/*                                                                      
 Copyright 2020 Blue Liang, liangkangnan@163.com
                                                                         
 Licensed under the Apache License, Version 2.0 (the "License");         
 you may not use this file except in compliance with the License.        
 You may obtain a copy of the License at                                 
                                                                         
     http://www.apache.org/licenses/LICENSE-2.0                          
                                                                         
 Unless required by applicable law or agreed to in writing, software    
 distributed under the License is distributed on an "AS IS" BASIS,       
 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 See the License for the specific language governing permissions and     
 limitations under the License.                                          
 */

`include "defines.v"


// core local interruptor module
// 核心中断管理、仲裁模块
module clint(

    input wire clk,
    input wire rst,

    // from id
    input wire[`INT_BUS] int_flag_i,
    input wire[`InstBus] inst_i,
    input wire[`InstAddrBus] inst_addr_i,

    // from ctrl
    input wire[`Hold_Flag_Bus] hold_flag_i,

    // from csr_reg
    input wire[`RegBus] data_i,

    // to csr_reg
    output reg we_o,
    output reg[`MemAddrBus] waddr_o,
    output reg[`MemAddrBus] raddr_o,
    output reg[`RegBus] data_o,

    // to ex
    output reg[`InstAddrBus] int_addr_o,
    output reg int_assert_o

    );


    // 状态定义
    localparam STATE_IDLE      = 4'b0001;
    localparam STATE_ASSERT    = 4'b0010;
    localparam STATE_WAIT_MRET = 4'b0100;
    localparam STATE_MRET      = 4'b1000;

    reg[3:0] state;
    reg[3:0] next_state;


    // 状态更新
    always @ (posedge clk) begin
        if (rst == `RstEnable) begin
            state <= STATE_IDLE;
        end else begin
            state <= next_state;
        end
    end

    // 状态切换
    always @ (*) begin
        if (rst == `RstEnable) begin
            next_state <= STATE_IDLE;
        end else begin
            case (state)
                STATE_IDLE: begin
                    // 目前只要外设有中断信号发出就立马响应.
                    // 后续增加中断优先级(嵌套)时需要修改这里的逻辑
                    if (int_flag_i != `INT_NONE) begin
                        next_state <= STATE_ASSERT;
                    end else begin
                        next_state <= STATE_IDLE;
                    end
                end
                STATE_ASSERT: begin
                    next_state <= STATE_WAIT_MRET;
                end
                STATE_WAIT_MRET: begin
                    if (inst_i == `INST_MRET) begin
                        next_state <= STATE_MRET;
                    end else begin
                        next_state <= STATE_WAIT_MRET;
                    end
                end
                STATE_MRET: begin
                    next_state <= STATE_IDLE;
                end
                default: begin
                    next_state <= STATE_IDLE;
                end
            endcase
        end
    end

    // 根据不同的状态，读取对应的CSR寄存器
    always @ (*) begin
        if (rst == `RstEnable) begin
            raddr_o <= `ZeroWord;
        end else begin
            case (state)
                STATE_IDLE: begin
                    raddr_o <= {20'h0, `CSR_MTVEC};
                end
                STATE_ASSERT: begin
                    raddr_o <= {20'h0, `CSR_MTVEC};
                end
                STATE_WAIT_MRET: begin
                    raddr_o <= {20'h0, `CSR_MEPC};
                end
                STATE_MRET: begin
                    raddr_o <= {20'h0, `CSR_MEPC};
                end
                default: begin
                    raddr_o <= {20'h0, `CSR_MTVEC};
                end
            endcase
        end
    end

    // 发出中断信号
    // 中断响应和中断返回时都要发
    always @ (posedge clk) begin
        if (rst == `RstEnable) begin
            int_assert_o <= `INT_DEASSERT;
            int_addr_o <= `ZeroWord;
        end else begin
            case (state)
                STATE_ASSERT: begin
                    int_assert_o <= `INT_ASSERT;
                    int_addr_o <= data_i;
                end
                STATE_MRET: begin
                    int_assert_o <= `INT_ASSERT;
                    int_addr_o <= data_i;
                end
                default: begin
                    int_assert_o <= `INT_DEASSERT;
                    int_addr_o <= `ZeroWord;
                end
            endcase
        end
    end

    // 根据不同的状态，写对应的CSR寄存器
    always @ (posedge clk) begin
        if (rst == `RstEnable) begin
            we_o <= `WriteDisable;
            waddr_o <= `ZeroWord;
            data_o <= `ZeroWord;
        end else begin
            if (state == STATE_ASSERT) begin
                we_o <= `WriteEnable;
                waddr_o <= {20'h0, `CSR_MEPC};
                data_o <= inst_addr_i;
            end else begin
                we_o <= `WriteEnable;
                waddr_o <= {20'h0, `CSR_MCAUSE};
                data_o <= {24'h0, int_flag_i};
            end
        end
    end

endmodule