Simple RiSC Core with 16 bit instructions

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`define ADD 3'd0
`define ADDI    3'd1
`define NAND    3'd2
`define LUI 3'd3
`define SW  3'd4
`define LW  3'd5
`define BNE 3'd6
`define JALR    3'd7
`define EXTEND  3'd7
 
`define IOP 15:13   // opcode
`define IRA 12:10   // rA 
`define IRB 9:7 // rB 
`define IRC 2:0 // rC 
`define IIM 6:0 // immediate (7-bit, to be sign-extended)
`define ILI 9:0 // large unsigned immediate (10-bit, to be 0-extended)
`define ISB 6   // immediate's sign bit
 
`define ZERO        16'd0
 
`define HALTINSTRUCTION { `EXTEND, 3'd0, 3'd0, 3'd7, 4'd1 }
 
module RiSC (clk);
    input       clk;
    reg     [15:0]  rf[0:7];
    reg     [15:0]  pc;
    reg     [15:0]  m[0:65535];
    reg     [15:0] m1;
    reg      [2:0] iop,ira,irb,irc;
    reg      [6:0] iim;
    reg      [9:0] ili;
    wire      isb;
    reg      [15:0] imm, imma;
    
    
    always @(negedge clk) begin
    rf[0] <= `ZERO;
    end
assign isb = m[pc[6]];
    always @(posedge clk) begin
    m1 = m[pc];
    iop = m[pc[15:13]];
        ira = m[pc[12:10]];
        irb = m[pc[9:7]];
        irc = m[pc[2:0]];
    iim = m[pc[6:0]];
    ili = m[pc[9:0]];
    
    imm = {ili,5'b0};
    
    if( iop == ADD) begin 
    rf[ira] <= rf[irb] + rf [irc];
    pc <= pc + 1;
    end
    
    else if (iop == ADDI) begin
    if(!isb) begin  
    rf[ira] <= rf[irb] + iim;
    pc <= pc+1;
    end
    else if(isb) begin
    rf[ira] <= rf[irb] - iim;
     pc <= pc+1;
    end
    end
    
    else if (iop == NAND) begin
    rf[ira] <= ~(rf[irb] && rf[irc]);
    pc <= pc+1;
    end
    
    else if (iop == LUI) begin
    imm = {ili,5'b0};
    rf[ira] <= imm;
    pc <= pc + 1;
    end 
    
    else if (iop == SW) begin
    if (!isb) begin
    imma <= iim + rf[irb];
    m[imma] <= rf[ira];
    pc <= pc+1;
    end
    else if (isb) begin
    imma <= rf[irb] - {10'b0,iim[5:0]};
    m[imma] <= rf[ira];
    pc <= pc+1;
    end 
    end
 
    else if (iop == LW) begin
    if (!isb) begin
    imma <= iim + rf[irb];
    rf[ira] <=  m[imma] ;
    pc <= pc+1;
    end
    else if (isb) begin
    imma <= rf[irb] - {10'b0,iim[5:0]};
    rf[ira]  <= m[imma] ;
    pc <= pc+1;
    end 
    end
    
    else if (iop == BNE) begin
    if (rf[ira] != rf[irb])
    begin
    if (!isb)
    pc <= pc + 1 + iim;
    else if (isb)
    pc <= pc + 1 + {10'b0,iim[5:0]};
    end
    else 
    pc <= pc+1;
    end 
    
    else if (iop == JALR) begin
    pc <= rf [irb];
   ira <= pc + 1;
    end
        
    else if (iop == `HALTINSTRUCTION) begin
    pc <= 0;
    $stop;
    end 
    end
endmodule