E203是两级流水线结构，第一级是IFU进行取指操作，第二级包括译码、执行、交付和写回等功能。架构图如下：
https://www.cnblogs.com/images/likecs_com/mikewolf2002/1519640/o_pipeline.jpg
译码模块就是把机器码翻译成对应的输出功能。E203支持RV32IMAC,它的译码器模块是纯的组合电路实现，相对比较简单。只要熟悉了RiscV的指令规范，很容易看懂。
译码模块的输入信号来自于IFU模块，包括以下信号：
input [`E203_INSTR_SIZE-1:0] i_instr, //来自IFU的32位指令 input [`E203_PC_SIZE-1:0] i_pc, //来自IFU的当前指令PC input i_prdt_taken, //预测需要跳转，来自IFU模块的分支预测单元，对分支跳转指令有用,如果当前指令为分支跳转指令，该信号会被传递到相应的info_bus input i_misalgn, // 表示当前指令遇到了分支跳转异常 input i_buserr, // 表面当前指令遭遇了取指存储器访问错误 input i_muldiv_b2b, // 前后临接的mul和div指令时置1,如果当前指令为乘法除法指令，该信号传递到相应的info_bus input dbg_mode, //debug模式下，忽视dret指令异常的情况
译码模块的输出包括以下信号：
output dec_rs1x0, // 该指令源操作数1为x0 output dec_rs2x0, // 该指令源操作数2为x0 output dec_rs1en, //该指令需要读取源操作数1 output dec_rs2en, //该指令需要读取源操作数2 output dec_rdwen, //该指令需要写结果操作数到目的寄存器 output [`E203_RFIDX_WIDTH-1:0] dec_rs1idx, //源操作数1的寄存器索引 output [`E203_RFIDX_WIDTH-1:0] dec_rs2idx, //源操作数2的寄存器索引 output [`E203_RFIDX_WIDTH-1:0] dec_rdidx, //目的寄存器索引 output [`E203_DECINFO_WIDTH-1:0] dec_info, //该指令的其它信息，被打包成一组宽信号，称之为信息总线(info bus). output [`E203_XLEN-1:0] dec_imm, //该指令使用的立即数值。 output [`E203_PC_SIZE-1:0] dec_pc, // 该指令的pc值，等于输入的i_pc output dec_misalgn, //指令遇到了分支跳转异常，等于输入的misalgn output dec_buserr, //指令遭遇了取指存储器访问错误，等于输入的i_buserr output dec_ilegl, //解码后，发现该指令是非法指令 output dec_mulhsu, //指令为 mulh或mulhsu或mulhu，这些乘法指令都把结果的高32位放到目的寄存器 output dec_mul , //指令为乘法指令 output dec_div , //指令为除法指令 output dec_rem ,//指数为取余数指令 output dec_divu ,//指令为无符号数除法指令 output dec_remu ,//指令为无符号数取余数指令 output dec_rv32, //该指令为32位指令，为0的话则是16位的压缩指令 output dec_bjp, //该指令为跳转指令，jal 或者jalr或者bxx指令 output dec_jal, //该指令为jal指令 output dec_jalr, //该指令为jalr指令 output dec_bxx, //该指令为bxx output [`E203_RFIDX_WIDTH-1:0] dec_jalr_rs1idx, //jalr指令中rs1寄存器索引 output [`E203_XLEN-1:0] dec_bjp_imm //bjp指令中的立即数
下面代码判断指令是32位指令还是16位指令的代码。操作码的低2位11,且3到5位不为111，则为rv32指令，否则为16位指令，因为E203不支持rv64，所以用一位信号就可以表示rv32和rvc了。
wire [32-1:0] rv32_instr = i_instr; wire [16-1:0] rv16_instr = i_instr[15:0]; //指令的低7位为操作码 wire [6:0] opcode = rv32_instr[6:0]; wire opcode_1_0_00 = (opcode[1:0] == 2'b00); wire opcode_1_0_01 = (opcode[1:0] == 2'b01); wire opcode_1_0_10 = (opcode[1:0] == 2'b10); wire opcode_1_0_11 = (opcode[1:0] == 2'b11); //为32位指令或者16位指令 wire rv32 = (~(i_instr[4:2] == 3'b111)) & opcode_1_0_11;
下面代码为取出32位指令和16位指令的关键编码段。RVC的编码比较复杂，有些指令寄存器为3位表示，有些为5位表示，立即数通常都有旋转，旋转格式还很多。具体参见https://www.cnblogs.com/mikewolf2002/p/9884789.html
wire [4:0] rv32_rd = rv32_instr[11:7]; // 目的寄存器索引 wire [2:0] rv32_func3 = rv32_instr[14:12];//func3段 wire [4:0] rv32_rs1 = rv32_instr[19:15];//源操作数1寄存器索引 wire [4:0] rv32_rs2 = rv32_instr[24:20];//源操作数2寄存器索引 wire [6:0] rv32_func7 = rv32_instr[31:25];//func7段 //CR 15-12 func4, 11-7 rd/rs1, 6-2 rs2, 0-1 opcode //CI 15-13 func3, 12 imm,11-7 rd/rs1,6-2 imm, 0-1 opcode wire [4:0] rv16_rd = rv32_rd; wire [4:0] rv16_rs1 = rv16_rd; wire [4:0] rv16_rs2 = rv32_instr[6:2]; //CIW,CL,CS,CA,CB,CJ //rdd=rd', rss1=rs1', rss2=rs2' short register, x8-x15, f8-f15 wire [4:0] rv16_rdd = {2'b01,rv32_instr[4:2]}; wire [4:0] rv16_rss1 = {2'b01,rv32_instr[9:7]}; wire [4:0] rv16_rss2 = rv16_rdd; wire [2:0] rv16_func3 = rv32_instr[15:13]
下面的代码产生指令中的关键信息判断，用于后面的代码复用。
// We generate the signals and reused them as much as possible to save gatecounts
wire opcode_4_2_000 = (opcode[4:2] == 3'b000);
wire opcode_4_2_001 = (opcode[4:2] == 3'b001);
wire opcode_4_2_010 = (opcode[4:2] == 3'b010);
wire opcode_4_2_011 = (opcode[4:2] == 3'b011);
wire opcode_4_2_100 = (opcode[4:2] == 3'b100);
wire opcode_4_2_101 = (opcode[4:2] == 3'b101);
wire opcode_4_2_110 = (opcode[4:2] == 3'b110);
wire opcode_4_2_111 = (opcode[4:2] == 3'b111);
wire opcode_6_5_00 = (opcode[6:5] == 2'b00);
wire opcode_6_5_01 = (opcode[6:5] == 2'b01);
wire opcode_6_5_10 = (opcode[6:5] == 2'b10);
wire opcode_6_5_11 = (opcode[6:5] == 2'b11);
wire rv32_func3_000 = (rv32_func3 == 3'b000);
wire rv32_func3_001 = (rv32_func3 == 3'b001);
wire rv32_func3_010 = (rv32_func3 == 3'b010);
wire rv32_func3_011 = (rv32_func3 == 3'b011);
wire rv32_func3_100 = (rv32_func3 == 3'b100);
wire rv32_func3_101 = (rv32_func3 == 3'b101);
wire rv32_func3_110 = (rv32_func3 == 3'b110);
wire rv32_func3_111 = (rv32_func3 == 3'b111);
wire rv16_func3_000 = (rv16_func3 == 3'b000);
wire rv16_func3_001 = (rv16_func3 == 3'b001);
wire rv16_func3_010 = (rv16_func3 == 3'b010);
wire rv16_func3_011 = (rv16_func3 == 3'b011);
wire rv16_func3_100 = (rv16_func3 == 3'b100);
wire rv16_func3_101 = (rv16_func3 == 3'b101);
wire rv16_func3_110 = (rv16_func3 == 3'b110);
wire rv16_func3_111 = (rv16_func3 == 3'b111);
wire rv32_func7_0000000 = (rv32_func7 == 7'b0000000);
wire rv32_func7_0100000 = (rv32_func7 == 7'b0100000);
wire rv32_func7_0000001 = (rv32_func7 == 7'b0000001);
wire rv32_func7_0000101 = (rv32_func7 == 7'b0000101);
wire rv32_func7_0001001 = (rv32_func7 == 7'b0001001);
wire rv32_func7_0001101 = (rv32_func7 == 7'b0001101);
wire rv32_func7_0010101 = (rv32_func7 == 7'b0010101);
wire rv32_func7_0100001 = (rv32_func7 == 7'b0100001);
wire rv32_func7_0010001 = (rv32_func7 == 7'b0010001);
wire rv32_func7_0101101 = (rv32_func7 == 7'b0101101);
wire rv32_func7_1111111 = (rv32_func7 == 7'b1111111);
wire rv32_func7_0000100 = (rv32_func7 == 7'b0000100);
wire rv32_func7_0001000 = (rv32_func7 == 7'b0001000);
wire rv32_func7_0001100 = (rv32_func7 == 7'b0001100);
wire rv32_func7_0101100 = (rv32_func7 == 7'b0101100);
wire rv32_func7_0010000 = (rv32_func7 == 7'b0010000);
wire rv32_func7_0010100 = (rv32_func7 == 7'b0010100);
wire rv32_func7_1100000 = (rv32_func7 == 7'b1100000);
wire rv32_func7_1110000 = (rv32_func7 == 7'b1110000);
wire rv32_func7_1010000 = (rv32_func7 == 7'b1010000);
wire rv32_func7_1101000 = (rv32_func7 == 7'b1101000);
wire rv32_func7_1111000 = (rv32_func7 == 7'b1111000);
wire rv32_func7_1010001 = (rv32_func7 == 7'b1010001);
wire rv32_func7_1110001 = (rv32_func7 == 7'b1110001);
wire rv32_func7_1100001 = (rv32_func7 == 7'b1100001);
wire rv32_func7_1101001 = (rv32_func7 == 7'b1101001);
wire rv32_rs1_x0 = (rv32_rs1 == 5'b00000);
wire rv32_rs2_x0 = (rv32_rs2 == 5'b00000);
wire rv32_rs2_x1 = (rv32_rs2 == 5'b00001);
wire rv32_rd_x0 = (rv32_rd == 5'b00000);
wire rv32_rd_x2 = (rv32_rd == 5'b00010);
wire rv16_rs1_x0 = (rv16_rs1 == 5'b00000);
wire rv16_rs2_x0 = (rv16_rs2 == 5'b00000);
wire rv16_rd_x0 = (rv16_rd == 5'b00000);
wire rv16_rd_x2 = (rv16_rd == 5'b00010);
wire rv32_rs1_x31 = (rv32_rs1 == 5'b11111);
wire rv32_rs2_x31 = (rv32_rs2 == 5'b11111);
wire rv32_rd_x31 = (rv32_rd == 5'b11111);