tiny-gpu入门4: ALU模块分析

ALU模块代码如下：

bash 复制代码

// ARITHMETIC-LOGIC UNIT
// > Executes computations on register values
// > In this minimal implementation, the ALU supports the 4 basic arithmetic operations
// > Each thread in each core has it's own ALU
// > ADD, SUB, MUL, DIV instructions are all executed here
module alu (
    input wire clk,
    input wire reset,
    input wire enable, // If current block has less threads then block size, some ALUs will be inactive

    input reg [2:0] core_state,

    input reg [1:0] decoded_alu_arithmetic_mux,
    input reg decoded_alu_output_mux,

    input reg [7:0] rs,
    input reg [7:0] rt,
    output wire [7:0] alu_out
);
    localparam ADD = 2'b00,
        SUB = 2'b01,
        MUL = 2'b10,
        DIV = 2'b11;

    reg [7:0] alu_out_reg;
    assign alu_out = alu_out_reg;

    always @(posedge clk) begin 
        if (reset) begin 
            alu_out_reg <= 8'b0;
        end else if (enable) begin
            // Calculate alu_out when core_state = EXECUTE
            if (core_state == 3'b101) begin 
                if (decoded_alu_output_mux == 1) begin 
                    // Set values to compare with NZP register in alu_out[2:0]
                    alu_out_reg <= {5'b0, (rs - rt > 0), (rs - rt == 0), (rs - rt < 0)};
                end else begin 
                    // Execute the specified arithmetic instruction
                    case (decoded_alu_arithmetic_mux)
                        ADD: begin 
                            alu_out_reg <= rs + rt;
                        end
                        SUB: begin 
                            alu_out_reg <= rs - rt;
                        end
                        MUL: begin 
                            alu_out_reg <= rs * rt;
                        end
                        DIV: begin 
                            alu_out_reg <= rs / rt;
                        end
                    endcase
                end
            end
        end
    end
endmodule

ALU会基于控制信号：[2:0] core_state、decoded_alu_output_mux和[1:0]decoded_alu_arithmetic_mux，对寄存器rs和rt的值执行具体的计算。

core_state为执行态（EXECUTE）的话开始执行具体的操作，所有状态如下：

bash 复制代码

    localparam IDLE = 3'b000, // Waiting to start
        FETCH = 3'b001,       // Fetch instructions from program memory
        DECODE = 3'b010,      // Decode instructions into control signals
        REQUEST = 3'b011,     // Request data from registers or memory
        WAIT = 3'b100,        // Wait for response from memory if necessary
        EXECUTE = 3'b101,     // Execute ALU and PC calculations
        UPDATE = 3'b110,      // Update registers, NZP, and PC
        DONE = 3'b111;        // Done executing this block

只有EXECUTE态才执行操作。

接下来有两个分支以及两个控制信号：decoded_alu_output_mux和[1:0]decoded_alu_arithmetic_mux，decoded_alu_output_mux用来控制ALU的操作，decoded_alu_arithmetic_mux控制执行的是具体的运算指令（即加减乘除）。

decoded_alu_output_mux为0则ALU会执行具体的运算指令，为1的话则会执行BRnzp指令，如果NZP寄存器符合指令中的' NZP '条件，则分支指令跳转到另一行程序内存，以这种方式来实现循环和条件指令的执行(CMP指令结果判定)。

最终结果输出到alu_out 。