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9086/system/decoder.v

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/* 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 "exec_state_def.v"
`include "alu_header.v"
`include "ucode_header.v"
`include "error_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 [`ERROR_BITS:0] DECODER_SIGNALS, output reg [`EXEC_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_alu_sel1,output reg [1:0]in_alu_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 memio_address_select
,output reg MEM_OR_IO
);
// 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_BITS-1:0] ERROR;
reg 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);
`define invalid_instruction next_state=`EXEC_DONE;ERROR<=`ERR_UNIMPL_INSTRUCTION;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 EXEC_DE_LOAD_8_PARAM
`define normal_instruction seq_addr_entry<=`UCODE_NO_INSTRUCTION;ERROR<=`ERR_NO_ERROR;HALT<=0;MEM_OR_IO=0;
`define normal_microcoded ERROR<=`ERR_NO_ERROR;HALT<=0;MEM_OR_IO=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 @( FLAGS or 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; //TODO: move the decoding done in BIU here and use theat size checker
Wbit=CIR[8:8];
IN_MOD=3'b011;
in_alu_sel1=2'b00;
in_alu_sel2=2'b01;
OUT_MOD=3'b011;
MEM_OR_IO=0;
reg_read_port2_addr={Wbit,3'b000};
reg_write_addr={Wbit,3'b000};
ALU_1OP=`ALU_OP_ADD;
memio_address_select=0;
if(Wbit)
next_state=`EXEC_DE_LOAD_16_PARAM;
else
next_state=`EXEC_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_alu_sel1=2'b00;
if(IN_MOD==3'b011)begin
in_alu_sel2=2'b01;
reg_read_port2_addr={Wbit,RM};
reg_write_addr={Wbit,RM};
end else begin
in_alu_sel2=2'b00;
end
OUT_MOD=IN_MOD;
MEM_OR_IO=0;
memio_address_select=0;
case({Sbit,Wbit})
2'b00,2'b11:begin
next_state=`EXEC_DE_LOAD_8_PARAM;
end
2'b01:begin
next_state=`EXEC_DE_LOAD_16_PARAM;
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];
if ( {Sbit,Wbit} == 2'b10 )begin
`invalid_instruction
end
in_alu_sel1=2'b00;
OUT_MOD=3'b100;
MEM_OR_IO=0;
ALU_1OP=`ALU_OP_SUB;
memio_address_select=0;
if(IN_MOD==3'b011)begin
/*compare register with param*/
in_alu_sel2=2'b01;
reg_read_port2_addr={Wbit,RM};
next_state=`EXEC_DE_LOAD_8_PARAM;
end else begin
/*compare register indirect access
* with param */
in_alu_sel2=2'b00;
next_state=`EXEC_DE_LOAD_16_PARAM; /*will then 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 */
opcode_size=0;
IN_MOD=3'b011;
in_alu_sel1=2'b00;
in_alu_sel2=2'b00;
OUT_MOD=3'b011;
MEM_OR_IO=0;
reg_write_addr={1'b0,CIR[10:8]};
PARAM1[7:0]=CIR[7:0];
PARAM2=0;
ALU_1OP=`ALU_OP_ADD;
next_state=`EXEC_WRITE_ENTRY;
`normal_instruction;
memio_address_select=0;
end
11'b1011_1???_??? : begin
/*MOV - Move Immediate word to register*/
Wbit=CIR[11:11]; /*IS 1 */
opcode_size=0;
IN_MOD=3'b011;
in_alu_sel1=2'b00;
in_alu_sel2=2'b00;
OUT_MOD=3'b011;
MEM_OR_IO=0;
reg_write_addr={1'b1,CIR[10:8]};
ALU_1OP=`ALU_OP_ADD;
PARAM2=0;
next_state=`EXEC_DE_LOAD_16_PARAM;
`normal_instruction;
memio_address_select=0;
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;
RM=CIR[2:0];
Wbit=CIR[8:8];
in_alu_sel1=2'b00;
PARAM1=0;
MEM_OR_IO=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_alu_sel2=2'b01;
reg_read_port2_addr={Wbit,RM};
next_state=`EXEC_WRITE_ENTRY;
end else begin
/*Mem to Reg*/
in_alu_sel2=2'b00;
next_state=`EXEC_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_alu_sel2=2'b01;
reg_write_addr={Wbit,RM};
next_state=`EXEC_WRITE_ENTRY;
end else begin
/*Reg to Mem*/
in_alu_sel2=2'b00;
next_state=`EXEC_DE_LOAD_REG_TO_PARAM;
end
reg_read_port2_addr={Wbit,CIR[5:3]};
end
ALU_1OP=`ALU_OP_ADD;
`normal_instruction;
memio_address_select=0;
end
11'b0100_????_???:begin//DEC
/* DEC - Decrement Register */
/* | 0 1 0 0 1 REG | */
/* INC - Increment Register */
/* | 0 1 0 0 0 REG | */
opcode_size=0;
Wbit=1;
in_alu_sel1=2'b01;
in_alu_sel2=2'b00;
OUT_MOD=3'b011;
MEM_OR_IO=0;
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=`EXEC_WRITE_ENTRY;
`normal_instruction;
memio_address_select=0;
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> */
opcode_size=1;
Wbit=CIR[8:8];
IN_MOD={1'b0,CIR[7:6]};
RM=CIR[2:0];
in_alu_sel2=(IN_MOD==3'b011)? 2'b01 : 2'b00;
in_alu_sel1=2'b00;/* number 1 */
PARAM1=1;
OUT_MOD=IN_MOD;
MEM_OR_IO=0;
/*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=`EXEC_WRITE_ENTRY;
else
next_state=`EXEC_MEMIO_READ;
`normal_instruction;
memio_address_select=0;
end
11'b1111_0100_??? : begin
/* HLT - Halt */
/* 1 1 1 1 0 1 0 0 | */
opcode_size=0;
IN_MOD=3'b011;
HALT<=1;
ERROR<=`ERR_NO_ERROR;
MEM_OR_IO=0;
seq_addr_entry<=`UCODE_NO_INSTRUCTION;
next_state=`EXEC_HALT;
memio_address_select=0;
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;
IN_MOD=3'b011;
in_alu_sel1=2'b00;
in_alu_sel2=2'b01;
reg_read_port2_addr={Wbit,3'b000};
OUT_MOD=3'b100;
ALU_1OP=`ALU_OP_SUB;
MEM_OR_IO=0;
if(Wbit==1)
next_state=`EXEC_DE_LOAD_16_PARAM;
else begin
PARAM1[7:0]=CIR[7:0];
next_state=`EXEC_WRITE_ENTRY;
end
`normal_instruction;
memio_address_select=0;
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 |*/
/* .... */
Wbit=1;
opcode_size=0;
in_alu_sel1=2'b10;
in_alu_sel2=2'b00;
PARAM2={{8{CIR[7:7]}},CIR[7:0]};
ALU_1OP=`ALU_OP_ADD_SIGNED_B;
MEM_OR_IO=0;
OUT_MOD=3'b101;
case(CIR[11:9])
3'b000: begin
/* Jump on (not) Overflow */
if(FLAGS[11:11]==CIR[8:8])
next_state=`EXEC_NEXT_INSTRUCTION;
else begin
next_state=`EXEC_WRITE_ENTRY;
end
end
3'b010: begin
/* Jump on (not) Zero */
if(FLAGS[6:6]==CIR[8:8])
next_state=`EXEC_NEXT_INSTRUCTION;
else
next_state=`EXEC_WRITE_ENTRY;
end
3'b100: begin
/* Jump on (not) Sign */
if(FLAGS[7:7]==CIR[8:8])
next_state=`EXEC_NEXT_INSTRUCTION;
else
next_state=`EXEC_WRITE_ENTRY;
end
3'b101: begin
/* Jump on (not) Parity */
if(FLAGS[2:2]==CIR[8:8])
next_state=`EXEC_NEXT_INSTRUCTION;
else
next_state=`EXEC_WRITE_ENTRY;
end
default:begin
`invalid_instruction; /*We don't support that condition*/
end
endcase
`normal_instruction;
memio_address_select=0;
end
11'b1110_1011_???:begin
/* JMP - Unconditional jump direct within segment (short) */
/* | 1 1 1 0 1 0 1 1 | IP-INC-LO | */
opcode_size=0;
Wbit=1;
in_alu_sel1=2'b10;
in_alu_sel2=2'b00;
PARAM2={{8{CIR[7:7]}},CIR[7:0]};
ALU_1OP=`ALU_OP_ADD_SIGNED_B;
OUT_MOD=3'b101;
MEM_OR_IO=0;
next_state=`EXEC_WRITE_ENTRY;
`normal_instruction;
memio_address_select=0;
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
opcode_size=0;
Wbit=1;
Sbit=1;
PARAM2=2; //subtract from sp
seq_addr_entry<=`UCODE_CALL_ENTRY;
`normal_microcoded
memio_address_select=0;
end
11'b1100_0011_???:begin
/* RET - Return from call within segment */
/* | 1 1 0 0 0 0 1 1 | */
// Microcode instruction
opcode_size=0;
Wbit=1;
Sbit=0;
PARAM1=2;
seq_addr_entry<=`UCODE_RET_ENTRY;
`normal_microcoded
memio_address_select=0;
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;
Wbit=CIR[8:8];
Sbit=0;
RM=3'b101;
seq_addr_entry<=`UCODE_STOS_ENTRY;
`normal_microcoded
PARAM2=(Wbit==1)?2:1;
memio_address_select=0;
end
11'b0101_0???_???:begin
/* PUSH - SP-=2; [SP]=REG */
/* | 0 1 0 1 0 REG | */
opcode_size=0;
Wbit=1;
Sbit=0;
PARAM2=2;
reg_read_port2_addr={1'b1,CIR[10:8]};
seq_addr_entry<=`UCODE_PUSH_ENTRY;
`normal_microcoded
memio_address_select=0;
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]};
MEM_OR_IO=0;
if(Wbit==1)begin
next_state=`EXEC_DE_LOAD_16_PARAM;
end else begin
next_state=`EXEC_DE_LOAD_8_PARAM;
end
in_alu_sel1=2'b00; /* PARAM1 */
ALU_1OP=`ALU_OP_AND;
case(IN_MOD)
3'b011:begin
in_alu_sel2=2'b01;
reg_read_port2_addr={Wbit,RM};
end
default:begin
`invalid_instruction
end
endcase
OUT_MOD=3'b100;/*NULL*/
`normal_instruction;
memio_address_select=0;
end
11'b0101_1???_???:begin
/* POP - REG=[SP]; SP+=2 */
/* | 0 1 0 1 1 REG | */
opcode_size=0;
Wbit=1;
Sbit=0;
PARAM1=2;
reg_write_addr={1'b1,CIR[10:8]};
seq_addr_entry<=`UCODE_POP_ENTRY;
`normal_microcoded
memio_address_select=0;
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;
Wbit=1;
IN_MOD={1'b0,CIR[7:6]};
RM=CIR[2:0];
MEM_OR_IO=0;
in_alu_sel1=2'b11;
if (IN_MOD==3'b011)begin
in_alu_sel2=2'b01;
reg_read_port2_addr={Wbit,RM};
next_state=`EXEC_WRITE_ENTRY;
end else begin
in_alu_sel2=2'b00;
next_state=`EXEC_MEMIO_READ;
end
ALU_1OP=`ALU_OP_ADD;
OUT_MOD=3'b101;
`normal_instruction;
memio_address_select=0;
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_alu_sel1=2'b00;
in_alu_sel2=2'b11;
MEM_OR_IO=0;
if(Wbit==1)begin
next_state=`EXEC_DE_LOAD_16_PARAM;
end else begin
next_state=`EXEC_DE_LOAD_8_PARAM;
end
OUT_MOD={1'b0,CIR[7:6]};
IN_MOD=3'b011;
RM=CIR[2:0];
`normal_instruction;
memio_address_select=0;
end
11'b1100_1101_???:begin
/* INT - execute interrupt handler */
/* 1 1 0 0 1 1 0 1 | DATA |*/
// [skipped] 1) push flags
// [skipped] 2) clear trap and interrupt enable flag
// [skipped] 3) push CS
// [skipped] 4) fetch CS from interrupt table
// 5) push ProgCount
// 6) fetch ProgCount from interrupt table
opcode_size=0;
Wbit=1;
Sbit=0;
PARAM2=2;
seq_addr_entry<=`UCODE_INT_ENTRY;
`normal_microcoded
memio_address_select=0;
end
11'b1110_011?_???:begin
/* OUT - write AL or AX to a defined output port */
/* | 1 1 1 0 0 1 1 W | DATA 8 | */
memio_address_select=1;
Wbit=CIR[8:8];
opcode_size=0;
in_alu_sel1=2'b00;
in_alu_sel2=2'b11;
reg_read_port1_addr={Wbit,3'b000};
next_state=`EXEC_DE_LOAD_8_PARAM;
MEM_OR_IO=1;
HALT <= 0;
PARAM1=0;
OUT_MOD={3'b000};
IN_MOD=3'b011;
end
11'b1100_1111_???:begin
/* IRET - Return from interrupt */
/* | 1 1 0 0 1 1 1 1 | */
// Since we only push one thing on the stack
// on INT we can just reuse the code from RET
opcode_size=0;
Wbit=1;
Sbit=0;
PARAM1=2;
seq_addr_entry<=`UCODE_RET_ENTRY;
`normal_microcoded
memio_address_select=0;
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[8:6])
3'b000: next_state=`EXEC_WRITE_ENTRY;
3'b001: next_state=`EXEC_DE_LOAD_16_PARAM;
3'b010: next_state=`EXEC_DE_LOAD_8_PARAM;
3'b011: next_state=`EXEC_MEMIO_READ;
3'b100: next_state=`EXEC_MEMIO_READ_SETADDR;
default: begin end /*impossible*/
endcase
if(ucode_data[36:36]==0) /*Set reg write address*/
reg_write_addr = ucode_data[12:9 ];
in_alu_sel1 = ucode_data[14:13];
in_alu_sel2 = ucode_data[16:15];
OUT_MOD = ucode_data[19:17];
/*1:1 map essentially but I want to keep the spec for these bits separate
* from the alu op select bits*/
case(ucode_data[22:20])
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;
3'b111: ALU_1OP=`ALU_OP_SHIFT_LEFT;
default: begin end
endcase
if(ucode_data[34:34]==0) /* Set reg read port 1 address */
reg_read_port1_addr=ucode_data[26:23];
IN_MOD=ucode_data[29:27];
if(ucode_data[35:35]==0) /* Set reg read port 1 address */
reg_read_port2_addr=ucode_data[33:30];
if(ucode_data[37:37]==1) /* Overwrite Wbit */
Wbit=ucode_data[38:38];
memio_address_select=ucode_data[39:39];
MEM_OR_IO=0;
HALT <= 0;
end
end
`undef invalid_instruction
endmodule
// verilator lint_on BLKSEQ
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