/* decoder.v - Implementation of instruction opcode decoding logic

   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 "ucode_header.v"

module microcode(
	input [`UCODE_ADDR_BITS-1:0] ADDR,
	output [`UCODE_DATA_BITS-1:0] DATA
);

initial begin
	string ucode_path;
	if($value$plusargs("MICROCODE=%s",ucode_path))begin
		$readmemb(ucode_path,ucode_rom,0,`UCODE_SIZE-1);
	end else begin
		$display("Please supply microcode rom file as a runtime vvp argument +MICROCODE=<path>");
		$finish;
	end
end

reg [`UCODE_DATA_BITS-1:0] ucode_rom [ 0:`UCODE_SIZE-1 ];

assign DATA=ucode_rom[ADDR];

endmodule

// verilator lint_off UNUSEDSIGNAL
module decoder(
	input wire [15:0] CIR,input wire [15:0] FLAGS, output wire [2:0] INSTRUCTION_INFO, output wire [1:0]DECODER_SIGNALS,output reg [`PROC_STATE_BITS-1:0]next_state
	,output reg [2:0]IN_MOD, output reg [2:0]RM, output reg [15:0] PARAM1,output reg [15:0] PARAM2
	,output reg [1:0]in_alu1_sel1,output reg [1:0]in_alu1_sel2,output reg [2:0]OUT_MOD
	,output wire [11:0]REGISTER_FILE_CONTROL
	,output reg [2:0]ALU_1OP
	,output reg [`UCODE_ADDR_BITS-1:0] seq_addr_entry, input wire SIMPLE_MICRO, input wire [`UCODE_ADDR_BITS-1:0] seq_addr_input
	,output reg [2:0]instruction_size
);
// verilator lint_on UNUSEDSIGNAL

reg [3:0]reg_read_port1_addr;
reg [3:0]reg_read_port2_addr;
reg [3:0]reg_write_addr;
assign REGISTER_FILE_CONTROL={reg_write_addr,reg_read_port1_addr,reg_read_port2_addr};

/* For correct fetching of instructions and global options for the alu */
reg Wbit,Sbit,opcode_size;
assign INSTRUCTION_INFO={Wbit,Sbit,opcode_size};

reg ERROR, HALT;
assign DECODER_SIGNALS={ERROR,HALT};



// verilator lint_off UNUSEDSIGNAL
wire [`UCODE_DATA_BITS-1:0] ucode_data;
// verilator lint_on UNUSEDSIGNAL

microcode ucode(seq_addr_input,ucode_data);

/* 0 0 0 0  0 0 0 0 | 0 0 0 0  0 0 0 0 | 0 0 0 0  0 0 0 0 | 0 0 0 0  0 0 0 0 */

`define invalid_instruction next_state=`PROC_IF_STATE_ENTRY;ERROR<=1;IN_MOD=3'b011;seq_addr_entry<=`UCODE_NO_INSTRUCTION;


//TODO: A possible optimisation for instruction with 8bit parameter and
//opcode_size=0 would be to set PARAM1 here instead of sending execution over
//to PROC_DE_LOAD_8_PARAM

`define normal_instruction seq_addr_entry<=`UCODE_NO_INSTRUCTION;ERROR<=0;HALT<=0;

// I use blocking for basically putting names on the different fields of CIR and
// then branching off of that instead of the raw bits. otherwise the code
// would be identical
// verilator lint_off BLKSEQ
always @( CIR or SIMPLE_MICRO or seq_addr_input ) begin
	if (SIMPLE_MICRO==0)begin
		casez({CIR[15:8],CIR[5:3]})
			11'b0000_010?_??? : begin
				/*     ADD - Add Immediate word/byte to accumulator       */
				/* 0 0 0 0  0 1 0 W |       DATA       |     DATA if W   |*/
				opcode_size=0;
				Wbit=CIR[8:8];
				if(Wbit)begin
					instruction_size=3;
				end else begin
					instruction_size=2;
				end
				IN_MOD=3'b011;
				in_alu1_sel1=2'b00;
				in_alu1_sel2=2'b01;
				OUT_MOD=3'b011;
				reg_read_port2_addr={Wbit,3'b000};
				reg_write_addr={Wbit,3'b000};
				ALU_1OP=`ALU_OP_ADD;
				if(Wbit)
					next_state=`PROC_DE_LOAD_16_PARAM;
				else
					next_state=`PROC_DE_LOAD_8_PARAM;
				`normal_instruction;
			end
			11'b1000_00??_101, /* SUB */
				11'b1000_00??_000 : /* ADD */ begin
				/*                                  ADD - Add Immediate word/byte to register/memory                                      */
				/* 1 0 0 0  0 0 S W | MOD 0 0  0  R/M  |     < DISP LO >  |     < DISP HI >   |      DATA        |      DATA if W    |    */
				/*                                  SUB - Subtract immediate word/byte from register/memory                               */
				/* 1 0 0 0  0 0 S W | MOD 1 0  1  R/M  |     < DISP LO >  |     < DISP HI >   |      DATA        |      DATA if W    |    */
				opcode_size=1;
				Wbit=CIR[8:8];
				Sbit=CIR[9:9];
				IN_MOD={1'b0,CIR[7:6]};
				RM=CIR[2:0];
				in_alu1_sel1=2'b00;
				if(IN_MOD==3'b011)begin
					in_alu1_sel2=2'b01;
					reg_read_port2_addr={Wbit,RM};
					reg_write_addr={Wbit,RM};
				end else begin
					in_alu1_sel2=2'b00;
				end
				OUT_MOD=IN_MOD;
				case({Sbit,Wbit})
					2'b00,2'b11:begin
						next_state=`PROC_DE_LOAD_8_PARAM;
						instruction_size=3;
					end
					2'b01:begin
						next_state=`PROC_DE_LOAD_16_PARAM;
						instruction_size=4;
					end
					default:begin
						`invalid_instruction
					end
				endcase
				case(CIR[5:3])
					3'b000: ALU_1OP=`ALU_OP_ADD;
					3'b101: ALU_1OP=`ALU_OP_SUB_REVERSE;
					default:begin
						/*Should be impossible*/
						`invalid_instruction
					end
				endcase
				`normal_instruction;
			end
			11'b1000_00??_111 : begin
				/*                                  CMP - compare Immediate with register / memory                                        */
				/* 1 0 0 0  0 0 S W | MOD 1 1  1  R/M  |     < DISP LO >  |     < DISP HI >   |      DATA        |      DATA if W    |    */
				opcode_size=1;
				Wbit=CIR[8:8];
				Sbit=CIR[9:9];
				IN_MOD={1'b0,CIR[7:6]};
				RM=CIR[2:0];
				case({Sbit,Wbit})
					2'b00,2'b11:begin
						instruction_size=3;
					end
					2'b01:begin
						instruction_size=4;
					end
					2'b10:begin
						`invalid_instruction
					end
				endcase
				in_alu1_sel1=2'b00;
				OUT_MOD=3'b100;
				ALU_1OP=`ALU_OP_SUB;
				if(IN_MOD==3'b011)begin
					/*compare register with param*/
					in_alu1_sel2=2'b01;
					reg_read_port2_addr={Wbit,RM};
					next_state=`PROC_DE_LOAD_8_PARAM;
				end else begin
					/*compare register indirect access
					* with param */
					in_alu1_sel2=2'b00;
					next_state=`PROC_DE_LOAD_16_PARAM; /*will the call MEMIO_READ*/
				end
				`normal_instruction;
			end
			11'b1011_0???_??? : begin
				/*        MOV - Move Immediate byte to register            */
				/* 1 0 1 1  W  REG  |       DATA       |     DATA if W    |*/
				Wbit=CIR[11:11]; /* IS 0 */
				instruction_size=2;
				opcode_size=0;
				IN_MOD=3'b011;
				in_alu1_sel1=2'b00;
				in_alu1_sel2=2'b00;
				OUT_MOD=3'b011;
				reg_write_addr={1'b0,CIR[10:8]};
				PARAM1[7:0]=CIR[7:0];
				PARAM2=0;
				ALU_1OP=`ALU_OP_ADD;
				next_state=`PROC_EX_STATE_ENTRY;
				`normal_instruction;
			end
			11'b1011_1???_??? : begin
				/*MOV - Move Immediate word to register*/
				Wbit=CIR[11:11]; /*IS 1 */
				instruction_size=3;
				opcode_size=0;
				IN_MOD=3'b011;
				in_alu1_sel1=2'b00;
				in_alu1_sel2=2'b00;
				OUT_MOD=3'b011;
				reg_write_addr={1'b1,CIR[10:8]};
				ALU_1OP=`ALU_OP_ADD;
				PARAM2=0;
				next_state=`PROC_DE_LOAD_16_PARAM;
				`normal_instruction;
			end
			11'b1000_10??_??? : begin
				/*                      MOV - Reg/Mem to/from register                        */
				/* 1 0 0 0  1 0 D W | MOD   REG    RM  |    < DISP LO >   |    < DISP HI >   |*/
				opcode_size=1;
				instruction_size=2;
				RM=CIR[2:0];
				Wbit=CIR[8:8];
				in_alu1_sel1=2'b00;
				PARAM1=0;
				if(CIR[9:9] == 1)begin
					/* Mem/Reg to reg */
					IN_MOD={1'b0,CIR[7:6]};
					if(IN_MOD==3'b011)begin
						/*Reg to Reg*/
						in_alu1_sel2=2'b01;
						reg_read_port2_addr={Wbit,RM};
						next_state=`PROC_EX_STATE_ENTRY;
					end else begin
						/*Mem to Reg*/
						in_alu1_sel2=2'b00;
						next_state=`PROC_MEMIO_READ;
					end
					OUT_MOD=3'b011;
					reg_write_addr={Wbit,CIR[5:3]};
				end else begin
					/* Reg to Mem/Reg */
					IN_MOD=3'b011;
					OUT_MOD={1'b0,CIR[7:6]};
					if(IN_MOD==3'b011)begin
						/*Reg to Reg*/
						in_alu1_sel2=2'b01;
						reg_write_addr={Wbit,RM};
						next_state=`PROC_EX_STATE_ENTRY;
					end else begin
						/*Reg to Mem*/
						in_alu1_sel2=2'b00;
						next_state=`PROC_DE_LOAD_REG_TO_PARAM;
					end
					reg_read_port2_addr={Wbit,CIR[5:3]};
				end
				ALU_1OP=`ALU_OP_ADD;

				`normal_instruction;
			end
			11'b0100_????_???:begin//DEC
				/* DEC - Decrement Register */
				/*   | 0 1 0 0  1  REG  |   */
				/* INC - Increment Register */
				/*   | 0 1 0 0  0  REG  |   */
				instruction_size=1;
				opcode_size=0;
				Wbit=1;
				in_alu1_sel1=2'b01;
				in_alu1_sel2=2'b00;
				OUT_MOD=3'b011;
				IN_MOD=3'b011;
				PARAM2=1;
				reg_read_port1_addr={1'b1,CIR[10:8]};
				reg_write_addr={1'b1,CIR[10:8]};
				if(CIR[11:11]==0)
					ALU_1OP=`ALU_OP_ADD;
				else
					ALU_1OP=`ALU_OP_SUB;
				next_state=`PROC_EX_STATE_ENTRY;
				`normal_instruction;
			end
			11'b1111_111?_00? : begin
				/*                          INC - Register/Memory                            */
				/* 1 1 1 1  1 1 1 W | MOD 0 0  0  R/M  |    < DISP LO>    |    < DISP HI>    */
				/*                          DEC - Register/Memory                            */
				/* 1 1 1 1  1 1 1 W | MOD 0 0  1  R/M  |    < DISP LO>    |    < DISP HI>    */
				instruction_size=2;
				opcode_size=1;
				Wbit=CIR[8:8];
				IN_MOD={1'b0,CIR[7:6]};
				RM=CIR[2:0];
				in_alu1_sel2=(IN_MOD==3'b011)? 2'b01 : 2'b00;
				in_alu1_sel1=2'b00;/* number 1 */
				PARAM1=1;
				OUT_MOD=IN_MOD;

				/*in case IN_MOD=011 */
				reg_read_port2_addr={1'b0,RM};
				reg_write_addr={1'b0,RM};

				ALU_1OP=(CIR[3:3]==1)?`ALU_OP_SUB_REVERSE:`ALU_OP_ADD;
				if ( IN_MOD == 3'b011 )
					next_state=`PROC_EX_STATE_ENTRY;
				else
					next_state=`PROC_MEMIO_READ;
				`normal_instruction;
			end
			11'b1111_0100_??? : begin
				/*     HLT - Halt     */
				/* 1 1 1 1  0 1 0 0 | */
				instruction_size=1;
				opcode_size=0;
				IN_MOD=3'b011;
				HALT<=1;
				ERROR<=0;
				seq_addr_entry<=`UCODE_NO_INSTRUCTION;
				next_state=`PROC_HALT_STATE;

			end
			11'b0011_110?_??? : begin
				/*         CMP - Compare Immediate with accumulator        */
				/* 0 0 1 1  1 1 0 W |       DATA       |     DATA if W    |*/
				/*                                                         */
				/* NOTE: 8086 doc doesn't show the third byte but the      */
				/* W flag and my assembler seem to disagree                */
				Wbit=CIR[8:8];
				opcode_size=0;
				if(Wbit)begin
					instruction_size=3;
				end else begin
					instruction_size=2;
				end
				IN_MOD=3'b011;
				in_alu1_sel1=2'b00;
				in_alu1_sel2=2'b01;
				reg_read_port2_addr={Wbit,3'b000};
				OUT_MOD=3'b100;
				ALU_1OP=`ALU_OP_SUB;
				if(Wbit==1)
					next_state=`PROC_DE_LOAD_16_PARAM;
				else begin
					PARAM1[7:0]=CIR[7:0];
					next_state=`PROC_EX_STATE_ENTRY;
				end
				`normal_instruction;
			end
			11'b0111_????_???:begin
				/*     Conditional relative jumps       */
				/*        JE/JZ - Jump on Zero          */
				/* 0 1 1 1  0 1 0 0 |     IP-INC8      |*/
				/*         JS - Jump on Sign            */
				/* 0 1 1 1  1 0 0 0 |     IP-INC8      |*/
				/*       JNS -Jump on not Sign          */
				/* 0 1 1 1  1 0 0 1 |     IP-INC8      |*/
				/*                ....                  */
				instruction_size=2;
				Wbit=1;
				opcode_size=0;
				in_alu1_sel1=2'b10;
				in_alu1_sel2=2'b00;
				PARAM2={{8{CIR[7:7]}},CIR[7:0]};
				ALU_1OP=`ALU_OP_ADD_SIGNED_B;
				OUT_MOD=3'b101;
				case(CIR[11:9])
					3'b000: begin
						/* Jump on (not) Overflow */
						if(FLAGS[11:11]==CIR[8:8])
							next_state=`PROC_IF_STATE_ENTRY;
						else begin
							next_state=`PROC_EX_STATE_ENTRY;
						end
					end
					3'b010: begin
						/* Jump on (not) Zero */
						if(FLAGS[6:6]==CIR[8:8])
							next_state=`PROC_IF_STATE_ENTRY;
						else
							next_state=`PROC_EX_STATE_ENTRY;
					end
					3'b100: begin
						/* Jump on (not) Sign */
						if(FLAGS[7:7]==CIR[8:8])
							next_state=`PROC_IF_STATE_ENTRY;
						else
							next_state=`PROC_EX_STATE_ENTRY;
					end
					3'b101: begin
						/* Jump on (not) Parity */
						if(FLAGS[2:2]==CIR[8:8])
							next_state=`PROC_IF_STATE_ENTRY;
						else
							next_state=`PROC_EX_STATE_ENTRY;
					end
					default:begin
						`invalid_instruction; /*We don't support that condition*/
					end
				endcase
				`normal_instruction;
			end
			11'b1110_1011_???:begin
				/* JMP - Unconditional jump direct within segment (short) */
				/*        | 1 1 1 0  1 0 1 1 |     IP-INC-LO    |         */
				instruction_size=2;
				opcode_size=0;
				Wbit=1;
				in_alu1_sel1=2'b10;
				in_alu1_sel2=2'b00;
				PARAM2={{8{CIR[7:7]}},CIR[7:0]};
				ALU_1OP=`ALU_OP_ADD_SIGNED_B;
				OUT_MOD=3'b101;
				next_state=`PROC_EX_STATE_ENTRY;
				`normal_instruction;
			end
			11'b1100_1101_???:begin
				/*    INT - execute interrupt handler   */
				/* 1 1 0 0  1 1 0 1 |       DATA       |*/
				instruction_size=2;
				opcode_size=0;
				/* Emulate MS-DOS print routines */
				if(CIR[7:0]==8'h21 && register_file.registers[0][15:8]==8'h02)begin
					$write("%s" ,register_file.registers[2][7:0]); /*TODO:Could trigger erroneously while CIR is not final*/
				end
				next_state=`PROC_IF_STATE_ENTRY;
				`normal_instruction;
			end
			11'b1110_1000_???:begin
				/*            CALL - Direct call within segment            */
				/* 1 1 1 0  1 0 0 0 |    IP-INC-LO     |    IP-INC-HI     |*/

				// Microcode instruction
				instruction_size=3;
				opcode_size=0;
				Wbit=1;
				Sbit=1;
				PARAM2=2; //subtract from sp
				seq_addr_entry<=`UCODE_CALL_ENTRY;
			end
			11'b1100_0011_???:begin
				/*  RET - Return from call within segment */
				/*           | 1 1 0 0  0 0 1 1 |         */

				// Microcode instruction
				instruction_size=1;
				opcode_size=0;
				Wbit=1;
				Sbit=0;
				PARAM1=2;
				seq_addr_entry<=`UCODE_RET_ENTRY;
			end
			11'b1010_101?_???:begin
				/*  STOS - Write byte/word to [DI] and increment accordingly */
				/*                     | 1 0 1 0  1 0 1 W |                  */
				opcode_size=0;
				instruction_size=1;
				Wbit=CIR[8:8];
				Sbit=0;
				RM=3'b101;
				seq_addr_entry<=`UCODE_STOS_ENTRY;
				PARAM2=(Wbit==1)?2:1;
			end
			11'b0101_0???_???:begin
				/*  PUSH -  SP-=2; [SP]=REG   */
				/*    | 0 1 0 1  0  REG  |    */
				opcode_size=0;
				instruction_size=1;
				Wbit=1;
				Sbit=0;
				PARAM2=2;
				reg_read_port2_addr={1'b1,CIR[10:8]};
				seq_addr_entry<=`UCODE_PUSH_ENTRY;
			end
			11'b1111_011?_000:begin
				/*                                     TEST - Bitwise AND affecting only flags                                     */
				/* 1 1 1 1  0 1 1 W | MOD 0 0  0  R/M  |    < DISP-LO >   |    < DISP-HI >   |       DATA       |    DATA if W     */
				opcode_size=1;
				Wbit=CIR[8:8];
				IN_MOD={1'b0,CIR[7:6]};
				RM={CIR[2:0]};
				if(Wbit==1)begin
					instruction_size=4;
					next_state=`PROC_DE_LOAD_16_PARAM;
				end else begin
					instruction_size=3;
					next_state=`PROC_DE_LOAD_8_PARAM;
				end
				in_alu1_sel1=2'b00; /* PARAM1 */
				ALU_1OP=`ALU_OP_AND;
				case(IN_MOD)
					3'b011:begin
						in_alu1_sel2=2'b01;
						reg_read_port2_addr={Wbit,RM};
					end
					default:begin
						`invalid_instruction
					end
				endcase
				OUT_MOD=3'b100;/*NULL*/
				`normal_instruction;
			end
			11'b0101_1???_???:begin
				/*  POP - REG=[SP]; SP+=2   */
				/*    | 0 1 0 1  1  REG  |    */
				opcode_size=0;
				instruction_size=1;
				Wbit=1;
				Sbit=0;
				PARAM1=2;
				reg_write_addr={1'b1,CIR[10:8]};
				seq_addr_entry<=`UCODE_POP_ENTRY;
			end
			11'b1111_1111_100:begin
				/*            JMP - Unconditional indirect within segment jump               */
				/* 1 1 1 1  1 1 1 1 | MOD 1 0  0  R/M  |   < DISP-LO >    |    < DISP-HI >   */
				opcode_size=1;
				instruction_size=2;
				Wbit=1;
				IN_MOD={1'b0,CIR[7:6]};
				RM=CIR[2:0];
				in_alu1_sel1=2'b11;
				if (IN_MOD==3'b011)begin
					in_alu1_sel2=2'b01;
					reg_read_port2_addr={Wbit,RM};
					next_state=`PROC_EX_STATE_ENTRY;
				end else begin
					in_alu1_sel2=2'b00;
					next_state=`PROC_MEMIO_READ;
				end
				ALU_1OP=`ALU_OP_ADD;
				OUT_MOD=3'b101;
				`normal_instruction;
			end
			11'b1100_011?_000:begin
				/*                                MOV - Move immediate to register/memory                                          */
				/* 1 1 0 0  0 1 1 W | MOD 0 0  0  R/M  |    < DISP-LO >   |    < DISP-HI >   |       DATA       |    DATA if W     */
				Wbit=CIR[8:8];
				opcode_size=1;
				in_alu1_sel1=2'b00;
				in_alu1_sel2=2'b11;
				if(Wbit==1)begin
					instruction_size=4;
					next_state=`PROC_DE_LOAD_16_PARAM;
				end else begin
					instruction_size=3;
					next_state=`PROC_DE_LOAD_8_PARAM;
				end

				OUT_MOD={1'b0,CIR[7:6]};
				IN_MOD=3'b011;
				RM=CIR[2:0];
				`normal_instruction;
			end
			default:begin
				`invalid_instruction
			end
		endcase
	end else begin
		/*Microcode output*/
		//Sbit, Wbit, opcode_size and the others are still latched
		//from when we ordered the switch to microcode
		seq_addr_entry <= ucode_data[`UCODE_ADDR_BITS-1:0];
		case(ucode_data[7:6])
			2'b00: next_state=`PROC_EX_STATE_ENTRY;
			2'b01: next_state=`PROC_DE_LOAD_16_PARAM;
			2'b10: next_state=`PROC_DE_LOAD_8_PARAM;
			2'b11: next_state=`PROC_MEMIO_READ;
		endcase
		if(ucode_data[35:35]==0)
			reg_write_addr=ucode_data[11:8 ];
		in_alu1_sel1  =ucode_data[13:12];
		in_alu1_sel2  =ucode_data[15:14];
		OUT_MOD       =ucode_data[18:16];
		/*1:1 map essentially but I want to keep the spec for these bits separate
		* from the alu op select bits*/
		case(ucode_data[21:19])
			3'b000: ALU_1OP=`ALU_OP_ADD;
			3'b001: ALU_1OP=`ALU_OP_SUB;
			3'b010: ALU_1OP=`ALU_OP_AND;
			3'b011: ALU_1OP=`ALU_OP_OR;
			3'b100: ALU_1OP=`ALU_OP_XOR;
			3'b101: ALU_1OP=`ALU_OP_ADD_SIGNED_B;
			3'b110: ALU_1OP=`ALU_OP_SUB_REVERSE;
			default: begin end
		endcase
		if(ucode_data[33:33]==0)
			reg_read_port1_addr=ucode_data[25:22];
		IN_MOD=ucode_data[28:26];
		if(ucode_data[34:34]==0)
			reg_read_port2_addr=ucode_data[32:29];
		if(ucode_data[37:37]==1)
			Wbit=ucode_data[36:36];
	end
end
`undef invalid_instruction

endmodule
// verilator lint_on BLKSEQ