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// alu op decode:
// Decodes instruction to corresponding alu_op if alu_ctrl is set to ALU_CTRL_OP
// otherwise, if it is ALU_CTRL_ADD, then its sets alu_op to ALU_OP_ADD.
// This is used if the cpu needs to add two values for example to calculate
// the next address of the pc.
module alu_op_decode (
input [6:0] opcode,
input [2:0] funct3,
input [6:0] funct7,
input alu_ctrl,
output reg [3:0] alu_op
);
`include "include/consts.vh"
always @ (*) begin
if (alu_ctrl == ALU_CTRL_ADD) begin
alu_op = ALU_OP_ADD;
end else if (opcode == OPCODE_REG || opcode == OPCODE_IMM) begin // instruction is of r-type or i-type
case (funct3)
FUNCT3_ALU_ADD_SUB: alu_op = (opcode == OPCODE_REG && funct7 == FUNCT7_ALU_SUB) ? ALU_OP_SUB : ALU_OP_ADD;
FUNCT3_ALU_SLL: alu_op = ALU_OP_SLL;
FUNCT3_ALU_SLT: alu_op = ALU_OP_SLT;
FUNCT3_ALU_SLTU: alu_op = ALU_OP_SLTU;
FUNCT3_ALU_XOR: alu_op = ALU_OP_XOR;
FUNCT3_ALU_SR: alu_op = funct7 == FUNCT7_ALU_SRL ? ALU_OP_SRL : ALU_OP_SRA;
FUNCT3_ALU_OR: alu_op = ALU_OP_OR;
FUNCT3_ALU_AND: alu_op = ALU_OP_AND;
default: alu_op = ALU_OP_ADD;
endcase
end else if (opcode == OPCODE_BRANCH) begin // instruction is of b-type
case (funct3)
// compare equality
FUNCT3_BRANCH_BEQ: alu_op = ALU_OP_SUB;
FUNCT3_BRANCH_BNE: alu_op = ALU_OP_SUB;
// compare signed
FUNCT3_BRANCH_BLT: alu_op = ALU_OP_SLT;
FUNCT3_BRANCH_BGE: alu_op = ALU_OP_SLT;
// compare unsigned
FUNCT3_BRANCH_BLTU: alu_op = ALU_OP_SLTU;
FUNCT3_BRANCH_BGEU: alu_op = ALU_OP_SLTU;
default: alu_op = ALU_OP_ADD;
endcase
end else begin
alu_op = ALU_OP_ADD;
end
end
endmodule
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