module uc(
        clk,
        rst,
        ri,
        ind,
        regs_addr,
        regs_oe,
        regs_we,
        alu_oe,
        alu_carry,
        alu_opcode,
        ram_oe,
        ram_we,
        io_oe,
        io_we,
        cp_oe,
        cp_we,
        ind_sel,
        ind_oe,
        ind_we,
        am_oe,
        am_we,
        aie_oe,
        aie_we,
        t1_oe,
        t1_we,
        t2_oe,
        t2_we,
        ri_oe,
        ri_we,
        disp_state
    );

parameter word_width =          16;
parameter state_width =         16;

`define ADC                     0
`define SBB1                    1
`define SBB2                    2
`define NOT                     3
`define AND                     4
`define OR                      5
`define XOR                     6
`define SHL                     7
`define SHR                     8
`define SAR                     9

`define RA                      0
`define RB                      1
`define RC                      2
`define IS                      3
`define XA                      4
`define XB                      5
`define BA                      6
`define BB                      7

input                           clk;
input                           rst;
input [word_width-1 : 0]        ri;
input [word_width-1 : 0]        ind;
output reg                      alu_oe;
output reg                      alu_carry;
output reg[3 : 0]               alu_opcode;
output reg                      ram_oe;
output reg                      ram_we;
output reg                      io_oe;
output reg                      io_we;
output reg[2 : 0]               regs_addr;
output reg                      regs_oe;
output reg                      regs_we;
output reg                      cp_oe;
output reg                      cp_we;
output reg                      ind_sel;        // controls IND register input (0 = bus, 1 = alu flags)
output reg                      ind_oe;
output reg                      ind_we;
output reg                      am_oe;
output reg                      am_we;
output reg                      aie_oe;
output reg                      aie_we;
output reg                      t1_oe;
output reg                      t1_we;
output reg                      t2_oe;
output reg                      t2_we;
output reg                      ri_oe;          // controls RI register output which generates the offset for Jcond instructions
output reg                      ri_we;
output[state_width-1 : 0]       disp_state;

wire [0:6]                      cop;
wire                            d;
wire [0:1]                      mod;
wire [0:2]                      rg;
wire [0:2]                      rm;

assign cop  = {ri[0], ri[1], ri[2], ri[3], ri[4], ri[5], ri[6]};
assign d    = {ri[7]};
assign mod  = {ri[8], ri[9]};
assign rg   = {ri[10], ri[11], ri[12]};
assign rm   = {ri[13], ri[14], ri[15]};

`define reset                   'h00            // reset state
`define fetch                   'h10            // load instruction to instruction register
`define decode                  'h20            // analyze loaded instruction
`define addr_sum                'h30            // computes address of the form [By+Xz] with y,z in {A, B}
`define addr_reg                'h34            // computes address of the form [yz] with y in {X, B} and z in {A, B}
`define load_src_reg            'h40            // load source operand from register
`define load_src_mem            'h44            // load source operand from memory
`define load_dst_reg            'h50            // load destination operand from register
`define load_dst_mem            'h54            // load destination operand from memory
`define exec_1op                'h60            // execute 1 operand instructions
`define exec_2op                'h64            // execute 2 operand instructions
`define store_reg               'h70            // store result to register
`define store_mem               'h74            // store result to memory
`define inc_cp                  'h80            // increment program counter

reg [state_width-1 : 0] state = `reset, state_next;
reg [state_width-1 : 0] decoded_src, decoded_src_next;      // stores decoded source operand load state
reg [state_width-1 : 0] decoded_dst, decoded_dst_next;      // stores decoded destination operand load state
reg [state_width-1 : 0] decoded_exec, decoded_exec_next;    // stores decoded execute state
reg [state_width-1 : 0] decoded_store, decoded_store_next;  // stores decoded store state
reg decoded_d, decoded_d_next;                              // stores decoded direction bit

// FSM - sequential part
always @(posedge clk) begin
    state <= `reset;

    if(!rst) begin
        state <= state_next;

        if(state == `decode) begin
            decoded_src <= decoded_src_next;
            decoded_dst <= decoded_dst_next;
            decoded_exec <= decoded_exec_next;
            decoded_store <= decoded_store_next;
            decoded_d <= decoded_d_next;
        end
    end
end

// FSM - combinational part
always @(*) begin
    state_next = `reset;
    decoded_src_next = `reset;
    decoded_dst_next = `reset;
    decoded_exec_next = `reset;
    decoded_store_next = `reset;
    decoded_d_next = 0;
    alu_oe = 0;
    alu_carry = 0;
    alu_opcode = 0;
    ram_oe = 0;
    ram_we = 0;
    io_oe = 0;
    io_we = 0;
    regs_addr = 0;
    regs_oe = 0;
    regs_we = 0;
    cp_oe = 0;
    cp_we = 0;
    ind_sel = 0;
    ind_oe = 0;
    ind_we = 0;
    am_oe = 0;
    am_we = 0;
    aie_oe = 0;
    aie_we = 0;
    t1_oe = 0;
    t1_we = 0;
    t2_oe = 0;
    t2_we = 0;
    ri_oe = 0;
    ri_we = 0;

    case(state)
        `reset: begin
            state_next = `fetch;
        end

        `fetch: begin
            cp_oe = 1;
            am_we = 1;

            state_next = `fetch + 1;
        end

        `fetch + 'd1: begin
            am_oe = 1;

            state_next = `fetch + 2;
        end

        `fetch + 'd2: begin
            ram_oe = 1;
            ri_we = 1;

            state_next = `decode;
        end

        `decode: begin
            // decode location of operands and operation
            if(cop[0:3] == 4'b0001) begin           // one operand instructions
                decoded_d_next      = 0;
                decoded_dst_next    = mod == 2'b11 ? `load_dst_reg : `load_dst_mem;
                decoded_src_next    = decoded_dst_next;
                decoded_exec_next   = `exec_1op;
                decoded_store_next  = mod == 2'b11 ? `store_reg : `store_mem;
            end
            else if(cop[0:2] == 3'b010) begin       // two operand instructions
                decoded_d_next      = d;
                decoded_dst_next    = (mod == 2'b11) || (d == 1) ? `load_dst_reg : `load_dst_mem;
                decoded_src_next    = (mod == 2'b11) || (d == 0) ? `load_src_reg : `load_src_mem;
                decoded_exec_next   = `exec_2op;
                decoded_store_next  = !cop[3] ? `inc_cp : ((mod == 2'b11) || (d == 1) ? `store_reg : `store_mem);
            end

            // decode address calculation mode
            case(mod)
                2'b00: begin
                    state_next = rm[0] ? `addr_reg : `addr_sum;
                end

                2'b11: begin
                    state_next = decoded_src_next;
                end
            endcase
        end

        `addr_sum: begin
            regs_addr = rm[1] ? `BB : `BA;
            regs_oe = 1;
            t1_we = 1;

            state_next = `addr_sum + 1;
        end

        `addr_sum + 'd1: begin
            regs_addr = rm[2] ? `XB : `XA;
            regs_oe = 1;
            t2_we = 1;

            state_next = `addr_sum + 2;
        end

        `addr_sum + 'd2: begin
            t1_oe = 1;
            t2_oe = 1;
            alu_carry = 0;
            alu_opcode = `ADC;
            alu_oe = 1;
            if(decoded_d)
                t2_we = 1;
            else
                t1_we = 1;

            state_next = decoded_src;
        end

        `addr_reg: begin
            regs_addr = rm;
            regs_oe = 1;
            if(decoded_d)
                t2_we = 1;
            else
                t1_we = 1;

            state_next = decoded_src;
        end

        `load_src_reg: begin
            regs_addr = decoded_d ? rm : rg;
            regs_oe = 1;
            t2_we = 1;

            state_next = decoded_dst;
        end

        `load_src_mem: begin
            t1_oe = 0;
            t2_oe = 1;
            alu_opcode = `OR;
            alu_oe = 1;
            am_we = 1;

            state_next = `load_src_mem + 1;
        end

        `load_src_mem + 'd1: begin
            am_oe = 1;

            state_next = `load_src_mem + 2;
        end

        `load_src_mem + 'd2: begin
            ram_oe = 1;
            t2_we = 1;

            state_next = decoded_dst;
        end

        `load_dst_reg: begin
            regs_addr = decoded_d ? rg : rm;
            regs_oe = 1;
            t1_we = 1;

            state_next = decoded_exec;
        end

        `load_dst_mem: begin
            t1_oe = 1;
            t2_oe = 0;
            alu_opcode = `OR;
            alu_oe = 1;
            am_we = 1;

            state_next = `load_dst_mem + 1;
        end

        `load_dst_mem + 'd1: begin
            am_oe = 1;

            state_next = `load_dst_mem + 2;
        end

        `load_dst_mem + 'd2: begin
            ram_oe = 1;
            t1_we = 1;

            state_next = decoded_exec;
        end

        `exec_1op: begin
            t1_oe = 1;
            case(cop[4:6])
                3'b000: begin                               // INC
                    alu_carry = 1;
                    alu_opcode = `ADC;
                end
                3'b001: begin                               // DEC
                    alu_carry = 1;
                    alu_opcode = `SBB1;
                end
                3'b010: begin                               // NEG
                    alu_carry = 0;
                    alu_opcode = `SBB2;
                end
                3'b011: begin                               // NOT
                    alu_opcode = `NOT;
                end
                3'b100: alu_opcode = `SHL;                  // SHL/SAL
                3'b101: alu_opcode = `SHR;                  // SHR
                3'b110: alu_opcode = `SAR;                  // SAR
            endcase
            alu_oe = 1;
            t1_we = 1;
            ind_sel = 1;
            ind_we = 1;

            state_next = decoded_store;
        end

        `exec_2op: begin
            t1_oe = 1;
            t2_oe = 1;
            case(cop[4:6])
                3'b000: begin                               // ADD
                    alu_carry = 0;
                    alu_opcode = `ADC;
                end
                3'b001: begin                               // ADC
                    alu_carry = ind[0];
                    alu_opcode = `ADC;
                end
                3'b010: begin                               // SUB/CMP
                    alu_carry = 0;
                    alu_opcode = `SBB1;
                end
                3'b011: begin                               // SBB
                    alu_carry = ind[0];
                    alu_opcode = `SBB1;
                end
                3'b100: alu_opcode = `AND;                  // AND/TEST
                3'b101: alu_opcode = `OR;                   // OR
                3'b110: alu_opcode = `XOR;                  // XOR
            endcase
            alu_oe = 1;
            t1_we = 1;
            ind_sel = 1;
            ind_we = 1;

            state_next = decoded_store;
        end

        `store_reg: begin
            t1_oe = 1;
            t2_oe = 0;
            alu_opcode = `OR;
            alu_oe = 1;
            regs_addr = decoded_d ? rg : rm;
            regs_we = 1;

            state_next = `inc_cp;
        end

        `store_mem: begin
            t1_oe = 1;
            t2_oe = 0;
            alu_opcode = `OR;
            alu_oe = 1;
            am_oe = 1;
            ram_we = 1;

            state_next = `store_mem + 1;
        end

        `store_mem + 'd1: begin
            state_next = `inc_cp;
        end

        `inc_cp: begin
            cp_oe = 1;
            t1_we = 1;

            state_next = `inc_cp + 1;
        end

        `inc_cp + 'd1: begin
            t1_oe = 1;
            cp_we = 1;
            alu_oe = 1;
            alu_carry = 1;
            alu_opcode = `ADC;

            state_next = `fetch;
        end

        default: ;
    endcase
end

assign disp_state = state;

endmodule