tiny-gpu入门4: ALU模块分析
ALU模块代码如下：
// 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 inactiveinput 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 = EXECUTEif (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 instructioncase (decoded_alu_arithmetic_mux)ADD: begin alu_out_reg <= rs + rt;endSUB: begin alu_out_reg <= rs - rt;endMUL: begin alu_out_reg <= rs * rt;endDIV: begin alu_out_reg <= rs / rt;endendcaseendendendend
endmoduleALU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：
localparam IDLE = 3'b000, // Waiting to startFETCH = 3'b001, // Fetch instructions from program memoryDECODE = 3'b010, // Decode instructions into control signalsREQUEST = 3'b011, // Request data from registers or memoryWAIT = 3'b100, // Wait for response from memory if necessaryEXECUTE = 3'b101, // Execute ALU and PC calculationsUPDATE = 3'b110, // Update registers, NZP, and PCDONE = 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 。