_1;

reg [ 31 : 0 ] ex_insn_cml_1;

reg [ 3 - 1 : 0 ] branch_op_cml_1;

reg [ 5 - 1 : 0 ] rf_addrw_cml_1;

reg [ 4 - 1 : 0 ] alu_op_cml_1;

reg [ 2 - 1 : 0 ] mac_op_cml_1;

reg [ 2 - 1 : 0 ] shrot_op_cml_1;

reg [ 3 - 1 : 0 ] rfwb_op_cml_1;

reg [ 31 : 0 ] wb_insn_cml_1;

reg [ 4 - 1 : 0 ] lsu_op_cml_1;

reg [ 4 - 1 : 0 ] comp_op_cml_1;

reg [ 15 : 0 ] spr_addrimm_cml_1;

reg  wbforw_valid_cml_1;

reg  sig_syscall_cml_1;

reg  sig_trap_cml_1;

reg  ex_macrc_op_cml_1;

reg  except_illegal_cml_1;

reg [ 3 - 1 : 0 ] pre_branch_op_cml_1;

reg [ 31 : 0 ] id_insn_cml_1;

reg [ 5 - 1 : 0 ] wb_rfaddrw_cml_1;

reg  sel_imm_cml_1;

//

// I/O

//

input                                   clk;

input                                   rst;

input                                   id_freeze;

input                                   ex_freeze;

input                                   wb_freeze;

input                                   flushpipe;

input   [31:0]                           if_insn;

output  [31:0]                           ex_insn;

output  [`OR1200_BRANCHOP_WIDTH-1:0]             branch_op;

input                                           branch_taken;

output  [`OR1200_REGFILE_ADDR_WIDTH-1:0] rf_addrw;

output  [`OR1200_REGFILE_ADDR_WIDTH-1:0] rf_addra;

output  [`OR1200_REGFILE_ADDR_WIDTH-1:0] rf_addrb;

output                                  rf_rda;

output                                  rf_rdb;

output  [`OR1200_ALUOP_WIDTH-1:0]                alu_op;

output  [`OR1200_MACOP_WIDTH-1:0]                mac_op;

output  [`OR1200_SHROTOP_WIDTH-1:0]              shrot_op;

output  [`OR1200_RFWBOP_WIDTH-1:0]               rfwb_op;

output  [31:0]                           wb_insn;

output  [31:0]                           simm;

output  [31:2]                          branch_addrofs;

output  [31:0]                           lsu_addrofs;

output  [`OR1200_SEL_WIDTH-1:0]          sel_a;

output  [`OR1200_SEL_WIDTH-1:0]          sel_b;

output  [`OR1200_LSUOP_WIDTH-1:0]                lsu_op;

output  [`OR1200_COMPOP_WIDTH-1:0]               comp_op;

output  [`OR1200_MULTICYCLE_WIDTH-1:0]           multicycle;

output  [4:0]                            cust5_op;

output  [5:0]                            cust5_limm;

output  [15:0]                           spr_addrimm;

input                                   wbforw_valid;

input                                   du_hwbkpt;

output                                  sig_syscall;

output                                  sig_trap;

output                                  force_dslot_fetch;

output                                  no_more_dslot;

output                                  ex_void;

output                                  id_macrc_op;

output                                  ex_macrc_op;

output                                  rfe;

output                                  except_illegal;

//

// Internal wires and regs

//

reg     [`OR1200_BRANCHOP_WIDTH-1:0]             pre_branch_op;

reg     [`OR1200_BRANCHOP_WIDTH-1:0]             branch_op;

reg     [`OR1200_ALUOP_WIDTH-1:0]                alu_op;

`ifdef OR1200_MAC_IMPLEMENTED

reg     [`OR1200_MACOP_WIDTH-1:0]                mac_op;

reg                                     ex_macrc_op;

`else

wire    [`OR1200_MACOP_WIDTH-1:0]                mac_op;

wire                                    ex_macrc_op;

`endif

reg     [`OR1200_SHROTOP_WIDTH-1:0]              shrot_op;

reg     [31:0]                           id_insn;

reg     [31:0]                           ex_insn;

reg     [31:0]                           wb_insn;

reg     [`OR1200_REGFILE_ADDR_WIDTH-1:0] rf_addrw;

reg     [`OR1200_REGFILE_ADDR_WIDTH-1:0] wb_rfaddrw;

reg     [`OR1200_RFWBOP_WIDTH-1:0]               rfwb_op;

reg     [31:0]                           lsu_addrofs;

reg     [`OR1200_SEL_WIDTH-1:0]          sel_a;

reg     [`OR1200_SEL_WIDTH-1:0]          sel_b;

reg                                     sel_imm;

reg     [`OR1200_LSUOP_WIDTH-1:0]                lsu_op;

reg     [`OR1200_COMPOP_WIDTH-1:0]               comp_op;

reg     [`OR1200_MULTICYCLE_WIDTH-1:0]           multicycle;

reg                                     imm_signextend;

reg     [15:0]                           spr_addrimm;

reg                                     sig_syscall;

reg                                     sig_trap;

reg                                     except_illegal;

wire                                    id_void;

//

// Register file read addresses

//

assign rf_addra = if_insn[20:16];

assign rf_addrb = if_insn[15:11];

assign rf_rda = if_insn[31];

assign rf_rdb = if_insn[30];

//

// Force fetch of delay slot instruction when jump/branch is preceeded by load/store

// instructions

//

// SIMON

// assign force_dslot_fetch = ((|pre_branch_op) & (|lsu_op));

assign force_dslot_fetch = 1'b0;

assign no_more_dslot = |branch_op & !id_void & branch_taken | (branch_op == `OR1200_BRANCHOP_RFE);

assign id_void = (id_insn[31:26] == `OR1200_OR32_NOP) & id_insn[16];

// SynEDA CoreMultiplier

// assignment(s): ex_void

// replace(s): ex_insn

assign ex_void = (ex_insn_cml_1[31:26] == `OR1200_OR32_NOP) & ex_insn_cml_1[16];

//

// Sign/Zero extension of immediates

//

// SynEDA CoreMultiplier

// assignment(s): simm

// replace(s): id_insn

assign simm = (imm_signextend == 1'b1) ? {{16{id_insn_cml_1[15]}}, id_insn_cml_1[15:0]} : {{16'b0}, id_insn_cml_1[15:0]};

//

// Sign extension of branch offset

//

assign branch_addrofs = {{4{ex_insn[25]}}, ex_insn[25:0]};

//

// l.macrc in ID stage

//

`ifdef OR1200_MAC_IMPLEMENTED

// SynEDA CoreMultiplier

// assignment(s): id_macrc_op

// replace(s): id_insn

assign id_macrc_op = (id_insn_cml_1[31:26] == `OR1200_OR32_MOVHI) & id_insn_cml_1[16];

`else

assign id_macrc_op = 1'b0;

`endif

//

// cust5_op, cust5_limm (L immediate)

//

assign cust5_op = ex_insn[4:0];

assign cust5_limm = ex_insn[10:5];

//

//

//

// SynEDA CoreMultiplier

// assignment(s): rfe

// replace(s): branch_op, pre_branch_op

assign rfe = (pre_branch_op_cml_1 == `OR1200_BRANCHOP_RFE) | (branch_op_cml_1 == `OR1200_BRANCHOP_RFE);

//

// Generation of sel_a

//

// SynEDA CoreMultiplier

// assignment(s): sel_a

// replace(s): rf_addrw, rfwb_op, wbforw_valid, id_insn, wb_rfaddrw

always @(rf_addrw_cml_1 or id_insn_cml_1 or rfwb_op_cml_1 or wbforw_valid_cml_1 or wb_rfaddrw_cml_1)

        if ((id_insn_cml_1[20:16] == rf_addrw_cml_1) && rfwb_op_cml_1[0])

                sel_a = `OR1200_SEL_EX_FORW;

        else if ((id_insn_cml_1[20:16] == wb_rfaddrw_cml_1) && wbforw_valid_cml_1)

                sel_a = `OR1200_SEL_WB_FORW;

        else

                sel_a = `OR1200_SEL_RF;

//

// Generation of sel_b

//

// SynEDA CoreMultiplier

// assignment(s): sel_b

// replace(s): rf_addrw, rfwb_op, wbforw_valid, id_insn, wb_rfaddrw, sel_imm

always @(rf_addrw_cml_1 or sel_imm_cml_1 or id_insn_cml_1 or rfwb_op_cml_1 or wbforw_valid_cml_1 or wb_rfaddrw_cml_1)

        if (sel_imm_cml_1)

                sel_b = `OR1200_SEL_IMM;

        else if ((id_insn_cml_1[15:11] == rf_addrw_cml_1) && rfwb_op_cml_1[0])

                sel_b = `OR1200_SEL_EX_FORW;

        else if ((id_insn_cml_1[15:11] == wb_rfaddrw_cml_1) && wbforw_valid_cml_1)

                sel_b = `OR1200_SEL_WB_FORW;

        else

                sel_b = `OR1200_SEL_RF;

//

// l.macrc in EX stage

//

`ifdef OR1200_MAC_IMPLEMENTED

// SynEDA CoreMultiplier

// assignment(s): ex_macrc_op

// replace(s): ex_macrc_op

always @(posedge clk or posedge rst) begin

        if (rst)

                ex_macrc_op <= #1 1'b0;

        else begin  ex_macrc_op <= ex_macrc_op_cml_1; if (!ex_freeze & id_freeze | flushpipe)

                ex_macrc_op <= #1 1'b0;

        else if (!ex_freeze)

                ex_macrc_op <= #1 id_macrc_op; end

end

`else

assign ex_macrc_op = 1'b0;

`endif

//

// Decode of spr_addrimm

//

// SynEDA CoreMultiplier

// assignment(s): spr_addrimm

// replace(s): spr_addrimm, id_insn

always @(posedge clk or posedge rst) begin

        if (rst)

                spr_addrimm <= #1 16'h0000;

        else begin  spr_addrimm <= spr_addrimm_cml_1; if (!ex_freeze & id_freeze | flushpipe)

                spr_addrimm <= #1 16'h0000;

        else if (!ex_freeze) begin

                case (id_insn_cml_1[31:26])     // synopsys parallel_case

                        // l.mfspr

                        `OR1200_OR32_MFSPR:

                                spr_addrimm <= #1 id_insn_cml_1[15:0];

                        // l.mtspr

                        default:

                                spr_addrimm <= #1 {id_insn_cml_1[25:21], id_insn_cml_1[10:0]};

                endcase

        end end

end

//

// Decode of multicycle

//

// SynEDA CoreMultiplier

// assignment(s): multicycle

// replace(s): id_insn

always @(id_insn_cml_1) begin

  case (id_insn_cml_1[31:26])           // synopsys parallel_case

`ifdef UNUSED

    // l.lwz

    `OR1200_OR32_LWZ:

      multicycle = `OR1200_TWO_CYCLES;

    // l.lbz

    `OR1200_OR32_LBZ:

      multicycle = `OR1200_TWO_CYCLES;

    // l.lbs

    `OR1200_OR32_LBS:

      multicycle = `OR1200_TWO_CYCLES;

    // l.lhz

    `OR1200_OR32_LHZ:

      multicycle = `OR1200_TWO_CYCLES;

    // l.lhs

    `OR1200_OR32_LHS:

      multicycle = `OR1200_TWO_CYCLES;

    // l.sw

    `OR1200_OR32_SW:

      multicycle = `OR1200_TWO_CYCLES;

    // l.sb

    `OR1200_OR32_SB:

      multicycle = `OR1200_TWO_CYCLES;

    // l.sh

    `OR1200_OR32_SH:

      multicycle = `OR1200_TWO_CYCLES;

`endif

    // ALU instructions except the one with immediate

    `OR1200_OR32_ALU:

      multicycle = id_insn_cml_1[`OR1200_ALUMCYC_POS];

    // Single cycle instructions

    default: begin

      multicycle = `OR1200_ONE_CYCLE;

    end

  endcase

end

//

// Decode of imm_signextend

//

// SynEDA CoreMultiplier

// assignment(s): imm_signextend

// replace(s): id_insn

always @(id_insn_cml_1) begin

  case (id_insn_cml_1[31:26])           // synopsys parallel_case

        // l.addi

        `OR1200_OR32_ADDI:

                imm_signextend = 1'b1;

        // l.addic

        `OR1200_OR32_ADDIC:

                imm_signextend = 1'b1;

        // l.xori

        `OR1200_OR32_XORI:

                imm_signextend = 1'b1;

        // l.muli

`ifdef OR1200_MULT_IMPLEMENTED

        `OR1200_OR32_MULI:

                imm_signextend = 1'b1;

`endif

        // l.maci

`ifdef OR1200_MAC_IMPLEMENTED

        `OR1200_OR32_MACI:

                imm_signextend = 1'b1;

`endif

        // SFXX insns with immediate

        `OR1200_OR32_SFXXI:

                imm_signextend = 1'b1;

        // Instructions with no or zero extended immediate

        default: begin

                imm_signextend = 1'b0;

        end

endcase

end

//

// LSU addr offset

//

always @(lsu_op or ex_insn) begin

        lsu_addrofs[10:0] = ex_insn[10:0];

        case(lsu_op)    // synopsys parallel_case

                `OR1200_LSUOP_SW, `OR1200_LSUOP_SH, `OR1200_LSUOP_SB :

                        lsu_addrofs[31:11] = {{16{ex_insn[25]}}, ex_insn[25:21]};

                default :

                        lsu_addrofs[31:11] = {{16{ex_insn[15]}}, ex_insn[15:11]};

        endcase

end

//

// Register file write address

//

// SynEDA CoreMultiplier

// assignment(s): rf_addrw

// replace(s): rf_addrw, pre_branch_op, id_insn

always @(posedge clk or posedge rst) begin

        if (rst)

                rf_addrw <= #1 5'd0;

        else begin  rf_addrw <= rf_addrw_cml_1; if (!ex_freeze & id_freeze)

                rf_addrw <= #1 5'd00;

        else if (!ex_freeze)

                case (pre_branch_op_cml_1)      // synopsys parallel_case

                        `OR1200_BRANCHOP_JR, `OR1200_BRANCHOP_BAL:

                                rf_addrw <= #1 5'd09;   // link register r9

                        default:

                                rf_addrw <= #1 id_insn_cml_1[25:21];

                endcase end

end

//

// rf_addrw in wb stage (used in forwarding logic)

//

// SynEDA CoreMultiplier

// assignment(s): wb_rfaddrw

// replace(s): rf_addrw, wb_rfaddrw

always @(posedge clk or posedge rst) begin

        if (rst)

                wb_rfaddrw <= #1 5'd0;

        else begin  wb_rfaddrw <= wb_rfaddrw_cml_1; if (!wb_freeze)

                wb_rfaddrw <= #1 rf_addrw_cml_1; end

end

//

// Instruction latch in id_insn

//

// SynEDA CoreMultiplier

// assignment(s): id_insn

// replace(s): id_insn

always @(posedge clk or posedge rst) begin

        if (rst)

                id_insn <= #1 {`OR1200_OR32_NOP, 26'h041_0000};

        else begin  id_insn <= id_insn_cml_1; if (flushpipe)

                id_insn <= #1 {`OR1200_OR32_NOP, 26'h041_0000};        // id_insn[16] must be 1

        else if (!id_freeze) begin

                id_insn <= #1 if_insn;

`ifdef OR1200_VERBOSE

// synopsys translate_off

                $display("%t: id_insn <= %h", $time, if_insn);

// synopsys translate_on

`endif

        end end

end

//

// Instruction latch in ex_insn

//

// SynEDA CoreMultiplier

// assignment(s): ex_insn

// replace(s): ex_insn, id_insn

always @(posedge clk or posedge rst) begin

        if (rst)

                ex_insn <= #1 {`OR1200_OR32_NOP, 26'h041_0000};

        else begin  ex_insn <= ex_insn_cml_1; if (!ex_freeze & id_freeze | flushpipe)

                ex_insn <= #1 {`OR1200_OR32_NOP, 26'h041_0000}; // ex_insn[16] must be 1

        else if (!ex_freeze) begin

                ex_insn <= #1 id_insn_cml_1;

`ifdef OR1200_VERBOSE

// synopsys translate_off

                $display("%t: ex_insn <= %h", $time, id_insn);

// synopsys translate_on

`endif

        end end

end

//

// Instruction latch in wb_insn

//

// SynEDA CoreMultiplier

// assignment(s): wb_insn

// replace(s): ex_insn, wb_insn

always @(posedge clk or posedge rst) begin

        if (rst)

                wb_insn <= #1 {`OR1200_OR32_NOP, 26'h041_0000};

        else begin  wb_insn <= wb_insn_cml_1; if (flushpipe)

                wb_insn <= #1 {`OR1200_OR32_NOP, 26'h041_0000}; // wb_insn[16] must be 1

        else if (!wb_freeze) begin

                wb_insn <= #1 ex_insn_cml_1;

        end end

end

//

// Decode of sel_imm

//

// SynEDA CoreMultiplier

// assignment(s): sel_imm

// replace(s): sel_imm

always @(posedge clk or posedge rst) begin

        if (rst)

                sel_imm <= #1 1'b0;

        else begin  sel_imm <= sel_imm_cml_1; if (!id_freeze) begin

          case (if_insn[31:26])         // synopsys parallel_case

            // j.jalr

            `OR1200_OR32_JALR:

              sel_imm <= #1 1'b0;

            // l.jr

            `OR1200_OR32_JR:

              sel_imm <= #1 1'b0;

            // l.rfe

            `OR1200_OR32_RFE:

              sel_imm <= #1 1'b0;

            // l.mfspr

            `OR1200_OR32_MFSPR:

              sel_imm <= #1 1'b0;

            // l.mtspr

            `OR1200_OR32_MTSPR:

              sel_imm <= #1 1'b0;

            // l.sys, l.brk and all three sync insns

            `OR1200_OR32_XSYNC:

              sel_imm <= #1 1'b0;

            // l.mac/l.msb

`ifdef OR1200_MAC_IMPLEMENTED

            `OR1200_OR32_MACMSB:

              sel_imm <= #1 1'b0;

`endif

            // l.sw

            `OR1200_OR32_SW:

              sel_imm <= #1 1'b0;

            // l.sb

            `OR1200_OR32_SB:

              sel_imm <= #1 1'b0;

            // l.sh

            `OR1200_OR32_SH:

              sel_imm <= #1 1'b0;

            // ALU instructions except the one with immediate

            `OR1200_OR32_ALU:

              sel_imm <= #1 1'b0;

            // SFXX instructions

            `OR1200_OR32_SFXX:

              sel_imm <= #1 1'b0;

`ifdef OR1200_OR32_CUST5

            // l.cust5 instructions

            `OR1200_OR32_CUST5:

              sel_imm <= #1 1'b0;

`endif

            // l.nop

            `OR1200_OR32_NOP:

              sel_imm <= #1 1'b0;

            // All instructions with immediates

            default: begin

              sel_imm <= #1 1'b1;

            end

          endcase