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`timescale 1ns / 1ps

/*

 * File         : Hazard_Detection.v

 * Project      : University of Utah, XUM Project MIPS32 core

 * Creator(s)   : Grant Ayers (ayers@cs.utah.edu)

 *

 * Modification History:

 *   Rev   Date         Initials  Description of Change

 *   1.0   23-Jul-2011  GEA       Initial design.

 *   2.0   26-May-2012  GEA       Release version with CP0.

 *   2.01   1-Nov-2012  GEA       Fixed issue with Jal.

 *

 * Standards/Formatting:

 *   Verilog 2001, 4 soft tab, wide column.

 *

 * Description:

 *   Hazard Detection and Forward Control. This is the glue that allows a

 *   pipelined processor to operate efficiently and correctly in the presence

 *   of data, structural, and control hazards. For each pipeline stage, it

 *   detects whether that stage requires data that is still in the pipeline,

 *   and whether that data may be forwarded or if the pipeline must be stalled.

 *

 *   This module is heavily commented. Read below for more information.

 */

module Hazard_Detection(

    input  [7:0] DP_Hazards,

    input  [4:0] ID_Rs,

    input  [4:0] ID_Rt,

    input  [4:0] EX_Rs,

    input  [4:0] EX_Rt,

    input  [4:0] EX_RtRd,

    input  [4:0] MEM_RtRd,

    input  [4:0] WB_RtRd,

    input  EX_Link,

    input  EX_RegWrite,

    input  MEM_RegWrite,

    input  WB_RegWrite,

    input  MEM_MemRead,

    input  MEM_MemWrite,        // Needed for Store Conditional which writes to a register

    input  InstMem_Read,

    input  InstMem_Ready,

    input  Mfc0,                // Using fwd mux; not part of haz/fwd.

    input  IF_Exception_Stall,

    input  ID_Exception_Stall,

    input  EX_Exception_Stall,

    input  M_Stall_Controller,  // Determined by data memory controller

    output IF_Stall,

    output ID_Stall,

    output EX_Stall,

    output M_Stall,

    output WB_Stall,

    output [1:0] ID_RsFwdSel,

    output [1:0] ID_RtFwdSel,

    output [1:0] EX_RsFwdSel,

    output [1:0] EX_RtFwdSel,

    output M_WriteDataFwdSel

    );

    /* Hazard and Forward Detection

     *

     * Most instructions read from one or more registers. Normally this occurs in

     * the ID stage. However, frequently the register file in the ID stage is stale

     * when one or more forward stages in the pipeline (EX, MEM, or WB) contains

     * an instruction which will eventually update it but has not yet done so.

     *

     * A hazard condition is created when a forward pipeline stage is set to write

     * the same register that a current pipeline stage (e.g. in ID) needs to read.

     * The solution is to stall the current stage (and effectively all stages behind

     * it) or bypass (forward) the data from forward stages. Fortunately forwarding

     * works for most combinations of instructions.

     *

     * Hazard and Forward conditions are handled based on two simple rules:

     * "Wants" and "Needs." If an instruction "wants" data in a certain pipeline

     * stage, and that data is available further along in the pipeline, it will

     * be forwarded. If it "needs" data and the data is not yet available for forwarding,

     * the pipeline stage stalls. If it does not want or need data in a certain

     * stage, forwarding is disabled and a stall will not occur. This is important

     * for instructions which insert custom data, such as jal or movz.

     *

     * Currently, "Want" and "Need" conditions are defined for both Rs data and Rt

     * data (the two read registers in MIPS), and these conditions exist in the

     * ID and EX pipeline stages. This is a total of eight condition bits.

     *

     * A unique exception exists with Store instructions, which don't need the

     * "Rt" data until the MEM stage. Because data doesn't change in WB, and WB

     * is the only stage following MEM, forwarding is *always* possible from

     * WB to Mem. This unit handles this situation, and a condition bit is not

     * needed.

     *

     * When data is needed from the MEM stage by a previous stage (ID or EX), the

     * decision to forward or stall is based on whether MEM is accessing memory

     * (stall) or not (forward). Normally store instructions don't write to registers

     * and thus are never needed for a data dependence, so the signal 'MEM_MemRead'

     * is sufficient to determine. Because of the Store Conditional instruction,

     * however, 'MEM_MemWrite' must also be considered because it writes to a register.

     *

     */

    wire WantRsByID, NeedRsByID, WantRtByID, NeedRtByID, WantRsByEX, NeedRsByEX, WantRtByEX, NeedRtByEX;

    assign WantRsByID = DP_Hazards[7];

    assign NeedRsByID = DP_Hazards[6];

    assign WantRtByID = DP_Hazards[5];

    assign NeedRtByID = DP_Hazards[4];

    assign WantRsByEX = DP_Hazards[3];

    assign NeedRsByEX = DP_Hazards[2];

    assign WantRtByEX = DP_Hazards[1];

    assign NeedRtByEX = DP_Hazards[0];

    // Trick allowed by RegDst = 0 which gives Rt. MEM_Rt is only used on

    // Data Memory write operations (stores), and RegWrite is always 0 in this case.

    wire [4:0] MEM_Rt = MEM_RtRd;

    // Forwarding should not happen when the src/dst register is $zero

    wire EX_RtRd_NZ  = (EX_RtRd  != 5'b00000);

    wire MEM_RtRd_NZ = (MEM_RtRd != 5'b00000);

    wire WB_RtRd_NZ  = (WB_RtRd  != 5'b00000);

    // ID Dependencies

    wire Rs_IDEX_Match  = (ID_Rs == EX_RtRd)  & EX_RtRd_NZ  & (WantRsByID | NeedRsByID) & EX_RegWrite;

    wire Rt_IDEX_Match  = (ID_Rt == EX_RtRd)  & EX_RtRd_NZ  & (WantRtByID | NeedRtByID) & EX_RegWrite;

    wire Rs_IDMEM_Match = (ID_Rs == MEM_RtRd) & MEM_RtRd_NZ & (WantRsByID | NeedRsByID) & MEM_RegWrite;

    wire Rt_IDMEM_Match = (ID_Rt == MEM_RtRd) & MEM_RtRd_NZ & (WantRtByID | NeedRtByID) & MEM_RegWrite;

    wire Rs_IDWB_Match  = (ID_Rs == WB_RtRd)  & WB_RtRd_NZ  & (WantRsByID | NeedRsByID) & WB_RegWrite;

    wire Rt_IDWB_Match  = (ID_Rt == WB_RtRd)  & WB_RtRd_NZ  & (WantRtByID | NeedRtByID) & WB_RegWrite;

    // EX Dependencies

    wire Rs_EXMEM_Match = (EX_Rs == MEM_RtRd) & MEM_RtRd_NZ & (WantRsByEX | NeedRsByEX) & MEM_RegWrite;

    wire Rt_EXMEM_Match = (EX_Rt == MEM_RtRd) & MEM_RtRd_NZ & (WantRtByEX | NeedRtByEX) & MEM_RegWrite;

    wire Rs_EXWB_Match  = (EX_Rs == WB_RtRd)  & WB_RtRd_NZ  & (WantRsByEX | NeedRsByEX) & WB_RegWrite;

    wire Rt_EXWB_Match  = (EX_Rt == WB_RtRd)  & WB_RtRd_NZ  & (WantRtByEX | NeedRtByEX) & WB_RegWrite;

    // MEM Dependencies

    wire Rt_MEMWB_Match = (MEM_Rt == WB_RtRd) & WB_RtRd_NZ  & WB_RegWrite;

    // ID needs data from EX  : Stall

    wire ID_Stall_1 = (Rs_IDEX_Match  &  NeedRsByID);

    wire ID_Stall_2 = (Rt_IDEX_Match  &  NeedRtByID);

    // ID needs data from MEM : Stall if mem access

    wire ID_Stall_3 = (Rs_IDMEM_Match &  (MEM_MemRead | MEM_MemWrite) & NeedRsByID);

    wire ID_Stall_4 = (Rt_IDMEM_Match &  (MEM_MemRead | MEM_MemWrite) & NeedRtByID);

    // ID wants data from MEM : Forward if not mem access

    wire ID_Fwd_1   = (Rs_IDMEM_Match & ~(MEM_MemRead | MEM_MemWrite));

    wire ID_Fwd_2   = (Rt_IDMEM_Match & ~(MEM_MemRead | MEM_MemWrite));

    // ID wants/needs data from WB  : Forward

    wire ID_Fwd_3   = (Rs_IDWB_Match);

    wire ID_Fwd_4   = (Rt_IDWB_Match);

    // EX needs data from MEM : Stall if mem access

    wire EX_Stall_1 = (Rs_EXMEM_Match &  (MEM_MemRead | MEM_MemWrite) & NeedRsByEX);

    wire EX_Stall_2 = (Rt_EXMEM_Match &  (MEM_MemRead | MEM_MemWrite) & NeedRtByEX);

    // EX wants data from MEM : Forward if not mem access

    wire EX_Fwd_1   = (Rs_EXMEM_Match & ~(MEM_MemRead | MEM_MemWrite));

    wire EX_Fwd_2   = (Rt_EXMEM_Match & ~(MEM_MemRead | MEM_MemWrite));

    // EX wants/needs data from WB  : Forward

    wire EX_Fwd_3   = (Rs_EXWB_Match);

    wire EX_Fwd_4   = (Rt_EXWB_Match);

    // MEM needs data from WB : Forward

    wire MEM_Fwd_1  = (Rt_MEMWB_Match);

    // Stalls and Control Flow Final Assignments    

    assign WB_Stall = M_Stall;

    assign  M_Stall = IF_Stall | M_Stall_Controller;

    assign EX_Stall = (EX_Stall_1 | EX_Stall_2 | EX_Exception_Stall) | M_Stall;

    assign ID_Stall = (ID_Stall_1 | ID_Stall_2 | ID_Stall_3 | ID_Stall_4 | ID_Exception_Stall) | EX_Stall;

    assign IF_Stall = InstMem_Read | InstMem_Ready | IF_Exception_Stall;

    // Forwarding Control Final Assignments

    assign ID_RsFwdSel = (ID_Fwd_1) ? 2'b01 : ((ID_Fwd_3) ? 2'b10 : 2'b00);

    assign ID_RtFwdSel = (Mfc0) ? 2'b11 : ((ID_Fwd_2) ? 2'b01 : ((ID_Fwd_4) ? 2'b10 : 2'b00));

    assign EX_RsFwdSel = (EX_Link) ? 2'b11 : ((EX_Fwd_1) ? 2'b01 : ((EX_Fwd_3) ? 2'b10 : 2'b00));

    assign EX_RtFwdSel = (EX_Link)  ? 2'b11 : ((EX_Fwd_2) ? 2'b01 : ((EX_Fwd_4) ? 2'b10 : 2'b00));

    assign M_WriteDataFwdSel = MEM_Fwd_1;

endmodule