从零开始设计RISC-V处理器——ALU的优化
作者：互联网
系列文章目录
（一）从零开始设计RISC-V处理器——指令系统
（二）从零开始设计RISC-V处理器——单周期处理器的设计
（三）从零开始设计RISC-V处理器——单周期处理器的仿真
（四）从零开始设计RISC-V处理器——ALU的优化
文章目录
前言
在前面的三篇文章中，我们已经完成了单周期CPU的设计，从这篇文章开始对这个单周期的CPU进行优化。
首先是对ALU部件进行优化，因为ALU是CPU最重要的部件之一，所有的运算都在这里进行。之前我们把ALU的运算部件分为算数运算单元（加法器），移位运算（移位器），小于置一（比较器），逻辑运算（逻辑门）几个部分。前面的设计中，加法器直接用verilog中的运算符“+”实现，移位器直接用verilog中的运算符“>>,<<”实现，这样相当于把加法器和移位器的设计交给综合工具自动完成，这样设计的ALU的性能是我们不可掌控的。因此，这篇文章就对加法器和移位器的优化做一个介绍。
一、加法器的设计
常见的加法器有很多种，有兴趣的读者可自行学习，此处主要介绍两种加法器：行波进位加法器和超前进位加法器。
行波进位加法器，是由n个一位全加器串联而成，结构非常简单，上一级的进位输出连接到下一级的进位输入。这种加法器结构简单，使用的逻辑资源很少，但是由于是串行计算，下一级的结果依赖于上一级的进位输出，因此对于多位的加法器，计算一次加法所需要的时间非常久。
如果能够提前把每一级的进位输出计算出来，那么就可以有效缩短延时，这便是所谓的超前进位加法器。
下面开始分析。
首先列出一位全加器的真值表
| Ai | Bi | Ci | Si | Ci+1 |
|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 1 | 0 |
| 0 | 1 | 0 | 1 | 0 |
| 0 | 1 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 | 0 |
| 1 | 0 | 1 | 0 | 1 |
| 1 | 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 | 1 |
由真值表可以得出以下关系：
Si=Ai ^ Bi ^ Ci;
Ci+1=Bi&Ci + Ai&Ci + Ai&Bi=Ai&Bi + (Ai + Bi)&Ci=Gi + Pi&Ci;
这里定义了两个新的信号：Gi=Ai&Bi, Pi=Ai | Bi;
Gi称为进位产生信号，Pi称为进位传递信号。
Gi为1时，一定产生进位，即Ci+1一定为1；
Gi为0且Pi为1时，进位传递，即把Ci的值传递给Ci+1,Ci为0时，Ci+1为0，Ci为1时，Ci+1为1。
将每一级的进位信号表示如下：
C0=Cin;
C1=A0&B0+(A0+B0)&C0=G0+P0&C0
C2=G1+P1&C1=G1+P1&(G0+P0&C0)=G1+P1&G0+P0&P1&C0
C3=G2+P2&C2=G2+P2&G1+P2&P1&G0+P2&P1&P0&C0
C4=G3+P3&G2+P3&P2&G1+P3&P2&P1&G0+P3&P2&P1&P0&C0
……
通过上式可以看到，以这样的方式展开，每一级的进位信号仅仅依赖于Pi,Gi以及C0，这样我们就可以根据加数A和被加数B，以及进位信号C0，同时求出每一级的进位信号Ci，同时可以求出每一级的Si。
通过以上分析，可以得出结论：
行波加法器，串行计算，结构简单（面积小），延时大
超前进位加法器，并行计算，结构复杂（面积大），延时小。
想要设计一个高性能的32位加法器，理论上可以同时并行生成所有的Si和Ci，但是这样会使电路结构非常复杂，极大的增大电路的扇入和扇出，这在实际设计时也是不可行的。
因此，我们采用组内并行和组间并行的方式实现一个32位加法器，即先实现一个4位超前进位加法器，再用4个4位超前进位加法器组间并行实现16位超前进位加法器，再用2个16位超前进位加法器组间并行实现32位超前进位加法器。
其结构如下：
实现代码如下：
//4位CLA部件
module cla_4(p,g,c_in,c,gx,px);
input[3:0] p,g;
input c_in;
output[4:1] c;
output gx,px;
assign c[1] = p[0]&c_in | g[0];
assign c[2] = p[1]&p[0]&c_in | p[1]&g[0] | g[1];
assign c[3] = p[2]&p[1]&p[0]&c_in | p[2]&p[1]&g[0] | p[2]&g[1] | g[2];
assign c[4] = gx | px&c_in;
assign px = p[3]&p[2]&p[1]&p[0];
assign gx = g[3] | p[3]&g[2] | p[3]&p[2]&g[1] | p[3]&p[2]&p[1]&g[0];
endmodule
//32位全加器
module cla_adder32(A,B,cin,result,cout);
input [31:0] A;
input [31:0] B;
input cin;
output[31:0] result;
output cout;
进位产生信号，进位传递信号
wire[31:0] TAG,TAP;
wire[32:1] TAC;
wire[15:0] TAG_0,TAP_0;
wire[3:0] TAG_1,TAP_1;
wire[8:1] TAC_1;
wire[4:1] TAC_2;
assign result = A ^ B ^ {TAC[31:1],cin};
进位产生信号，进位传递信号
assign TAG = A&B;
assign TAP = A|B;
cla_4 cla_0_0( .p(TAP[3:0]), .g(TAG[3:0]), .c_in(cin), .c(TAC[4:1]), .gx(TAG_0[0]),.px(TAP_0[0]));
cla_4 cla_0_1( .p(TAP[7:4]), .g(TAG[7:4]), .c_in(TAC_1[1]),.c(TAC[8:5]), .gx(TAG_0[1]),.px(TAP_0[1]));
cla_4 cla_0_2( .p(TAP[11:8]), .g(TAG[11:8]), .c_in(TAC_1[2]),.c(TAC[12:9]), .gx(TAG_0[2]),.px(TAP_0[2]));
cla_4 cla_0_3( .p(TAP[15:12]),.g(TAG[15:12]),.c_in(TAC_1[3]),.c(TAC[16:13]),.gx(TAG_0[3]),.px(TAP_0[3]));
cla_4 cla_0_4( .p(TAP[19:16]),.g(TAG[19:16]),.c_in(TAC_1[4]),.c(TAC[20:17]),.gx(TAG_0[4]),.px(TAP_0[4]));
cla_4 cla_0_5( .p(TAP[23:20]),.g(TAG[23:20]),.c_in(TAC_1[5]),.c(TAC[24:21]),.gx(TAG_0[5]),.px(TAP_0[5]));
cla_4 cla_0_6( .p(TAP[27:24]),.g(TAG[27:24]),.c_in(TAC_1[6]),.c(TAC[28:25]),.gx(TAG_0[6]),.px(TAP_0[6]));
cla_4 cla_0_7( .p(TAP[31:28]),.g(TAG[31:28]),.c_in(TAC_1[7]),.c(TAC[32:29]),.gx(TAG_0[7]),.px(TAP_0[7]));
cla_4 cla_1_0(.p(TAP_0[3:0]), .g(TAG_0[3:0]), .c_in(cin),.c(TAC_1[4:1]), .gx(TAG_1[0]),.px(TAP_1[0]));
cla_4 cla_1_1(.p(TAP_0[7:4]), .g(TAG_0[7:4]), .c_in(TAC_1[4]),.c(TAC_1[8:5]), .gx(TAG_1[1]),.px(TAP_1[1]));
assign TAG_1[3:2] = 2'b00;
assign TAP_1[3:2] = 2'b00;
cla_4 cla_2_0(.p(TAP_1[3:0]), .g(TAG_1[3:0]), .c_in(1'b0), .c(TAC_2[4:1]), .gx(),.px());
assign cout = TAC_2[2];
endmodule
二、移位器的设计
移位运算不涉及到时序，用组合逻辑电路的移位器实现。
移位的原理是使用不同的接线，错位ALU_SHIFT位就实现了移动ALU_SHIFT位，所以使用数据选择器实现。
module Shifter(input [31:0] ALU_DA,
input [4:0] ALU_SHIFT,
input [1:0] Shiftctr,
output reg [31:0] shift_result);
reg[31:0] SLL_M,SRL_M,SRA_M;
always@(*)//SRL
begin
case(ALU_SHIFT)
5'b00000:SRL_M[31:0]=ALU_DA[31:0];
5'b00001:SRL_M[31:0]={1'd0 ,ALU_DA[31:1]};
5'b00010:SRL_M[31:0]={2'd0 ,ALU_DA[31:2]};
5'b00011:SRL_M[31:0]={3'd0 ,ALU_DA[31:3]};
5'b00100:SRL_M[31:0]={4'd0 ,ALU_DA[31:4]};
5'b00101:SRL_M[31:0]={5'd0 ,ALU_DA[31:5]};
5'b00110:SRL_M[31:0]={6'd0 ,ALU_DA[31:6]};
5'b00111:SRL_M[31:0]={7'd0 ,ALU_DA[31:7]};
5'b01000:SRL_M[31:0]={8'd0 ,ALU_DA[31:8]};
5'b01001:SRL_M[31:0]={9'd0 ,ALU_DA[31:9]};
5'b01010:SRL_M[31:0]={10'd0,ALU_DA[31:10]};
5'b01011:SRL_M[31:0]={11'd0,ALU_DA[31:11]};
5'b01100:SRL_M[31:0]={12'd0,ALU_DA[31:12]};
5'b01101:SRL_M[31:0]={13'd0,ALU_DA[31:13]};
5'b01110:SRL_M[31:0]={14'd0,ALU_DA[31:14]};
5'b01111:SRL_M[31:0]={15'd0,ALU_DA[31:15]};
5'b10000:SRL_M[31:0]={16'd0,ALU_DA[31:16]};
5'b10001:SRL_M[31:0]={17'd0,ALU_DA[31:17]};
5'b10010:SRL_M[31:0]={18'd0,ALU_DA[31:18]};
5'b10011:SRL_M[31:0]={19'd0,ALU_DA[31:19]};
5'b10100:SRL_M[31:0]={20'd0,ALU_DA[31:20]};
5'b10101:SRL_M[31:0]={21'd0,ALU_DA[31:21]};
5'b10110:SRL_M[31:0]={22'd0,ALU_DA[31:22]};
5'b10111:SRL_M[31:0]={23'd0,ALU_DA[31:23]};
5'b11000:SRL_M[31:0]={24'd0,ALU_DA[31:24]};
5'b11001:SRL_M[31:0]={25'd0,ALU_DA[31:25]};
5'b11010:SRL_M[31:0]={26'd0,ALU_DA[31:26]};
5'b11011:SRL_M[31:0]={27'd0,ALU_DA[31:27]};
5'b11100:SRL_M[31:0]={28'd0,ALU_DA[31:28]};
5'b11101:SRL_M[31:0]={29'd0,ALU_DA[31:29]};
5'b11110:SRL_M[31:0]={30'd0,ALU_DA[31:30]};
5'b11111:SRL_M[31:0]={31'd0,ALU_DA[31]};
default: SRL_M[31:0]=ALU_DA[31:0];
endcase
end
always@(*) //SLL
begin
case(ALU_SHIFT)
5'b00000:SLL_M[31:0]=ALU_DA[31:0];
5'b00001:SLL_M[31:0]={ALU_DA[30:0],1'd0};
5'b00010:SLL_M[31:0]={ALU_DA[29:0],2'd0};
5'b00011:SLL_M[31:0]={ALU_DA[28:0],3'd0};
5'b00100:SLL_M[31:0]={ALU_DA[27:0],4'd0};
5'b00101:SLL_M[31:0]={ALU_DA[26:0],5'd0};
5'b00110:SLL_M[31:0]={ALU_DA[25:0],6'd0};
5'b00111:SLL_M[31:0]={ALU_DA[24:0],7'd0};
5'b01000:SLL_M[31:0]={ALU_DA[23:0],8'd0};
5'b01001:SLL_M[31:0]={ALU_DA[22:0],9'd0};
5'b01010:SLL_M[31:0]={ALU_DA[21:0],10'd0};
5'b01011:SLL_M[31:0]={ALU_DA[20:0],11'd0};
5'b01100:SLL_M[31:0]={ALU_DA[19:0],12'd0};
5'b01101:SLL_M[31:0]={ALU_DA[18:0],13'd0};
5'b01110:SLL_M[31:0]={ALU_DA[17:0],14'd0};
5'b01111:SLL_M[31:0]={ALU_DA[16:0],15'd0};
5'b10000:SLL_M[31:0]={ALU_DA[15:0],16'd0};
5'b10001:SLL_M[31:0]={ALU_DA[14:0],17'd0};
5'b10010:SLL_M[31:0]={ALU_DA[13:0],18'd0};
5'b10011:SLL_M[31:0]={ALU_DA[12:0],19'd0};
5'b10100:SLL_M[31:0]={ALU_DA[11:0],20'd0};
5'b10101:SLL_M[31:0]={ALU_DA[10:0],21'd0};
5'b10110:SLL_M[31:0]={ALU_DA[9:0] ,22'd0};
5'b10111:SLL_M[31:0]={ALU_DA[8:0] ,23'd0};
5'b11000:SLL_M[31:0]={ALU_DA[7:0] ,24'd0};
5'b11001:SLL_M[31:0]={ALU_DA[6:0] ,25'd0};
5'b11010:SLL_M[31:0]={ALU_DA[5:0] ,26'd0};
5'b11011:SLL_M[31:0]={ALU_DA[4:0] ,27'd0};
5'b11100:SLL_M[31:0]={ALU_DA[3:0] ,28'd0};
5'b11101:SLL_M[31:0]={ALU_DA[2:0] ,29'd0};
5'b11110:SLL_M[31:0]={ALU_DA[1:0] ,30'd0};
5'b11111:SLL_M[31:0]={ALU_DA[0],31'd0};
default: SLL_M[31:0]=ALU_DA[31:0];
endcase
end
always@(*) //SRA
begin
case(ALU_SHIFT)
5'b00000:SRA_M[31:0]=ALU_DA[31:0];
5'b00001:SRA_M[31:0]={{1{ALU_DA[31]}},ALU_DA[31:1]};
5'b00010:SRA_M[31:0]={{2{ALU_DA[31]}},ALU_DA[31:2]};
5'b00011:SRA_M[31:0]={{3{ALU_DA[31]}},ALU_DA[31:3]};
5'b00100:SRA_M[31:0]={{4{ALU_DA[31]}},ALU_DA[31:4]};
5'b00101:SRA_M[31:0]={{5{ALU_DA[31]}},ALU_DA[31:5]};
5'b00110:SRA_M[31:0]={{6{ALU_DA[31]}},ALU_DA[31:6]};
5'b00111:SRA_M[31:0]={{7{ALU_DA[31]}},ALU_DA[31:7]};
5'b01000:SRA_M[31:0]={{8{ALU_DA[31]}},ALU_DA[31:8]};
5'b01001:SRA_M[31:0]={{9{ALU_DA[31]}},ALU_DA[31:9]};
5'b01010:SRA_M[31:0]={{10{ALU_DA[31]}},ALU_DA[31:10]};
5'b01011:SRA_M[31:0]={{11{ALU_DA[31]}},ALU_DA[31:11]};
5'b01100:SRA_M[31:0]={{12{ALU_DA[31]}},ALU_DA[31:12]};
5'b01101:SRA_M[31:0]={{13{ALU_DA[31]}},ALU_DA[31:13]};
5'b01110:SRA_M[31:0]={{14{ALU_DA[31]}},ALU_DA[31:14]};
5'b01111:SRA_M[31:0]={{15{ALU_DA[31]}},ALU_DA[31:15]};
5'b10000:SRA_M[31:0]={{16{ALU_DA[31]}},ALU_DA[31:16]};
5'b10001:SRA_M[31:0]={{17{ALU_DA[31]}},ALU_DA[31:17]};
5'b10010:SRA_M[31:0]={{18{ALU_DA[31]}},ALU_DA[31:18]};
5'b10011:SRA_M[31:0]={{19{ALU_DA[31]}},ALU_DA[31:19]};
5'b10100:SRA_M[31:0]={{20{ALU_DA[31]}},ALU_DA[31:20]};
5'b10101:SRA_M[31:0]={{21{ALU_DA[31]}},ALU_DA[31:21]};
5'b10110:SRA_M[31:0]={{22{ALU_DA[31]}},ALU_DA[31:22]};
5'b10111:SRA_M[31:0]={{23{ALU_DA[31]}},ALU_DA[31:23]};
5'b11000:SRA_M[31:0]={{24{ALU_DA[31]}},ALU_DA[31:24]};
5'b11001:SRA_M[31:0]={{25{ALU_DA[31]}},ALU_DA[31:25]};
5'b11010:SRA_M[31:0]={{26{ALU_DA[31]}},ALU_DA[31:26]};
5'b11011:SRA_M[31:0]={{27{ALU_DA[31]}},ALU_DA[31:27]};
5'b11100:SRA_M[31:0]={{28{ALU_DA[31]}},ALU_DA[31:28]};
5'b11101:SRA_M[31:0]={{29{ALU_DA[31]}},ALU_DA[31:29]};
5'b11110:SRA_M[31:0]={{30{ALU_DA[31]}},ALU_DA[31:30]};
5'b11111:SRA_M[31:0]={{31{ALU_DA[31]}},ALU_DA[31]};
default: SRA_M[31:0]=ALU_DA[31:0];
endcase
end
always@(*) //SHIFT
begin
case(Shiftctr)
2'b00:shift_result=SLL_M;
2'b01:shift_result=SRL_M;
2'b10:shift_result=SRA_M;
default: shift_result=ALU_DA;
endcase
end
endmodule
三、仿真测试
单独的加法器和移位器的仿真请读者自行完成，在此不再赘述。
将改进后的ALU模块的代码展示如下：
module alu(
ALU_DA,
ALU_DB,
ALU_CTL,
ALU_ZERO,
ALU_OverFlow,
ALU_DC
);
input [31:0] ALU_DA;
input [31:0] ALU_DB;
input [3:0] ALU_CTL;
output ALU_ZERO;
output ALU_OverFlow;
output reg [31:0] ALU_DC;
//********************generate ctr***********************
wire SUBctr;
wire SIGctr;
wire Ovctr;
wire [1:0] Opctr;
wire [1:0] Logicctr;
wire [1:0] Shiftctr;
assign SUBctr = (~ ALU_CTL[3] & ~ALU_CTL[2] & ALU_CTL[1]) | ( ALU_CTL[3] & ~ALU_CTL[2]);
assign Opctr = ALU_CTL[3:2];
assign Ovctr = ALU_CTL[0] & ~ ALU_CTL[3] & ~ALU_CTL[2] ;
assign SIGctr = ALU_CTL[0];
assign Logicctr = ALU_CTL[1:0];
assign Shiftctr = ALU_CTL[1:0];
//********************************************************
//*********************logic op***************************
reg [31:0] logic_result;
always@(*) begin
case(Logicctr)
2'b00:logic_result = ALU_DA & ALU_DB;
2'b01:logic_result = ALU_DA | ALU_DB;
2'b10:logic_result = ALU_DA ^ ALU_DB;
2'b11:logic_result = ~(ALU_DA | ALU_DB);
endcase
end
//********************************************************
//************************shift op************************
wire [4:0] ALU_SHIFT;
wire [31:0] shift_result;
assign ALU_SHIFT=ALU_DB[4:0];
Shifter Shifter(.ALU_DA(ALU_DA),
.ALU_SHIFT(ALU_SHIFT),
.Shiftctr(Shiftctr),
.shift_result(shift_result));
//********************************************************
//************************add sub op**********************
wire [31:0] BIT_M,XOR_M;
wire ADD_carry,ADD_OverFlow;
wire [31:0] ADD_result;
assign BIT_M={32{SUBctr}};
assign XOR_M=BIT_M^ALU_DB;
Adder Adder(.A(ALU_DA),
.B(XOR_M),
.Cin(SUBctr),
.ALU_CTL(ALU_CTL),
.ADD_carry(ADD_carry),
.ADD_OverFlow(ADD_OverFlow),
.ADD_zero(ALU_ZERO),
.ADD_result(ADD_result));
assign ALU_OverFlow = ADD_OverFlow & Ovctr;
//********************************************************
//**************************slt op************************
wire [31:0] SLT_result;
wire LESS_M1,LESS_M2,LESS_S,SLT_M;
assign LESS_M1 = ADD_carry ^ SUBctr;
assign LESS_M2 = ADD_OverFlow ^ ADD_result[31];
assign LESS_S = (SIGctr==1'b0)?LESS_M1:LESS_M2;
assign SLT_result = (LESS_S)?32'h00000001:32'h00000000;
//********************************************************
//**************************ALU result********************
always @(*)
begin
case(Opctr)
2'b00:ALU_DC<=ADD_result;
2'b01:ALU_DC<=logic_result;
2'b10:ALU_DC<=SLT_result;
2'b11:ALU_DC<=shift_result;
endcase
end
//********************************************************
endmodule
//********************************************************
//*************************shifter************************
//`define BEHAVOR
module Shifter(input [31:0] ALU_DA,
input [4:0] ALU_SHIFT,
input [1:0] Shiftctr,
output reg [31:0] shift_result);
`ifdef BEHAVOR
wire [5:0] shift_n;
assign shift_n = 6'd32 - Shiftctr;
always@(*) begin
case(Shiftctr)
2'b00:shift_result = ALU_DA << ALU_SHIFT;
2'b01:shift_result = ALU_DA >> ALU_SHIFT;
2'b10:shift_result = ({32{ALU_DA[31]}} << shift_n) | (ALU_DA >> ALU_SHIFT);
default:shift_result = ALU_DA;
endcase
end
`else
reg[31:0] SLL_M,SRL_M,SRA_M;
always@(*)//SRL
begin
case(ALU_SHIFT)
5'b00000:SRL_M[31:0]=ALU_DA[31:0];
5'b00001:SRL_M[31:0]={1'd0 ,ALU_DA[31:1]};
5'b00010:SRL_M[31:0]={2'd0 ,ALU_DA[31:2]};
5'b00011:SRL_M[31:0]={3'd0 ,ALU_DA[31:3]};
5'b00100:SRL_M[31:0]={4'd0 ,ALU_DA[31:4]};
5'b00101:SRL_M[31:0]={5'd0 ,ALU_DA[31:5]};
5'b00110:SRL_M[31:0]={6'd0 ,ALU_DA[31:6]};
5'b00111:SRL_M[31:0]={7'd0 ,ALU_DA[31:7]};
5'b01000:SRL_M[31:0]={8'd0 ,ALU_DA[31:8]};
5'b01001:SRL_M[31:0]={9'd0 ,ALU_DA[31:9]};
5'b01010:SRL_M[31:0]={10'd0,ALU_DA[31:10]};
5'b01011:SRL_M[31:0]={11'd0,ALU_DA[31:11]};
5'b01100:SRL_M[31:0]={12'd0,ALU_DA[31:12]};
5'b01101:SRL_M[31:0]={13'd0,ALU_DA[31:13]};
5'b01110:SRL_M[31:0]={14'd0,ALU_DA[31:14]};
5'b01111:SRL_M[31:0]={15'd0,ALU_DA[31:15]};
5'b10000:SRL_M[31:0]={16'd0,ALU_DA[31:16]};
5'b10001:SRL_M[31:0]={17'd0,ALU_DA[31:17]};
5'b10010:SRL_M[31:0]={18'd0,ALU_DA[31:18]};
5'b10011:SRL_M[31:0]={19'd0,ALU_DA[31:19]};
5'b10100:SRL_M[31:0]={20'd0,ALU_DA[31:20]};
5'b10101:SRL_M[31:0]={21'd0,ALU_DA[31:21]};
5'b10110:SRL_M[31:0]={22'd0,ALU_DA[31:22]};
5'b10111:SRL_M[31:0]={23'd0,ALU_DA[31:23]};
5'b11000:SRL_M[31:0]={24'd0,ALU_DA[31:24]};
5'b11001:SRL_M[31:0]={25'd0,ALU_DA[31:25]};
5'b11010:SRL_M[31:0]={26'd0,ALU_DA[31:26]};
5'b11011:SRL_M[31:0]={27'd0,ALU_DA[31:27]};
5'b11100:SRL_M[31:0]={28'd0,ALU_DA[31:28]};
5'b11101:SRL_M[31:0]={29'd0,ALU_DA[31:29]};
5'b11110:SRL_M[31:0]={30'd0,ALU_DA[31:30]};
5'b11111:SRL_M[31:0]={31'd0,ALU_DA[31]};
default: SRL_M[31:0]=ALU_DA[31:0];
endcase
end
always@(*) //SLL
begin
case(ALU_SHIFT)
5'b00000:SLL_M[31:0]=ALU_DA[31:0];
5'b00001:SLL_M[31:0]={ALU_DA[30:0],1'd0};
5'b00010:SLL_M[31:0]={ALU_DA[29:0],2'd0};
5'b00011:SLL_M[31:0]={ALU_DA[28:0],3'd0};
5'b00100:SLL_M[31:0]={ALU_DA[27:0],4'd0};
5'b00101:SLL_M[31:0]={ALU_DA[26:0],5'd0};
5'b00110:SLL_M[31:0]={ALU_DA[25:0],6'd0};
5'b00111:SLL_M[31:0]={ALU_DA[24:0],7'd0};
5'b01000:SLL_M[31:0]={ALU_DA[23:0],8'd0};
5'b01001:SLL_M[31:0]={ALU_DA[22:0],9'd0};
5'b01010:SLL_M[31:0]={ALU_DA[21:0],10'd0};
5'b01011:SLL_M[31:0]={ALU_DA[20:0],11'd0};
5'b01100:SLL_M[31:0]={ALU_DA[19:0],12'd0};
5'b01101:SLL_M[31:0]={ALU_DA[18:0],13'd0};
5'b01110:SLL_M[31:0]={ALU_DA[17:0],14'd0};
5'b01111:SLL_M[31:0]={ALU_DA[16:0],15'd0};
5'b10000:SLL_M[31:0]={ALU_DA[15:0],16'd0};
5'b10001:SLL_M[31:0]={ALU_DA[14:0],17'd0};
5'b10010:SLL_M[31:0]={ALU_DA[13:0],18'd0};
5'b10011:SLL_M[31:0]={ALU_DA[12:0],19'd0};
5'b10100:SLL_M[31:0]={ALU_DA[11:0],20'd0};
5'b10101:SLL_M[31:0]={ALU_DA[10:0],21'd0};
5'b10110:SLL_M[31:0]={ALU_DA[9:0] ,22'd0};
5'b10111:SLL_M[31:0]={ALU_DA[8:0] ,23'd0};
5'b11000:SLL_M[31:0]={ALU_DA[7:0] ,24'd0};
5'b11001:SLL_M[31:0]={ALU_DA[6:0] ,25'd0};
5'b11010:SLL_M[31:0]={ALU_DA[5:0] ,26'd0};
5'b11011:SLL_M[31:0]={ALU_DA[4:0] ,27'd0};
5'b11100:SLL_M[31:0]={ALU_DA[3:0] ,28'd0};
5'b11101:SLL_M[31:0]={ALU_DA[2:0] ,29'd0};
5'b11110:SLL_M[31:0]={ALU_DA[1:0] ,30'd0};
5'b11111:SLL_M[31:0]={ALU_DA[0],31'd0};
default: SLL_M[31:0]=ALU_DA[31:0];
endcase
end
always@(*) //SRA
begin
case(ALU_SHIFT)
5'b00000:SRA_M[31:0]=ALU_DA[31:0];
5'b00001:SRA_M[31:0]={{1{ALU_DA[31]}},ALU_DA[31:1]};
5'b00010:SRA_M[31:0]={{2{ALU_DA[31]}},ALU_DA[31:2]};
5'b00011:SRA_M[31:0]={{3{ALU_DA[31]}},ALU_DA[31:3]};
5'b00100:SRA_M[31:0]={{4{ALU_DA[31]}},ALU_DA[31:4]};
5'b00101:SRA_M[31:0]={{5{ALU_DA[31]}},ALU_DA[31:5]};
5'b00110:SRA_M[31:0]={{6{ALU_DA[31]}},ALU_DA[31:6]};
5'b00111:SRA_M[31:0]={{7{ALU_DA[31]}},ALU_DA[31:7]};
5'b01000:SRA_M[31:0]={{8{ALU_DA[31]}},ALU_DA[31:8]};
5'b01001:SRA_M[31:0]={{9{ALU_DA[31]}},ALU_DA[31:9]};
5'b01010:SRA_M[31:0]={{10{ALU_DA[31]}},ALU_DA[31:10]};
5'b01011:SRA_M[31:0]={{11{ALU_DA[31]}},ALU_DA[31:11]};
5'b01100:SRA_M[31:0]={{12{ALU_DA[31]}},ALU_DA[31:12]};
5'b01101:SRA_M[31:0]={{13{ALU_DA[31]}},ALU_DA[31:13]};
5'b01110:SRA_M[31:0]={{14{ALU_DA[31]}},ALU_DA[31:14]};
5'b01111:SRA_M[31:0]={{15{ALU_DA[31]}},ALU_DA[31:15]};
5'b10000:SRA_M[31:0]={{16{ALU_DA[31]}},ALU_DA[31:16]};
5'b10001:SRA_M[31:0]={{17{ALU_DA[31]}},ALU_DA[31:17]};
5'b10010:SRA_M[31:0]={{18{ALU_DA[31]}},ALU_DA[31:18]};
5'b10011:SRA_M[31:0]={{19{ALU_DA[31]}},ALU_DA[31:19]};
5'b10100:SRA_M[31:0]={{20{ALU_DA[31]}},ALU_DA[31:20]};
5'b10101:SRA_M[31:0]={{21{ALU_DA[31]}},ALU_DA[31:21]};
5'b10110:SRA_M[31:0]={{22{ALU_DA[31]}},ALU_DA[31:22]};
5'b10111:SRA_M[31:0]={{23{ALU_DA[31]}},ALU_DA[31:23]};
5'b11000:SRA_M[31:0]={{24{ALU_DA[31]}},ALU_DA[31:24]};
5'b11001:SRA_M[31:0]={{25{ALU_DA[31]}},ALU_DA[31:25]};
5'b11010:SRA_M[31:0]={{26{ALU_DA[31]}},ALU_DA[31:26]};
5'b11011:SRA_M[31:0]={{27{ALU_DA[31]}},ALU_DA[31:27]};
5'b11100:SRA_M[31:0]={{28{ALU_DA[31]}},ALU_DA[31:28]};
5'b11101:SRA_M[31:0]={{29{ALU_DA[31]}},ALU_DA[31:29]};
5'b11110:SRA_M[31:0]={{30{ALU_DA[31]}},ALU_DA[31:30]};
5'b11111:SRA_M[31:0]={{31{ALU_DA[31]}},ALU_DA[31]};
default: SRA_M[31:0]=ALU_DA[31:0];
endcase
end
always@(*) //SHIFT
begin
case(Shiftctr)
2'b00:shift_result=SLL_M;
2'b01:shift_result=SRL_M;
2'b10:shift_result=SRA_M;
default: shift_result=ALU_DA;
endcase
end
`endif
endmodule
//*************************************************************
//***********************************adder*********************
//`define ALGORITHM
module Adder(input [31:0] A,
input [31:0] B,
input Cin,
input [3:0] ALU_CTL,
output ADD_carry,
output ADD_OverFlow,
output ADD_zero,
output [31:0] ADD_result);
`ifdef ALGORITHM
assign {ADD_carry,ADD_result}=A+B+Cin;
`else
cla_adder32 cla_adder32_inst1(.A(A),
.B(B),
.cin(Cin),
.cout(ADD_carry),
.result(ADD_result));
`endif
assign ADD_zero = ~(|ADD_result);
assign ADD_OverFlow=((ALU_CTL==4'b0001) & ~A[31] & ~B[31] & ADD_result[31])
| ((ALU_CTL==4'b0001) & A[31] & B[31] & ~ADD_result[31])
| ((ALU_CTL==4'b0011) & A[31] & ~B[31] & ~ADD_result[31])
| ((ALU_CTL==4'b0011) & ~A[31] & B[31] & ADD_result[31]);
endmodule
//************************************************************************************
//***************************************cla_adder************************************
4位CLA部件
module cla_4(p,g,c_in,c,gx,px);
input[3:0] p,g;
input c_in;
output[4:1] c;
output gx,px;
assign c[1] = p[0]&c_in | g[0];
assign c[2] = p[1]&p[0]&c_in | p[1]&g[0] | g[1];
assign c[3] = p[2]&p[1]&p[0]&c_in | p[2]&p[1]&g[0] | p[2]&g[1] | g[2];
assign c[4] = gx | px&c_in;
assign px = p[3]&p[2]&p[1]&p[0];
assign gx = g[3] | p[3]&g[2] | p[3]&p[2]&g[1] | p[3]&p[2]&p[1]&g[0];
endmodule
//32位全加器
module cla_adder32(A,B,cin,result,cout);
input [31:0] A;
input [31:0] B;
input cin;
output[31:0] result;
output cout;
进位产生信号，进位传递信号
wire[31:0] TAG,TAP;
wire[32:1] TAC;
wire[15:0] TAG_0,TAP_0;
wire[3:0] TAG_1,TAP_1;
wire[8:1] TAC_1;
wire[4:1] TAC_2;
assign result = A ^ B ^ {TAC[31:1],cin};
assign TAG = A&B;
assign TAP = A|B;
cla_4 cla_0_0( .p(TAP[3:0]), .g(TAG[3:0]), .c_in(cin), .c(TAC[4:1]), .gx(TAG_0[0]),.px(TAP_0[0]));
cla_4 cla_0_1( .p(TAP[7:4]), .g(TAG[7:4]), .c_in(TAC_1[1]),.c(TAC[8:5]), .gx(TAG_0[1]),.px(TAP_0[1]));
cla_4 cla_0_2( .p(TAP[11:8]), .g(TAG[11:8]), .c_in(TAC_1[2]),.c(TAC[12:9]), .gx(TAG_0[2]),.px(TAP_0[2]));
cla_4 cla_0_3( .p(TAP[15:12]),.g(TAG[15:12]),.c_in(TAC_1[3]),.c(TAC[16:13]),.gx(TAG_0[3]),.px(TAP_0[3]));
cla_4 cla_0_4( .p(TAP[19:16]),.g(TAG[19:16]),.c_in(TAC_1[4]),.c(TAC[20:17]),.gx(TAG_0[4]),.px(TAP_0[4]));
cla_4 cla_0_5( .p(TAP[23:20]),.g(TAG[23:20]),.c_in(TAC_1[5]),.c(TAC[24:21]),.gx(TAG_0[5]),.px(TAP_0[5]));
cla_4 cla_0_6( .p(TAP[27:24]),.g(TAG[27:24]),.c_in(TAC_1[6]),.c(TAC[28:25]),.gx(TAG_0[6]),.px(TAP_0[6]));
cla_4 cla_0_7( .p(TAP[31:28]),.g(TAG[31:28]),.c_in(TAC_1[7]),.c(TAC[32:29]),.gx(TAG_0[7]),.px(TAP_0[7]));
cla_4 cla_1_0(.p(TAP_0[3:0]), .g(TAG_0[3:0]), .c_in(cin),.c(TAC_1[4:1]), .gx(TAG_1[0]),.px(TAP_1[0]));
cla_4 cla_1_1(.p(TAP_0[7:4]), .g(TAG_0[7:4]), .c_in(TAC_1[4]),.c(TAC_1[8:5]), .gx(TAG_1[1]),.px(TAP_1[1]));
assign TAG_1[3:2] = 2'b00;
assign TAP_1[3:2] = 2'b00;
cla_4 cla_2_0(.p(TAP_1[3:0]), .g(TAG_1[3:0]), .c_in(1'b0), .c(TAC_2[4:1]), .gx(),.px());
assign cout = TAC_2[2];
endmodule
此处对改进后的CPU进行测试。
测试用例是上一篇文章中的跳转指令，移位运算指令和算数运算指令。
算数运算指令如下：
addi x1,x0,1
add x2,x1,x1
add x3,x2,x2
sub x3,x3,x1
addi x4,x0,-4
add x5,x4,x4
sub x6,x5,x3
sub x7,x6,x4
add x8,x7,x3
汇编器执行结果如下：
CPU执行结果如下：
移位运算指令如下：
addi x1,x0,0xff
addi x2,x0,4
sll x3,x1,x2
srl x4,x1,x2
sra x5,x1,x2
addi x6,x0,-0xff
sll x7,x6,x2
srl x8,x6,x2
sra x9,x6,x2
slli x11,x1,4
srli x12,x1,4
srai x13,x1,4
slli x11,x6,4
srli x12,x6,4
srai x13,x6,4
汇编器执行结果如下：
CPU执行结果如下：
跳转指令如下：
addi x1,x0,1
addi x2,x0,2
jal x31,label1
addi x3,x0,3
label1:
addi x4,x0,4
add x5,x2,x2
beq x4,x5,label2
addi x6,x0,6
label2:
bne x4,x5,label3
addi x7,x0,7
label3:
bne x7,x6,label4
addi x8,x0,8
label4:
addi x9,x0,0x30
jalr x10,x9,12
addi x11,x0,11
addi x12,x0,-12
addi x13,x0,-13
blt x13,x12,label5
addi x14,x0,-14
label5:
bltu x13,x12,label6
addi x15,x0,-15
label6:
bltu x12,x13,label7
addi x16,x0,-16
label7:
bge x12,x13,label8
addi x17,x0,-17
label8:
bge x1,x2,label9
addi x18,x0,-18
label9:
bgeu x12,x13,label10
addi x19,x0,-19
label10:
bgeu x13,x12,label11
addi x20,x0,-20
label11:
addi x21,x0,-20
addi x22,x0,-20
bge x21,x22,label12
addi x23,x0,-23
label12:
addi x24,x0,-24
汇编器执行结果如下：
CPU执行结果如下：
总结
以上就是今天要讲的内容，对ALU的加法器和移位两个运算部件进行了详细的设计。下一篇文章开始进行流水线处理器的设计。标签：SRA,31,RISC,DA,SLL,从零开始,ALU,d0 来源： https://blog.csdn.net/qq_45677520/article/details/122489700