/* processor.v - implementation of most functions of the 9086 processor

   This file is part of the 9086 project.

   Copyright (c) 2023 Efthymios Kritikos

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program 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 General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

`include "proc_state_def.v"
`include "alu_header.v"
`include "config.v"

module processor ( input clock, input reset, output reg [19:0] external_address_bus, inout [15:0] external_data_bus,output reg read, output reg write, output reg HALT,output reg ERROR);

/*if we don't read, output the register to have the bus stable by the write falling edge*/
reg [15:0] data_bus_output_register;
assign external_data_bus=read?data_bus_output_register:16'hz;

/*** Global Definitions ***/

reg [`PROC_STATE_BITS-1:0] state;

/* Decoder */
wire Wbit, Sbit, unaligning_instruction;
wire [`PROC_STATE_BITS-1:0] next_state;
wire [1:0]MOD;
wire [2:0]RM;
wire [15:0]DE_PARAM1;
wire [15:0]DE_PARAM2;
wire DE_ERROR,DE_HALT;
wire [3:0]DE_reg_read_port1_addr,DE_reg_write_addr;
wire opcode_size;
wire has_operands;
decoder decoder(
	CIR,FLAGS,Wbit,Sbit,unaligning_instruction,opcode_size,DE_ERROR,next_state
	,MOD,RM,DE_PARAM1,DE_PARAM2,DE_HALT,has_operands
	,in_alu1_sel1,in_alu1_sel2,out_alu1_sel
	,DE_reg_read_port1_addr,DE_reg_write_addr
	,ALU_1OP
);

// Registers
reg [19:0] ProgCount; /*TODO consider having single circuit to increment PC instead of having possible lots of adders all over the code*/
reg [15:0] CIR;
reg [15:0] PARAM1;
reg [15:0] PARAM2;
reg one_byte_instruction;
reg unaligned_access;

reg [15:0]FLAGS;

reg [15:0] BYTE_WRITE_TEMP_REG;//we read 16bits here if we want to change just 8 and leave the rest


/*** RESET LOGIC ***/
always @(negedge reset) begin
	if (reset==0) begin
		@(posedge clock);
		state=`PROC_HALT_STATE;
		ProgCount=0;//TODO: Reset Vector
		HALT=0;
		reg_write_we=1;
		unaligned_access=0;
		@(posedge reset)
		@(negedge clock);
		state=`PROC_IF_STATE_ENTRY;
		one_byte_instruction=0;
		ERROR=0;
	end
end

/*** ALU and EXEC stage logic ***/

//Architectural Register file
reg [3:0] reg_write_addr;
wire [15:0] reg_write_data;
reg reg_write_we;
reg [3:0] reg_read_port1_addr;
reg [15:0] reg_read_port1_data;
reg [1:0] reg_write_in_sel;
mux4 #(.WIDTH(16)) REG_FILE_WRITE_IN_MUX(
	ALU_1O,
	16'hz,
	16'hz,
	16'hz,
	reg_write_in_sel,
	reg_write_data);
register_file register_file(reg_write_addr,reg_write_data,reg_write_we,reg_read_port1_addr,reg_read_port1_data);

/****   ALU 1  ******/
reg [1:0] in_alu1_sel1;
reg [1:0] in_alu1_sel2;
/* out_alu1_sel : { EXTRA_FUNCTIONS_BIT[0:0], MOD_OR_EXTRA_FUNCTION[1:0] } */
reg [2:0] out_alu1_sel;

mux4 #(.WIDTH(16)) MUX16_1A(
	PARAM1,
	reg_read_port1_data,
	{ProgCount[14:0],unaligned_access},
	16'b0,
	in_alu1_sel1,
	ALU_1A);

mux4 #(.WIDTH(16)) MUX16_1B(
	PARAM2,
	reg_read_port1_data,
	{ProgCount[14:0],unaligned_access},
	16'b0,
	in_alu1_sel2,
	ALU_1B);

wire [15:0] ALU_1A;
wire [15:0] ALU_1B;
wire [15:0] ALU_1O;
reg [`ALU_OP_BITS-1:0]ALU_1OP;
wire [7:0] ALU_1FLAGS;
ALU ALU1(ALU_1A,ALU_1B,ALU_1O,ALU_1OP,ALU_1FLAGS,Wbit);

/*** Processor stages ***/
`define invalid_instruction state=`PROC_IF_STATE_ENTRY;ERROR=1;

always @(negedge clock) begin
	case(state)
		`PROC_IF_WRITE_CIR:begin
			if(unaligned_access)begin
				if(one_byte_instruction==1)begin /*TODO: have a read buffer so we can do this even with data reads */
					CIR <= {CIR[7:0],external_data_bus[15:8]};
					state=`PROC_DE_STATE_ENTRY;
				end else begin
					CIR[15:8] <= external_data_bus[7:0];
					state=`PROC_IF_STATE_EXTRA_FETCH_SET;
				end
			end else begin
				CIR <= external_data_bus;
				ProgCount=ProgCount+1;
				state=`PROC_DE_STATE_ENTRY;
			end
		end
		`PROC_IF_STATE_EXTRA_FETCH:begin
			CIR[7:0] <= external_data_bus[15:8];
			state=`PROC_DE_STATE_ENTRY;
		end
		`PROC_EX_STATE_EXIT:begin
			unaligned_access=unaligning_instruction^unaligned_access;
			case(out_alu1_sel) /*TODO: use RM*/
				3'b000,
					3'b001,
					3'b010 : begin
					case (RM) /* Duplicate code with write... */
						3'b000:begin
							/*[BX]+[SI]*/
							`invalid_instruction
						end
						3'b001:begin
							/*[BX]+[SI]*/
							`invalid_instruction
						end
						3'b010:begin
							/*[BP]+[SI]*/
							`invalid_instruction
						end
						3'b011:begin
							/*[BP]+[DI]*/
							`invalid_instruction
						end
						3'b100:begin
							/*[SI]*/
							reg_read_port1_addr=4'b1110;
							state=`PROC_MEMIO_WRITE;
						end
						3'b101:begin
							/*[DI]*/
							reg_read_port1_addr=4'b1111;
							state=`PROC_MEMIO_WRITE;
						end
						3'b110:begin
							/*d16 */
							`invalid_instruction
						end
						3'b111:begin
							/*[BX]*/
							reg_read_port1_addr=4'b1011;
							state=`PROC_MEMIO_WRITE;
						end
					endcase
				end
				3'b011:begin
					reg_write_we=0;
					state=`PROC_IF_STATE_ENTRY;
				end
				3'b101:begin
					ProgCount={5'b00000,ALU_1O[15:1]};
					unaligned_access=ALU_1O[0:0];
					state=`PROC_IF_STATE_ENTRY;
				end
				3'b100:begin
					state=`PROC_IF_STATE_ENTRY;
				end
				default:begin
					`invalid_instruction
				end
			endcase
		end
		`PROC_DE_LOAD_16_EXTRA_FETCH_SET:begin
			external_address_bus = ProgCount;
			state=`PROC_DE_LOAD_16_EXTRA_FETCH;
		end
		`PROC_MEMIO_READ_SETADDR:begin
			external_address_bus = {5'b00000,reg_read_port1_data[15:1]};
			state=reg_read_port1_data[0:0]?`PROC_MEMIO_GET_UNALIGNED_DATA:`PROC_MEMIO_GET_ALIGNED_DATA;
		end
		`PROC_MEMIO_PUT_BYTE:begin
			BYTE_WRITE_TEMP_REG=external_data_bus;
			state=`PROC_MEMIO_PUT_BYTE_STOP_READ;
		end
		`PROC_MEMIO_WRITE_EXIT:begin
			write=0;
			state=`PROC_IF_STATE_ENTRY;
		end
		`PROC_MEMIO_PUT_ALIGNED_DATA:begin
			read=1;
			data_bus_output_register={ALU_1O[7:0],ALU_1O[15:8]};
			state=`PROC_MEMIO_WRITE_EXIT;
		end
		`PROC_MEMIO_PUT_UNALIGNED_DATA:begin
			BYTE_WRITE_TEMP_REG=external_data_bus;
			state=`PROC_MEMIO_PUT_UNALIGNED_FIRST_BYTE;
		end
		`PROC_MEMIO_PUT_UNALIGNED_PREP_NEXT:begin
			write=0;
			state=`PROC_MEMIO_PUT_UNALIGNED_PREP_NEXT2;
			data_bus_output_register={BYTE_WRITE_TEMP_REG[15:8],ALU_1O[7:0]};
		end
		`PROC_MEMIO_PUT_UNALIGNED_PREP_NEXT3:begin
			BYTE_WRITE_TEMP_REG=external_data_bus;
			state=`PROC_MEMIO_PUT_UNALIGNED_PREP_NEXT4;
		end
		`PROC_MEMIO_GET_SECOND_BYTE:begin
			external_address_bus=external_address_bus+1;
			state=`PROC_MEMIO_GET_SECOND_BYTE1;
		end
		`PROC_DE_LOAD_8_PARAM_UNALIGNED:begin
				if({Sbit,Wbit}==2'b11)begin
					PARAM1 = {{8{external_data_bus[15:15]}},external_data_bus[15:8]};
				end else begin
					PARAM1[7:0] = external_data_bus[15:8];
				end
				state=`PROC_EX_STATE_ENTRY;
		end
		default:begin
		end
	endcase
end


always @(posedge clock) begin
	case(state)
		`PROC_HALT_STATE:begin
		end
		`PROC_IF_STATE_ENTRY:begin
			`ifdef DEBUG_PC_ADDRESS
				/* Weird (possible bug) where even though the
				* testbench stop the clock after ERROR gets
				* raised the logic for the rising edge still
				* gets triggered printing this debug message. */
				if(ERROR!=1)
					$display("Fetched instruction at %04x",{ProgCount[18:0],unaligned_access});
			`endif
			external_address_bus = ProgCount;
			read = 0;
			write = 1;
			reg_write_we=1;
			state=`PROC_IF_WRITE_CIR;
			reg_write_in_sel=2'b00;
		end
		`PROC_IF_STATE_EXTRA_FETCH_SET:begin
			ProgCount=ProgCount+1;
			external_address_bus = ProgCount;
			state=`PROC_IF_STATE_EXTRA_FETCH;
		end
		`PROC_DE_STATE_ENTRY:begin
			/* If we are unaligned, the address bus contains the
			 * ProgCount and points to the second word containing
			 * the nest unread byte in extenral_data_bus[7:0]. If
			 * we are aligned the address bus points to the first
			 * word of the instruction which contains no useful
			 * data anymore but the ProgCount has the correct
			 * address so update it now so that whatever the case
			 * external_data_bus contains at least some unknown data */
			one_byte_instruction=(!has_operands)&&(!opcode_size);
			external_address_bus = ProgCount;
			state=next_state;
			PARAM1=DE_PARAM1;
			PARAM2=DE_PARAM2;
			ERROR=DE_ERROR;
			HALT=DE_HALT;
			reg_read_port1_addr=DE_reg_read_port1_addr;
			reg_write_addr=DE_reg_write_addr;
		end
		`PROC_DE_LOAD_REG_TO_PARAM:begin
			PARAM1=reg_read_port1_data;
			state=`PROC_EX_STATE_ENTRY;
		end
		`PROC_DE_LOAD_8_PARAM:begin
			if(opcode_size==0)begin
				if({Sbit,Wbit}==2'b11)begin
					/*signed "16bit" read*/
					PARAM1 = {{8{CIR[7:7]}},CIR[7:0]};
				end else begin
					PARAM1[7:0] = CIR[7:0];
				end
				state=`PROC_EX_STATE_ENTRY;
			end else begin
				if(unaligned_access==1)begin
					if({Sbit,Wbit}==2'b11)begin
						/*signed "16bit" read*/
						PARAM1 = {{8{external_data_bus[7:7]}},external_data_bus[7:0]};
					end else begin
						PARAM1[7:0] = external_data_bus[7:0];
					end
					ProgCount=ProgCount+1;
					state=`PROC_EX_STATE_ENTRY;
				end else begin
					external_address_bus=ProgCount;
					state=`PROC_DE_LOAD_8_PARAM_UNALIGNED;
				end
			end
		end
		`PROC_DE_LOAD_16_PARAM:begin
			if(opcode_size==0)begin
				if(unaligned_access==1)begin
					PARAM1 = {external_data_bus[7:0],external_data_bus[15:8]};
					ProgCount=ProgCount+1;
					state=`PROC_EX_STATE_ENTRY;
				end else begin
					PARAM1 = {external_data_bus[15:8],CIR[7:0]};
					state=`PROC_EX_STATE_ENTRY;
				end
			end else begin
				ProgCount=ProgCount+1;
				if(unaligned_access==1)begin
					PARAM1[7:0] = external_data_bus[7:0];
					state=`PROC_DE_LOAD_16_EXTRA_FETCH_SET;
				end else begin
					PARAM1 = {external_data_bus[7:0],external_data_bus[15:8]};
					state=`PROC_EX_STATE_ENTRY;
				end
			end
		end
		`PROC_DE_LOAD_16_EXTRA_FETCH:begin
			PARAM1[15:8] = external_data_bus[15:8];
			state=`PROC_EX_STATE_ENTRY;
		end
		`RPOC_MEMIO_READ:begin
			/*Decode MOD R/M, read the data and place it to PARAM1*/
			case (RM)
				3'b000:begin
					/*[BX]+[SI]*/
					`invalid_instruction
				end
				3'b001:begin
					/*[BX]+[SI]*/
					`invalid_instruction
				end
				3'b010:begin
					/*[BP]+[SI]*/
					`invalid_instruction
				end
				3'b011:begin
					/*[BP]+[DI]*/
					`invalid_instruction
				end
				3'b100:begin
					/*[SI]*/
					reg_read_port1_addr=4'b1110;
					state=`PROC_MEMIO_READ_SETADDR;
				end
				3'b101:begin
					/*[DI]*/
					reg_read_port1_addr=4'b1111;
					state=`PROC_MEMIO_READ_SETADDR;
				end
				3'b110:begin
					/*d16 */
					`invalid_instruction
				end
				3'b111:begin
					/*[BX]*/
					reg_read_port1_addr=4'b1011;
					state=`PROC_MEMIO_READ_SETADDR;
				end
			endcase
			if(MOD!=2'b00)begin
				/*Actually check if 01 and add the 8bits or if 10 add the 16bits ....*/
				`invalid_instruction;
			end

		end
		`PROC_MEMIO_GET_ALIGNED_DATA:begin
			PARAM1=(Wbit==1)? {external_data_bus[7:0],external_data_bus[15:8]} : {8'b00000000,external_data_bus[15:8]} ;
			state=`PROC_EX_STATE_ENTRY;
		end
		`PROC_MEMIO_GET_UNALIGNED_DATA:begin
			PARAM1={8'b00000000,external_data_bus[7:0]};
			if(Wbit==1) begin
				state=`PROC_MEMIO_GET_SECOND_BYTE;
			end else begin
				state=`PROC_EX_STATE_ENTRY;
			end
		end
		`PROC_EX_STATE_ENTRY:begin
			FLAGS[7:0] = ALU_1FLAGS[7:0];
			state=`PROC_EX_STATE_EXIT;
		end
		`PROC_MEMIO_WRITE:begin
			/* ADDRESS: reg_read_port1_data   DATA:ALU1_O */
			`ifdef DEBUG_MEMORY_WRITES
				$display("Writing at %04x , %04x",reg_read_port1_data,ALU_1O);
			`endif
			external_address_bus = {5'b00000,reg_read_port1_data[15:1]};
			state = (Wbit==0) ? `PROC_MEMIO_PUT_BYTE : (reg_read_port1_data[0:0]?`PROC_MEMIO_PUT_UNALIGNED_DATA:`PROC_MEMIO_PUT_ALIGNED_DATA) ;
		end
		`PROC_MEMIO_PUT_BYTE_STOP_READ:begin
			read=1;
			state=`PROC_MEMIO_WRITE_EXIT;
			if(reg_read_port1_data[0:0]==0)
				data_bus_output_register={ALU_1O[7:0],BYTE_WRITE_TEMP_REG[7:0]};
			else
				data_bus_output_register={BYTE_WRITE_TEMP_REG[15:8],ALU_1O[7:0]};
		end
		`PROC_MEMIO_PUT_UNALIGNED_FIRST_BYTE:begin
			read=1;
			state=`PROC_MEMIO_PUT_UNALIGNED_PREP_NEXT;
			data_bus_output_register={BYTE_WRITE_TEMP_REG[15:8],ALU_1O[7:0]};
		end
		`PROC_MEMIO_PUT_UNALIGNED_PREP_NEXT2:begin
			external_address_bus=external_address_bus+1;
			write=1;
			read=0;
			state=`PROC_MEMIO_PUT_UNALIGNED_PREP_NEXT3;
		end
		`PROC_MEMIO_PUT_UNALIGNED_PREP_NEXT4:begin
			read=1;
			state=`PROC_MEMIO_WRITE_EXIT;
			data_bus_output_register={ALU_1O[15:8],BYTE_WRITE_TEMP_REG[7:0]};
		end
		`PROC_MEMIO_GET_SECOND_BYTE1:begin
			PARAM1[15:8]=external_data_bus[15:8];
			state=`PROC_EX_STATE_ENTRY;
		end
		default:begin
		end
	endcase
end


endmodule