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// Richard CPU
// Copyright (C) 2026 Richard Qin
// This program is free software: 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
// MERCHANTABILITY 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. If not, see <https://www.gnu.org/licenses/>.
// File path: rtl/core/if_id_reg.sv
// Description: IF/ID Pipeline Register
// Latches PC and instruction from IF stage and passes them to ID stage.
// Supports stall (freeze) and flush (insert NOP).
module if_id_reg
import rv64_pkg::*;
(
input logic clk,
input logic rst,
// Pipeline Control from Hazard Unit
input logic stall_en, // 1: Freeze pipeline register state
input logic flush_en, // 1: Clear register (insert NOP)
// Inputs from IF Stage
input logic [XLEN-1:0] if_pc, // PC from IF stage
input logic [INST_WIDTH-1:0] if_instr, // Fetched instruction from IF stage
input logic [2:0] if_instr_len,
input logic if_instr_access_fault,
input logic if_instr_page_fault,
// Outputs to ID Stage
output logic [XLEN-1:0] id_pc, // Passed to ID stage
output logic [INST_WIDTH-1:0] id_instr, // Passed to ID stage
output logic [2:0] id_instr_len,
output logic id_instr_access_fault,
output logic id_instr_page_fault
);
localparam logic [31:0] NOP_INSTR = 32'h0000_0013;
always_ff @(posedge clk) begin
if (rst) begin
id_pc <= '0;
id_instr <= NOP_INSTR;
id_instr_len <= 3'd4;
id_instr_access_fault <= 1'b0;
id_instr_page_fault <= 1'b0;
end else if (flush_en) begin
id_pc <= '0;
id_instr <= NOP_INSTR;
id_instr_len <= 3'd4;
id_instr_access_fault <= 1'b0;
id_instr_page_fault <= 1'b0;
end else if (~stall_en) begin
id_pc <= if_pc;
id_instr <= if_instr;
id_instr_len <= if_instr_len;
id_instr_access_fault <= if_instr_access_fault;
id_instr_page_fault <= if_instr_page_fault;
end
// If stall_en && !flush_en, maintain current state
end
endmodule
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