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// Incomplete behavioral model of MIPS pipeline
module mipspipe(clock);
// in_out
input clock;
// Instruction opcodes
parameter LW = 6'b100011, SW = 6'b101011, BEQ = 6'b000100, nop = 32'b00000_100000, ALUop = 6'b0;
reg [31:0] PC, // Program counter
Regs[0:31], // Register file
IMemory[0:1023], DMemory[0:1023], // Instruction and data memories
IFIDIR, IDEXA, IDEXB, IDEXIR, EXMEMIR, EXMEMB, // pipeline latches
EXMEMALUOut, MEMWBValue, MEMWBIR; // pipeline latches
wire [4:0] IDEXrs, IDEXrt, EXMEMrd, MEMWBrd, MEMWBrt; // fields of pipeline latches
wire [5:0] EXMEMop, MEMWBop, IDEXop; // opcodes
wire [31:0] Ain, Bin; // ALU inputs
// Define fields of pipeline latches
assign IDEXrs = IDEXIR[25:21]; // rs field
assign IDEXrt = IDEXIR[20:16]; // rt field
assign EXMEMrd = EXMEMIR[15:11]; // rd field
assign MEMWBrd = MEMWBIR[15:11]; // rd field
assign MEMWBrt = MEMWBIR[20:16]; // rt field -- for loads
assign EXMEMop = EXMEMIR[31:26]; // opcode
assign MEMWBop = MEMWBIR[31:26]; // opcode
assign IDEXop = IDEXIR[31:26]; // opcode
// Inputs to the ALU come directly from the ID/EX pipeline latches
assign Ain = IDEXA;
assign Bin = IDEXB;
reg [5:0] i; //used to initialize registers
reg [10:0] j,k; //used to initialize registers
initial begin
PC = 0;
IFIDIR = nop;
IDEXIR = nop;
EXMEMIR = nop;
MEMWBIR = nop; // no-ops placed in pipeline latches
// test some instructions
for (i=0;i<=31;i=i+1) Regs[i] = i; // initialize registers
IMemory[0] = 32'h8c210003;
IMemory[1] = 32'hac020000;
IMemory[2] = 32'h00642820;
for (j=3;j<=1023;j=j+1) IMemory[j] = nop;
DMemory[0] = 32'h00000000;
DMemory[1] = 32'hffffffff;
for (k=2;k<=1023;k=k+1) DMemory[k] = 0;
end
always @ (posedge clock)
begin
// FETCH: Fetch instruction & update PC
IFIDIR <= IMemory[PC>>2];
PC <= PC + 4;
// DECODE: Read registers
IDEXA <= Regs[IFIDIR[25:21]];
IDEXB <= Regs[IFIDIR[20:16]]; // get two registers
IDEXIR <= IFIDIR; // pass along IR
// EX: Address calculation or ALU operation
if ((IDEXop==LW) |(IDEXop==SW)) // address calculation
EXMEMALUOut <= IDEXA +{{16{IDEXIR[15]}}, IDEXIR[15:0]};
else if (IDEXop==ALUop) begin // ALU operation
case (IDEXIR[5:0]) // R-type instruction
32: EXMEMALUOut <= Ain + Bin; // add operation
default: ; // other R-type operations [to be implemented]
endcase
end
EXMEMIR <= IDEXIR; EXMEMB <= IDEXB; //pass along the IR & B
// MEM
if (EXMEMop==ALUop) MEMWBValue <= EXMEMALUOut; //pass along ALU result
else if (EXMEMop == LW) MEMWBValue <= DMemory[EXMEMALUOut>>2]; // load
else if (EXMEMop == SW) DMemory[EXMEMALUOut>>2] <=EXMEMB; // store
MEMWBIR <= EXMEMIR; //pass along IR
// WB
if ((MEMWBop==ALUop) & (MEMWBrd != 0)) // update registers if ALU operation and destination not 0
Regs[MEMWBrd] <= MEMWBValue; // ALU operation
else if ((MEMWBop == LW)& (MEMWBrt != 0)) // Update registers if load and destination not 0
Regs[MEMWBrt] <= MEMWBValue;
end
endmodule
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