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////////////////////////////////////////////////////////////////////////////
////                                                                    ////
//// T6507LP IP Core                                                    ////
////                                                                    ////
//// This file is part of the T6507LP project                           ////
//// http://www.opencores.org/cores/t6507lp/                            ////
////                                                                    ////
//// Description                                                        ////
//// 6507 FSM                                                           ////
////                                                                    ////
//// TODO:                                                              ////
//// - Fix absolute indexed mode                                        ////
//// - Code the relative mode                                           ////
//// - Code the indexed indirect mode                                   ////
//// - Code the indirect indexed mode                                   ////
//// - Code the absolute indirect mode                                  ////
////                                                                    ////
//// Author(s):                                                         ////
//// - Gabriel Oshiro Zardo, gabrieloshiro@gmail.com                    ////
//// - Samuel Nascimento Pagliarini (creep), snpagliarini@gmail.com     ////
////                                                                    ////
////////////////////////////////////////////////////////////////////////////
////                                                                    ////
//// Copyright (C) 2001 Authors and OPENCORES.ORG                       ////
////                                                                    ////
//// This source file may be used and distributed without               ////
//// restriction provided that this copyright statement is not          ////
//// removed from the file and that any derivative work contains        ////
//// the original copyright notice and the associated disclaimer.       ////
////                                                                    ////
//// This source file is free software; you can redistribute it         ////
//// and/or modify it under the terms of the GNU Lesser General         ////
//// Public License as published by the Free Software Foundation;       ////
//// either version 2.1 of the License, or (at your option) any         ////
//// later version.                                                     ////
////                                                                    ////
//// This source is distributed in the hope that it will be             ////
//// useful, but WITHOUT ANY WARRANTY; without even the implied         ////
//// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR            ////
//// PURPOSE. See the GNU Lesser General Public License for more        ////
//// details.                                                           ////
////                                                                    ////
//// You should have received a copy of the GNU Lesser General          ////
//// Public License along with this source; if not, download it         ////
//// from http://www.opencores.org/lgpl.shtml                           ////
////                                                                    ////
////////////////////////////////////////////////////////////////////////////
 
`timescale 1ns / 1ps
 
module t6507lp_fsm(clk, reset_n, alu_result, alu_status, data_in, address, control, data_out, alu_opcode, alu_a, alu_enable);
        parameter DATA_SIZE = 4'd8;
        parameter ADDR_SIZE = 4'd13;
 
        input clk;
        input reset_n;
        input [DATA_SIZE-1:0] alu_result;
        input [DATA_SIZE-1:0] alu_status;
        input [DATA_SIZE-1:0] data_in;
        output reg [ADDR_SIZE-1:0] address;
        output reg control; // one bit is enough? read = 0, write = 1
        output reg [DATA_SIZE-1:0] data_out;
        output reg [DATA_SIZE-1:0] alu_opcode;
        output reg [DATA_SIZE-1:0] alu_a;
        output reg alu_enable;
 
 
        // FSM states
        localparam RESET = 4'b1111;
        localparam FETCH_OP = 4'b0000;
        localparam FETCH_OP_CALC = 4'b0001;
        localparam FETCH_LOW = 4'b0010;
        localparam FETCH_HIGH = 4'b0011;
        localparam READ_MEM = 4'b0100;
        localparam DUMMY_WRT_CALC = 4'b0101;
        localparam WRITE_MEM = 4'b0110;
        localparam FETCH_OP_CALC_PARAM = 4'b0111;
 
        // OPCODES TODO: verify how this get synthesised
        `include "../T6507LP_Package.v"
 
        // control signals
        localparam MEM_READ = 1'b0;
        localparam MEM_WRITE = 1'b1;
 
        reg [ADDR_SIZE-1:0] pc;          // program counter
        reg [DATA_SIZE-1:0] sp;          // stack pointer
        reg [DATA_SIZE-1:0] ir;          // instruction register
        reg [ADDR_SIZE:0] temp_addr;     // temporary address
        reg [DATA_SIZE-1:0] temp_data;   // temporary data
 
        reg [3:0] state, next_state; // current and next state registers
        // TODO: not sure if this will be 4 bits wide. as of march 9th this was 4bit wide.
 
        // wiring that simplifies the FSM logic
        reg absolute;
        reg absolute_indexed;
        reg accumulator;
        reg immediate;
        reg implied;
        reg indirect;
        reg relative;
        reg zero_page;
        reg zero_page_indexed;
 
        // regs that store the type of operation. again, this simplifies the FSM a lot.
        reg read;
        reg read_modify_write;
        reg write;
        reg jump;
 
        wire [ADDR_SIZE-1:0] next_pc;
        assign next_pc = pc + 13'b0000000000001;
 
        always @ (posedge clk or negedge reset_n) begin // sequencial always block
                if (reset_n == 1'b0) begin
                        // all registers must assume default values
                        pc <= 0; // TODO: this is written somewhere. something about a reset vector. must be checked.
                        sp <= 0; // TODO: the default is not 0. maybe $0100 or something like that. must be checked.
                        ir <= 0;
                        temp_addr <= 0;
                        temp_data <= 0;
                        state <= RESET;
                        // registered outputs also receive default values
                        address <= 0;
                        control <= 0; // check if these 2 shouldnt be on the other always block along with the address
                        data_out <= 0;
                end
                else begin
                        state <= next_state;
                        control <= MEM_READ;
 
                        case (state)
                                RESET: begin
                                        // The processor was reset 
                                end
                                FETCH_OP: begin // this state is the simplest one. it is a simple fetch that must be done when the cpu was reset or
                                                // the last cycle was a memory write.
                                        pc <= next_pc;
                                        address <= next_pc;
                                        ir <= data_in;
                                end
                                FETCH_OP_CALC, FETCH_OP_CALC_PARAM: begin // this is the pipeline happening!
                                        pc <= next_pc;
                                        address <= next_pc;
                                        ir <= data_in;
                                end
                                FETCH_LOW: begin // in this state the opcode is already known so truly execution begins
                                        if (accumulator || implied) begin
                                                pc <= pc; // is this better?
                                                address <= pc;
                                        end
                                        else if (immediate) begin
                                                pc <= next_pc;
                                                address <= next_pc;
                                                temp_data <= data_in; // the follow-up byte is saved in temp_data 
                                        end
                                        else if (absolute) begin
                                                pc <= next_pc;
                                                address <= next_pc;
                                                temp_addr[7:0] <= data_in;
                                        end
                                end
                                FETCH_HIGH: begin
                                        if (jump) begin
                                                pc <= {data_in[4:0], temp_addr}; // PCL <= first byte, PCH <= second byte
                                                address <= {data_in[4:0], temp_addr};
                                        end
                                        else begin
                                                if (write) begin
                                                        pc <= next_pc;
                                                        temp_addr[12:8] <= data_in[4:0];
                                                        address <= {data_in[4:0],temp_addr[7:0]};
                                                        control <= MEM_WRITE;
                                                end
                                                else begin // read_modify_write or just read
                                                        pc <= next_pc;
                                                        temp_addr[12:8] <= data_in[4:0];
                                                        address <= {data_in[4:0],temp_addr[7:0]};
                                                end
                                        end
                                        //else begin
                                        //      $write("FETCHHIGH PROBLEM"); 
                                        //      $finish(0); 
                                        //end
                                end
                                READ_MEM: begin
                                        if (read_modify_write) begin
                                                pc <= pc;
                                                address <= temp_addr;
                                                control <= MEM_WRITE;
                                                temp_data <= data_in;
                                                data_out <= data_in; // writeback the same value
                                        end
                                        else begin
                                                pc <= pc;
                                                address <= pc;
                                                temp_data <= data_in;
                                        end
                                end
                                DUMMY_WRT_CALC: begin
                                        pc <= pc;
                                        address <= temp_addr;
                                        control <= MEM_WRITE;
                                        data_out <= alu_result;
                                end
                                WRITE_MEM: begin
                                        pc <= pc;
                                        address <= pc;
                                        data_out = 8'hzz;
                                end
                                default: begin
                                        $write("unknown state");        // TODO: check if synth really ignores this 2 lines. Otherwise wrap it with a `ifdef 
                                        $finish(0);
                                end
 
                        endcase
                end
        end
 
        always @ (*) begin // this is the next_state logic and the output logic always block
                //control = MEM_READ; 
                //data_out = 8'h00;
 
                alu_opcode = 8'h00;
                alu_a = 8'h00;
                alu_enable = 1'b0;
                //address = pc;
 
                next_state = RESET; // this prevents the latch
 
                case (state)
                        RESET: begin
                                next_state = FETCH_OP;
                        end
                        FETCH_OP: begin
                                next_state = FETCH_LOW;
                                //address = pc;
                        end
                        FETCH_OP_CALC: begin
                                next_state = FETCH_LOW;
                                alu_opcode = ir;
                                alu_enable = 1'b1;
                                //address = next_pc;
 
                        end
                        FETCH_OP_CALC_PARAM: begin
                                next_state = FETCH_LOW;
                                alu_opcode = ir;
                                alu_enable = 1'b1;
                                alu_a = temp_data;
                                //address = next_pc;
                        end
                        FETCH_LOW: begin
                                if (accumulator  || implied) begin
                                        alu_opcode = ir;
                                        alu_enable = 1'b1;
                                        next_state = FETCH_OP;
                                end
                                else if (immediate) begin
                                        next_state = FETCH_OP_CALC_PARAM;
                                end
                                else begin // at least the absolute address mode falls here
                                        next_state = FETCH_HIGH;
                                end
                                //address = next_pc;
 
                        end
                        FETCH_HIGH: begin
                                if (jump) begin
                                        next_state = FETCH_OP;
                                end
                                else if (read || read_modify_write) begin
                                        next_state = READ_MEM;
                                end
                                else if (write) begin
                                        next_state = WRITE_MEM;
                                end
                                else begin
                                        $write("unknown behavior");
                                        $finish(0);
                                end
                        end
                        READ_MEM: begin
                                if (read) begin
                                        next_state = FETCH_OP_CALC_PARAM;
                                end
                                else if (read_modify_write) begin
                                        next_state = DUMMY_WRT_CALC;
                                end
                        end
                        DUMMY_WRT_CALC: begin
                                alu_opcode = ir;
                                alu_enable = 1'b1;
                                alu_a = data_in;
                                next_state = WRITE_MEM;
                        end
                        WRITE_MEM: begin
                                next_state = FETCH_OP;
                        end
                        default: begin
                                next_state = RESET;
                        end
                endcase
        end
 
        // this always block is responsible for updating the address mode
        always @ (*) begin // 
                absolute = 1'b0;
                absolute_indexed = 1'b0;
                accumulator = 1'b0;
                immediate = 1'b0;
                implied = 1'b0;
                indirect = 1'b0;
                relative = 1'b0;
                zero_page = 1'b0;
                zero_page_indexed = 1'b0;
 
                read = 1'b0;
                read_modify_write = 1'b0;
                write = 1'b0;
                jump = 1'b0;
 
                case (ir)
                        BRK_IMP, CLC_IMP, CLD_IMP, CLI_IMP, CLV_IMP, DEX_IMP, DEY_IMP, INX_IMP, INY_IMP, NOP_IMP, PHA_IMP, PHP_IMP, PLA_IMP,
                        PLP_IMP, RTI_IMP, RTS_IMP, SEC_IMP, SED_IMP, SEI_IMP, TAX_IMP, TAY_IMP, TSX_IMP, TXA_IMP, TXS_IMP, TYA_IMP: begin
                                implied = 1'b1;
                        end
                        ASL_ACC, LSR_ACC, ROL_ACC, ROR_ACC: begin
                                accumulator = 1'b1;
                        end
                        ADC_IMM, AND_IMM, CMP_IMM, CPX_IMM, CPY_IMM, EOR_IMM, LDA_IMM, LDX_IMM, LDY_IMM, ORA_IMM, SBC_IMM: begin
                                immediate = 1'b1;
                        end
                        ADC_ZPG, AND_ZPG, ASL_ZPG, BIT_ZPG, CMP_ZPG, CPX_ZPG, CPY_ZPG, DEC_ZPG, EOR_ZPG, INC_ZPG, LDA_ZPG, LDX_ZPG, LDY_ZPG,
                        LSR_ZPG, ORA_ZPG, ROL_ZPG, ROR_ZPG, SBC_ZPG, STA_ZPG, STX_ZPG, STY_ZPG: begin
                                zero_page = 1'b1;
                        end
                        ADC_ZPX, AND_ZPX, ASL_ZPX, CMP_ZPX, DEC_ZPX, EOR_ZPX, INC_ZPX, LDA_ZPX, LDY_ZPX, LSR_ZPX, ORA_ZPX, ROL_ZPX, ROR_ZPX,
                        SBC_ZPX, STA_ZPX, LDX_ZPY, STX_ZPY, STY_ZPX: begin
                                zero_page_indexed = 1'b1;
                        end
                        BCC_REL, BCS_REL, BEQ_REL, BMI_REL, BNE_REL, BPL_REL, BVC_REL, BVS_REL: begin
                                relative = 1'b1;
                        end
                        ADC_ABS, AND_ABS, ASL_ABS, BIT_ABS, CMP_ABS, CPX_ABS, CPY_ABS, DEC_ABS, EOR_ABS, INC_ABS, JMP_ABS, JSR_ABS, LDA_ABS,
                        LDX_ABS, LDY_ABS, LSR_ABS, ORA_ABS, ROL_ABS, ROR_ABS, SBC_ABS, STA_ABS, STX_ABS, STY_ABS: begin
                                absolute = 1'b1;
                        end
                        ADC_ABX, AND_ABX, ASL_ABX, CMP_ABX, DEC_ABX, EOR_ABX, INC_ABX, LDA_ABX, LDY_ABX, LSR_ABX, ORA_ABX, ROL_ABX, ROR_ABX,
                        SBC_ABX, STA_ABX, ADC_ABY, AND_ABY, CMP_ABY, EOR_ABY, LDA_ABY, LDX_ABY, ORA_ABY, SBC_ABY, STA_ABY: begin
                                absolute_indexed = 1'b1;
                        end
                        ADC_IDX, AND_IDX, CMP_IDX, EOR_IDX, LDA_IDX, ORA_IDX, SBC_IDX, STA_IDX, ADC_IDY, AND_IDY, CMP_IDY, EOR_IDY, LDA_IDY,
                        ORA_IDY, SBC_IDY, STA_IDY: begin // all these opcodes are 8'hX1; TODO: optimize this
                                indirect = 1'b1;
                        end
                        default: begin
                                $write("unknown OPCODE!");
                                $finish();
                        end
                endcase
 
                case (ir)
                        ASL_ACC, ASL_ZPG, ASL_ZPX, ASL_ABS, ASL_ABX, LSR_ACC, LSR_ZPG, LSR_ZPX, LSR_ABS, LSR_ABX, ROL_ACC, ROL_ZPG, ROL_ZPX, ROL_ABS,
                        ROL_ABX, ROR_ACC, ROR_ZPG, ROR_ZPX, ROR_ABS, ROR_ABX, INC_ZPG, INC_ZPX, INC_ABS, INC_ABX, DEC_ZPG, DEC_ZPX, DEC_ABS,
                        DEC_ABX: begin
                                read_modify_write = 1'b1;
                        end
                        STA_ZPG, STA_ZPX, STA_ABS, STA_ABX, STA_ABY, STA_IDX, STA_IDY, STX_ZPG, STX_ZPY, STX_ABS, STY_ZPG, STY_ZPX, STY_ABS: begin
                                write = 1'b1;
                        end
                        default: begin // this should work fine since the previous case statement will detect the unknown/undocumented/unsupported opcodes
                                read = 1'b1;
                        end
                endcase
 
                if (ir == JMP_ABS || ir == JMP_IND) begin // the opcodes are 8'h4C and 8'h6C
                        jump = 1'b1;
                end
        end // no way
endmodule
 
 
 

Line No.	Rev	Author	Line
1	61	creep	`////////////////////////////////////////////////////////////////////////////`
2			`//// ////`
3			`//// T6507LP IP Core ////`
4			`//// ////`
5			`//// This file is part of the T6507LP project ////`
6			`//// http://www.opencores.org/cores/t6507lp/ ////`
7			`//// ////`
8			`//// Description ////`
9			`//// 6507 FSM ////`
10			`//// ////`
11			`//// TODO: ////`
12			`//// - Fix absolute indexed mode ////`
13			`//// - Code the relative mode ////`
14			`//// - Code the indexed indirect mode ////`
15			`//// - Code the indirect indexed mode ////`
16			`//// - Code the absolute indirect mode ////`
17			`//// ////`
18			`//// Author(s): ////`
19			`//// - Gabriel Oshiro Zardo, gabrieloshiro@gmail.com ////`
20			`//// - Samuel Nascimento Pagliarini (creep), snpagliarini@gmail.com ////`
21			`//// ////`
22			`////////////////////////////////////////////////////////////////////////////`
23			`//// ////`
24			`//// Copyright (C) 2001 Authors and OPENCORES.ORG ////`
25			`//// ////`
26			`//// This source file may be used and distributed without ////`
27			`//// restriction provided that this copyright statement is not ////`
28			`//// removed from the file and that any derivative work contains ////`
29			`//// the original copyright notice and the associated disclaimer. ////`
30			`//// ////`
31			`//// This source file is free software; you can redistribute it ////`
32			`//// and/or modify it under the terms of the GNU Lesser General ////`
33			`//// Public License as published by the Free Software Foundation; ////`
34			`//// either version 2.1 of the License, or (at your option) any ////`
35			`//// later version. ////`
36			`//// ////`
37			`//// This source is distributed in the hope that it will be ////`
38			`//// useful, but WITHOUT ANY WARRANTY; without even the implied ////`
39			`//// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR ////`
40			`//// PURPOSE. See the GNU Lesser General Public License for more ////`
41			`//// details. ////`
42			`//// ////`
43			`//// You should have received a copy of the GNU Lesser General ////`
44			`//// Public License along with this source; if not, download it ////`
45			`//// from http://www.opencores.org/lgpl.shtml ////`
46			`//// ////`
47			`////////////////////////////////////////////////////////////////////////////`
48
49			`timescale 1ns / 1ps
50
51	71	creep	`module t6507lp_fsm(clk, reset_n, alu_result, alu_status, data_in, address, control, data_out, alu_opcode, alu_a, alu_enable);`
52	68	creep	`parameter DATA_SIZE = 4'd8;`
53			`parameter ADDR_SIZE = 4'd13;`
54
55	71	creep	`input clk;`
56			`input reset_n;`
57	68	creep	`input [DATA_SIZE-1:0] alu_result;`
58			`input [DATA_SIZE-1:0] alu_status;`
59			`input [DATA_SIZE-1:0] data_in;`
60			`output reg [ADDR_SIZE-1:0] address;`
61	61	creep	`output reg control; // one bit is enough? read = 0, write = 1`
62	68	creep	`output reg [DATA_SIZE-1:0] data_out;`
63			`output reg [DATA_SIZE-1:0] alu_opcode;`
64			`output reg [DATA_SIZE-1:0] alu_a;`
65			`output reg alu_enable;`
66	61	creep
67	68	creep
68	61	creep	`// FSM states`
69	68	creep	`localparam RESET = 4'b1111;`
70	63	creep	`localparam FETCH_OP = 4'b0000;`
71	68	creep	`localparam FETCH_OP_CALC = 4'b0001;`
72			`localparam FETCH_LOW = 4'b0010;`
73			`localparam FETCH_HIGH = 4'b0011;`
74	71	creep	`localparam READ_MEM = 4'b0100;`
75			`localparam DUMMY_WRT_CALC = 4'b0101;`
76			`localparam WRITE_MEM = 4'b0110;`
77			`localparam FETCH_OP_CALC_PARAM = 4'b0111;`
78	61	creep
79			`// OPCODES TODO: verify how this get synthesised`
80			`include "../T6507LP_Package.v"
81
82			`// control signals`
83			`localparam MEM_READ = 1'b0;`
84			`localparam MEM_WRITE = 1'b1;`
85
86	68	creep	`reg [ADDR_SIZE-1:0] pc; // program counter`
87			`reg [DATA_SIZE-1:0] sp; // stack pointer`
88			`reg [DATA_SIZE-1:0] ir; // instruction register`
89	71	creep	`reg [ADDR_SIZE:0] temp_addr; // temporary address`
90	68	creep	`reg [DATA_SIZE-1:0] temp_data; // temporary data`
91	61	creep
92			`reg [3:0] state, next_state; // current and next state registers`
93			`// TODO: not sure if this will be 4 bits wide. as of march 9th this was 4bit wide.`
94
95			`// wiring that simplifies the FSM logic`
96			`reg absolute;`
97			`reg absolute_indexed;`
98			`reg accumulator;`
99			`reg immediate;`
100			`reg implied;`
101			`reg indirect;`
102			`reg relative;`
103			`reg zero_page;`
104			`reg zero_page_indexed;`
105
106			`// regs that store the type of operation. again, this simplifies the FSM a lot.`
107			`reg read;`
108			`reg read_modify_write;`
109			`reg write;`
110			`reg jump;`
111
112	68	creep	`wire [ADDR_SIZE-1:0] next_pc;`
113	63	creep	`assign next_pc = pc + 13'b0000000000001;`
114	61	creep
115	71	creep	`always @ (posedge clk or negedge reset_n) begin // sequencial always block`
116			`if (reset_n == 1'b0) begin`
117			`// all registers must assume default values`
118	68	creep	`pc <= 0; // TODO: this is written somewhere. something about a reset vector. must be checked.`
119			`sp <= 0; // TODO: the default is not 0. maybe $0100 or something like that. must be checked.`
120			`ir <= 0;`
121	71	creep	`temp_addr <= 0;`
122	68	creep	`temp_data <= 0;`
123			`state <= RESET;`
124	71	creep	`// registered outputs also receive default values`
125			`address <= 0;`
126			`control <= 0; // check if these 2 shouldnt be on the other always block along with the address`
127			`data_out <= 0;`
128	61	creep	`end`
129			`else begin`
130			`state <= next_state;`
131	71	creep	`control <= MEM_READ;`
132	61	creep
133			`case (state)`
134	68	creep	`RESET: begin`
135			`// The processor was reset`
136			`end`
137	61	creep	`FETCH_OP: begin // this state is the simplest one. it is a simple fetch that must be done when the cpu was reset or`
138			`// the last cycle was a memory write.`
139			`pc <= next_pc;`
140	71	creep	`address <= next_pc;`
141	70	creep	`ir <= data_in;`
142	61	creep	`end`
143	71	creep	`FETCH_OP_CALC, FETCH_OP_CALC_PARAM: begin // this is the pipeline happening!`
144	61	creep	`pc <= next_pc;`
145	71	creep	`address <= next_pc;`
146	70	creep	`ir <= data_in;`
147	61	creep	`end`
148	68	creep	`FETCH_LOW: begin // in this state the opcode is already known so truly execution begins`
149			`if (accumulator \|\| implied) begin`
150	70	creep	`pc <= pc; // is this better?`
151	71	creep	`address <= pc;`
152	61	creep	`end`
153	70	creep	`else if (immediate) begin`
154	68	creep	`pc <= next_pc;`
155	71	creep	`address <= next_pc;`
156	70	creep	`temp_data <= data_in; // the follow-up byte is saved in temp_data`
157	61	creep	`end`
158	71	creep	`else if (absolute) begin`
159			`pc <= next_pc;`
160			`address <= next_pc;`
161			`temp_addr[7:0] <= data_in;`
162			`end`
163	61	creep	`end`
164	71	creep	`FETCH_HIGH: begin`
165			`if (jump) begin`
166			`pc <= {data_in[4:0], temp_addr}; // PCL <= first byte, PCH <= second byte`
167			`address <= {data_in[4:0], temp_addr};`
168			`end`
169			`else begin`
170			`if (write) begin`
171			`pc <= next_pc;`
172			`temp_addr[12:8] <= data_in[4:0];`
173			`address <= {data_in[4:0],temp_addr[7:0]};`
174			`control <= MEM_WRITE;`
175			`end`
176			`else begin // read_modify_write or just read`
177			`pc <= next_pc;`
178			`temp_addr[12:8] <= data_in[4:0];`
179			`address <= {data_in[4:0],temp_addr[7:0]};`
180			`end`
181			`end`
182			`//else begin`
183			`// $write("FETCHHIGH PROBLEM");`
184			`// $finish(0);`
185			`//end`
186	61	creep	`end`
187	71	creep	`READ_MEM: begin`
188			`if (read_modify_write) begin`
189			`pc <= pc;`
190			`address <= temp_addr;`
191			`control <= MEM_WRITE;`
192			`temp_data <= data_in;`
193			`data_out <= data_in; // writeback the same value`
194			`end`
195			`else begin`
196			`pc <= pc;`
197			`address <= pc;`
198			`temp_data <= data_in;`
199			`end`
200	70	creep	`end`
201	71	creep	`DUMMY_WRT_CALC: begin`
202			`pc <= pc;`
203			`address <= temp_addr;`
204			`control <= MEM_WRITE;`
205			`data_out <= alu_result;`
206	70	creep	`end`
207	71	creep	`WRITE_MEM: begin`
208			`pc <= pc;`
209			`address <= pc;`
210			`data_out = 8'hzz;`
211	70	creep	`end`
212			`default: begin`
213			$write("unknown state"); // TODO: check if synth really ignores this 2 lines. Otherwise wrap it with a `ifdef
214			`$finish(0);`
215			`end`
216
217			`endcase`
218			`end`
219			`end`
220
221	71	creep	`always @ (*) begin // this is the next_state logic and the output logic always block`
222	70	creep	`//control = MEM_READ;`
223			`//data_out = 8'h00;`
224	71	creep
225			`alu_opcode = 8'h00;`
226			`alu_a = 8'h00;`
227			`alu_enable = 1'b0;`
228			`//address = pc;`
229	70	creep
230	71	creep	`next_state = RESET; // this prevents the latch`
231	68	creep
232	71	creep	`case (state)`
233			`RESET: begin`
234			`next_state = FETCH_OP;`
235			`end`
236			`FETCH_OP: begin`
237			`next_state = FETCH_LOW;`
238			`//address = pc;`
239			`end`
240			`FETCH_OP_CALC: begin`
241			`next_state = FETCH_LOW;`
242			`alu_opcode = ir;`
243			`alu_enable = 1'b1;`
244			`//address = next_pc;`
245
246			`end`
247			`FETCH_OP_CALC_PARAM: begin`
248			`next_state = FETCH_LOW;`
249			`alu_opcode = ir;`
250			`alu_enable = 1'b1;`
251			`alu_a = temp_data;`
252			`//address = next_pc;`
253			`end`
254			`FETCH_LOW: begin`
255			`if (accumulator \|\| implied) begin`
256			`alu_opcode = ir;`
257			`alu_enable = 1'b1;`
258			`next_state = FETCH_OP;`
259			`end`
260			`else if (immediate) begin`
261			`next_state = FETCH_OP_CALC_PARAM;`
262			`end`
263			`else begin // at least the absolute address mode falls here`
264			`next_state = FETCH_HIGH;`
265			`end`
266			`//address = next_pc;`
267
268			`end`
269			`FETCH_HIGH: begin`
270			`if (jump) begin`
271	68	creep	`next_state = FETCH_OP;`
272	61	creep	`end`
273	71	creep	`else if (read \|\| read_modify_write) begin`
274			`next_state = READ_MEM;`
275	61	creep	`end`
276	71	creep	`else if (write) begin`
277			`next_state = WRITE_MEM;`
278	68	creep	`end`
279	71	creep	`else begin`
280			`$write("unknown behavior");`
281			`$finish(0);`
282	61	creep	`end`
283	71	creep	`end`
284			`READ_MEM: begin`
285			`if (read) begin`
286			`next_state = FETCH_OP_CALC_PARAM;`
287	61	creep	`end`
288	71	creep	`else if (read_modify_write) begin`
289			`next_state = DUMMY_WRT_CALC;`
290			`end`
291			`end`
292			`DUMMY_WRT_CALC: begin`
293			`alu_opcode = ir;`
294			`alu_enable = 1'b1;`
295			`alu_a = data_in;`
296			`next_state = WRITE_MEM;`
297			`end`
298			`WRITE_MEM: begin`
299			`next_state = FETCH_OP;`
300			`end`
301			`default: begin`
302			`next_state = RESET;`
303			`end`
304			`endcase`
305	61	creep	`end`
306
307			`// this always block is responsible for updating the address mode`
308	68	creep	`always @ (*) begin //`
309	61	creep	`absolute = 1'b0;`
310			`absolute_indexed = 1'b0;`
311			`accumulator = 1'b0;`
312			`immediate = 1'b0;`
313			`implied = 1'b0;`
314			`indirect = 1'b0;`
315			`relative = 1'b0;`
316			`zero_page = 1'b0;`
317			`zero_page_indexed = 1'b0;`
318
319			`read = 1'b0;`
320			`read_modify_write = 1'b0;`
321			`write = 1'b0;`
322			`jump = 1'b0;`
323
324	70	creep	`case (ir)`
325			`BRK_IMP, CLC_IMP, CLD_IMP, CLI_IMP, CLV_IMP, DEX_IMP, DEY_IMP, INX_IMP, INY_IMP, NOP_IMP, PHA_IMP, PHP_IMP, PLA_IMP,`
326			`PLP_IMP, RTI_IMP, RTS_IMP, SEC_IMP, SED_IMP, SEI_IMP, TAX_IMP, TAY_IMP, TSX_IMP, TXA_IMP, TXS_IMP, TYA_IMP: begin`
327			`implied = 1'b1;`
328			`end`
329			`ASL_ACC, LSR_ACC, ROL_ACC, ROR_ACC: begin`
330			`accumulator = 1'b1;`
331			`end`
332			`ADC_IMM, AND_IMM, CMP_IMM, CPX_IMM, CPY_IMM, EOR_IMM, LDA_IMM, LDX_IMM, LDY_IMM, ORA_IMM, SBC_IMM: begin`
333			`immediate = 1'b1;`
334			`end`
335			`ADC_ZPG, AND_ZPG, ASL_ZPG, BIT_ZPG, CMP_ZPG, CPX_ZPG, CPY_ZPG, DEC_ZPG, EOR_ZPG, INC_ZPG, LDA_ZPG, LDX_ZPG, LDY_ZPG,`
336			`LSR_ZPG, ORA_ZPG, ROL_ZPG, ROR_ZPG, SBC_ZPG, STA_ZPG, STX_ZPG, STY_ZPG: begin`
337			`zero_page = 1'b1;`
338			`end`
339			`ADC_ZPX, AND_ZPX, ASL_ZPX, CMP_ZPX, DEC_ZPX, EOR_ZPX, INC_ZPX, LDA_ZPX, LDY_ZPX, LSR_ZPX, ORA_ZPX, ROL_ZPX, ROR_ZPX,`
340			`SBC_ZPX, STA_ZPX, LDX_ZPY, STX_ZPY, STY_ZPX: begin`
341			`zero_page_indexed = 1'b1;`
342			`end`
343			`BCC_REL, BCS_REL, BEQ_REL, BMI_REL, BNE_REL, BPL_REL, BVC_REL, BVS_REL: begin`
344			`relative = 1'b1;`
345			`end`
346			`ADC_ABS, AND_ABS, ASL_ABS, BIT_ABS, CMP_ABS, CPX_ABS, CPY_ABS, DEC_ABS, EOR_ABS, INC_ABS, JMP_ABS, JSR_ABS, LDA_ABS,`
347			`LDX_ABS, LDY_ABS, LSR_ABS, ORA_ABS, ROL_ABS, ROR_ABS, SBC_ABS, STA_ABS, STX_ABS, STY_ABS: begin`
348			`absolute = 1'b1;`
349			`end`
350			`ADC_ABX, AND_ABX, ASL_ABX, CMP_ABX, DEC_ABX, EOR_ABX, INC_ABX, LDA_ABX, LDY_ABX, LSR_ABX, ORA_ABX, ROL_ABX, ROR_ABX,`
351			`SBC_ABX, STA_ABX, ADC_ABY, AND_ABY, CMP_ABY, EOR_ABY, LDA_ABY, LDX_ABY, ORA_ABY, SBC_ABY, STA_ABY: begin`
352			`absolute_indexed = 1'b1;`
353			`end`
354			`ADC_IDX, AND_IDX, CMP_IDX, EOR_IDX, LDA_IDX, ORA_IDX, SBC_IDX, STA_IDX, ADC_IDY, AND_IDY, CMP_IDY, EOR_IDY, LDA_IDY,`
355			`ORA_IDY, SBC_IDY, STA_IDY: begin // all these opcodes are 8'hX1; TODO: optimize this`
356			`indirect = 1'b1;`
357			`end`
358	71	creep	`default: begin`
359			`$write("unknown OPCODE!");`
360			`$finish();`
361			`end`
362	70	creep	`endcase`
363	71	creep
364			`case (ir)`
365			`ASL_ACC, ASL_ZPG, ASL_ZPX, ASL_ABS, ASL_ABX, LSR_ACC, LSR_ZPG, LSR_ZPX, LSR_ABS, LSR_ABX, ROL_ACC, ROL_ZPG, ROL_ZPX, ROL_ABS,`
366			`ROL_ABX, ROR_ACC, ROR_ZPG, ROR_ZPX, ROR_ABS, ROR_ABX, INC_ZPG, INC_ZPX, INC_ABS, INC_ABX, DEC_ZPG, DEC_ZPX, DEC_ABS,`
367			`DEC_ABX: begin`
368			`read_modify_write = 1'b1;`
369			`end`
370			`STA_ZPG, STA_ZPX, STA_ABS, STA_ABX, STA_ABY, STA_IDX, STA_IDY, STX_ZPG, STX_ZPY, STX_ABS, STY_ZPG, STY_ZPX, STY_ABS: begin`
371			`write = 1'b1;`
372			`end`
373			`default: begin // this should work fine since the previous case statement will detect the unknown/undocumented/unsupported opcodes`
374			`read = 1'b1;`
375			`end`
376			`endcase`
377	61	creep
378	71	creep	`if (ir == JMP_ABS \|\| ir == JMP_IND) begin // the opcodes are 8'h4C and 8'h6C`
379	70	creep	`jump = 1'b1;`
380			`end`
381	63	creep	`end // no way`
382	61	creep	`endmodule`
383
384
385

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