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8bitworkshop/presets/verilog/cpu6502.v

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/* verilator lint_off CASEX */
/* verilator lint_off CASEINCOMPLETE */
/* verilator lint_off CASEOVERLAP */
/* verilator lint_off SYNCASYNCNET */
/*
* verilog model of 6502 CPU.
*
* (C) Arlet Ottens, <arlet@c-scape.nl>
*
* https://github.com/Arlet/verilog-6502/
*
* Feel free to use this code in any project (commercial or not), as long as you
* keep this message, and the copyright notice. This code is provided "as is",
* without any warranties of any kind.
*
* Note that not all 6502 interface signals are supported (yet). The goal
* is to create an Acorn Atom model, and the Atom didn't use all signals on
* the main board.
*
* The data bus is implemented as separate read/write buses. Combine them
* on the output pads if external memory is required.
*
* Also see: https://github.com/sehugg/mango_one
*/
module cpu6502( clk, reset, AB, DI, DO, WE, IRQ, NMI, RDY );
input clk; // CPU clock
input reset; // reset signal
output reg [15:0] AB; // address bus
input [7:0] DI; // data in, read bus
output [7:0] DO; // data out, write bus
output WE; // write enable
input IRQ; // interrupt request
input NMI; // non-maskable interrupt request
input RDY; // Ready signal. Pauses CPU when RDY=0
/*
* internal signals
*/
reg [15:0] PC; // Program Counter
reg [7:0] ABL; // Address Bus Register LSB
reg [7:0] ABH; // Address Bus Register MSB
wire [7:0] ADD; // Adder Hold Register (registered in ALU)
reg [7:0] DIHOLD; // Hold for Data In
reg DIHOLD_valid; //
wire [7:0] DIMUX; //
reg [7:0] IRHOLD; // Hold for Instruction register
reg IRHOLD_valid; // Valid instruction in IRHOLD
reg [7:0] AXYS[3:0]; // A, X, Y and S register file
reg C = 0; // carry flag (init at zero to avoid X's in ALU sim)
reg Z = 0; // zero flag
reg I = 0; // interrupt flag
reg D = 0; // decimal flag
reg V = 0; // overflow flag
reg N = 0; // negative flag
wire AZ; // ALU Zero flag
wire AV; // ALU overflow flag
wire AN; // ALU negative flag
wire HC; // ALU half carry
reg [7:0] AI; // ALU Input A
reg [7:0] BI; // ALU Input B
wire [7:0] DI; // Data In
wire [7:0] IR; // Instruction register
reg [7:0] DO; // Data Out
reg WE; // Write Enable
reg CI; // Carry In
wire CO; // Carry Out
wire [7:0] PCH = PC[15:8];
wire [7:0] PCL = PC[7:0];
reg NMI_edge = 0; // captured NMI edge
reg [1:0] regsel; // Select A, X, Y or S register
wire [7:0] regfile = AXYS[regsel]; // Selected register output
parameter
SEL_A = 2'd0,
SEL_S = 2'd1,
SEL_X = 2'd2,
SEL_Y = 2'd3;
/*
* define some signals for watching in simulator output
*/
`ifdef SIM
wire [7:0] A = AXYS[SEL_A]; // Accumulator
wire [7:0] X = AXYS[SEL_X]; // X register
wire [7:0] Y = AXYS[SEL_Y]; // Y register
wire [7:0] S = AXYS[SEL_S]; // Stack pointer
`endif
wire [7:0] P = { N, V, 2'b11, D, I, Z, C };
/*
* instruction decoder/sequencer
*/
reg [5:0] state;
/*
* control signals
*/
reg PC_inc; // Increment PC
reg [15:0] PC_temp; // intermediate value of PC
reg [1:0] src_reg; // source register index
reg [1:0] dst_reg; // destination register index
reg index_y; // if set, then Y is index reg rather than X
reg load_reg; // loading a register (A, X, Y, S) in this instruction
reg inc; // increment
reg write_back; // set if memory is read/modified/written
reg load_only; // LDA/LDX/LDY instruction
reg store; // doing store (STA/STX/STY)
reg adc_sbc; // doing ADC/SBC
reg compare; // doing CMP/CPY/CPX
reg shift; // doing shift/rotate instruction
reg rotate; // doing rotate (no shift)
reg backwards; // backwards branch
reg cond_true; // branch condition is true
reg [2:0] cond_code; // condition code bits from instruction
reg shift_right; // Instruction ALU shift/rotate right
reg alu_shift_right; // Current cycle shift right enable
reg [3:0] op; // Main ALU operation for instruction
reg [3:0] alu_op; // Current cycle ALU operation
reg adc_bcd; // ALU should do BCD style carry
reg adj_bcd; // results should be BCD adjusted
/*
* some flip flops to remember we're doing special instructions. These
* get loaded at the DECODE state, and used later
*/
reg bit_ins; // doing BIT instruction
reg plp; // doing PLP instruction
reg php; // doing PHP instruction
reg clc; // clear carry
reg sec; // set carry
reg cld; // clear decimal
reg sed; // set decimal
reg cli; // clear interrupt
reg sei; // set interrupt
reg clv; // clear overflow
reg brk; // doing BRK
reg res; // in reset
/*
* ALU operations
*/
parameter
OP_OR = 4'b1100,
OP_AND = 4'b1101,
OP_EOR = 4'b1110,
OP_ADD = 4'b0011,
OP_SUB = 4'b0111,
OP_ROL = 4'b1011,
OP_A = 4'b1111;
/*
* Microcode state machine. Basically, every addressing mode has its own
* path through the state machine. Additional information, such as the
* operation, source and destination registers are decoded in parallel, and
* kept in separate flops.
*/
parameter
ABS0 = 6'd0, // ABS - fetch LSB
ABS1 = 6'd1, // ABS - fetch MSB
ABSX0 = 6'd2, // ABS, X - fetch LSB and send to ALU (+X)
ABSX1 = 6'd3, // ABS, X - fetch MSB and send to ALU (+Carry)
ABSX2 = 6'd4, // ABS, X - Wait for ALU (only if needed)
BRA0 = 6'd5, // Branch - fetch offset and send to ALU (+PC[7:0])
BRA1 = 6'd6, // Branch - fetch opcode, and send PC[15:8] to ALU
BRA2 = 6'd7, // Branch - fetch opcode (if page boundary crossed)
BRK0 = 6'd8, // BRK/IRQ - push PCH, send S to ALU (-1)
BRK1 = 6'd9, // BRK/IRQ - push PCL, send S to ALU (-1)
BRK2 = 6'd10, // BRK/IRQ - push P, send S to ALU (-1)
BRK3 = 6'd11, // BRK/IRQ - write S, and fetch @ fffe
DECODE = 6'd12, // IR is valid, decode instruction, and write prev reg
FETCH = 6'd13, // fetch next opcode, and perform prev ALU op
INDX0 = 6'd14, // (ZP,X) - fetch ZP address, and send to ALU (+X)
INDX1 = 6'd15, // (ZP,X) - fetch LSB at ZP+X, calculate ZP+X+1
INDX2 = 6'd16, // (ZP,X) - fetch MSB at ZP+X+1
INDX3 = 6'd17, // (ZP,X) - fetch data
INDY0 = 6'd18, // (ZP),Y - fetch ZP address, and send ZP to ALU (+1)
INDY1 = 6'd19, // (ZP),Y - fetch at ZP+1, and send LSB to ALU (+Y)
INDY2 = 6'd20, // (ZP),Y - fetch data, and send MSB to ALU (+Carry)
INDY3 = 6'd21, // (ZP),Y) - fetch data (if page boundary crossed)
JMP0 = 6'd22, // JMP - fetch PCL and hold
JMP1 = 6'd23, // JMP - fetch PCH
JMPI0 = 6'd24, // JMP IND - fetch LSB and send to ALU for delay (+0)
JMPI1 = 6'd25, // JMP IND - fetch MSB, proceed with JMP0 state
JSR0 = 6'd26, // JSR - push PCH, save LSB, send S to ALU (-1)
JSR1 = 6'd27, // JSR - push PCL, send S to ALU (-1)
JSR2 = 6'd28, // JSR - write S
JSR3 = 6'd29, // JSR - fetch MSB
PULL0 = 6'd30, // PLP/PLA - save next op in IRHOLD, send S to ALU (+1)
PULL1 = 6'd31, // PLP/PLA - fetch data from stack, write S
PULL2 = 6'd32, // PLP/PLA - prefetch op, but don't increment PC
PUSH0 = 6'd33, // PHP/PHA - send A to ALU (+0)
PUSH1 = 6'd34, // PHP/PHA - write A/P, send S to ALU (-1)
READ = 6'd35, // Read memory for read/modify/write (INC, DEC, shift)
REG = 6'd36, // Read register for reg-reg transfers
RTI0 = 6'd37, // RTI - send S to ALU (+1)
RTI1 = 6'd38, // RTI - read P from stack
RTI2 = 6'd39, // RTI - read PCL from stack
RTI3 = 6'd40, // RTI - read PCH from stack
RTI4 = 6'd41, // RTI - read PCH from stack
RTS0 = 6'd42, // RTS - send S to ALU (+1)
RTS1 = 6'd43, // RTS - read PCL from stack
RTS2 = 6'd44, // RTS - write PCL to ALU, read PCH
RTS3 = 6'd45, // RTS - load PC and increment
WRITE = 6'd46, // Write memory for read/modify/write
ZP0 = 6'd47, // Z-page - fetch ZP address
ZPX0 = 6'd48, // ZP, X - fetch ZP, and send to ALU (+X)
ZPX1 = 6'd49; // ZP, X - load from memory
`ifdef SIM
/*
* easy to read names in simulator output
*/
reg [8*6-1:0] statename;
always @*
case( state )
DECODE: statename = "DECODE";
REG: statename = "REG";
ZP0: statename = "ZP0";
ZPX0: statename = "ZPX0";
ZPX1: statename = "ZPX1";
ABS0: statename = "ABS0";
ABS1: statename = "ABS1";
ABSX0: statename = "ABSX0";
ABSX1: statename = "ABSX1";
ABSX2: statename = "ABSX2";
INDX0: statename = "INDX0";
INDX1: statename = "INDX1";
INDX2: statename = "INDX2";
INDX3: statename = "INDX3";
INDY0: statename = "INDY0";
INDY1: statename = "INDY1";
INDY2: statename = "INDY2";
INDY3: statename = "INDY3";
READ: statename = "READ";
WRITE: statename = "WRITE";
FETCH: statename = "FETCH";
PUSH0: statename = "PUSH0";
PUSH1: statename = "PUSH1";
PULL0: statename = "PULL0";
PULL1: statename = "PULL1";
PULL2: statename = "PULL2";
JSR0: statename = "JSR0";
JSR1: statename = "JSR1";
JSR2: statename = "JSR2";
JSR3: statename = "JSR3";
RTI0: statename = "RTI0";
RTI1: statename = "RTI1";
RTI2: statename = "RTI2";
RTI3: statename = "RTI3";
RTI4: statename = "RTI4";
RTS0: statename = "RTS0";
RTS1: statename = "RTS1";
RTS2: statename = "RTS2";
RTS3: statename = "RTS3";
BRK0: statename = "BRK0";
BRK1: statename = "BRK1";
BRK2: statename = "BRK2";
BRK3: statename = "BRK3";
BRA0: statename = "BRA0";
BRA1: statename = "BRA1";
BRA2: statename = "BRA2";
JMP0: statename = "JMP0";
JMP1: statename = "JMP1";
JMPI0: statename = "JMPI0";
JMPI1: statename = "JMPI1";
endcase
//always @( PC )
// $display( "%t, PC:%04x IR:%02x A:%02x X:%02x Y:%02x S:%02x C:%d Z:%d V:%d N:%d P:%02x", $time, PC, IR, A, X, Y, S, C, Z, V, N, P );
`endif
/*
* Program Counter Increment/Load. First calculate the base value in
* PC_temp.
*/
always @*
case( state )
DECODE: if( (~I & IRQ) | NMI_edge )
PC_temp = { ABH, ABL };
else
PC_temp = PC;
JMP1,
JMPI1,
JSR3,
RTS3,
RTI4: PC_temp = { DIMUX, ADD };
BRA1: PC_temp = { ABH, ADD };
BRA2: PC_temp = { ADD, PCL };
BRK2: PC_temp = res ? 16'hfffc :
NMI_edge ? 16'hfffa : 16'hfffe;
default: PC_temp = PC;
endcase
/*
* Determine wether we need PC_temp, or PC_temp + 1
*/
always @*
case( state )
DECODE: if( (~I & IRQ) | NMI_edge )
PC_inc = 0;
else
PC_inc = 1;
ABS0,
ABSX0,
FETCH,
BRA0,
BRA2,
BRK3,
JMPI1,
JMP1,
RTI4,
RTS3: PC_inc = 1;
BRA1: PC_inc = CO ^~ backwards;
default: PC_inc = 0;
endcase
/*
* Set new PC
*/
always @(posedge clk)
if( RDY )
PC <= PC_temp + {15'b0, PC_inc};
/*
* Address Generator
*/
parameter
ZEROPAGE = 8'h00,
STACKPAGE = 8'h01;
always @*
case( state )
ABSX1,
INDX3,
INDY2,
JMP1,
JMPI1,
RTI4,
ABS1: AB = { DIMUX, ADD };
BRA2,
INDY3,
ABSX2: AB = { ADD, ABL };
BRA1: AB = { ABH, ADD };
JSR0,
PUSH1,
RTS0,
RTI0,
BRK0: AB = { STACKPAGE, regfile };
BRK1,
JSR1,
PULL1,
RTS1,
RTS2,
RTI1,
RTI2,
RTI3,
BRK2: AB = { STACKPAGE, ADD };
INDY1,
INDX1,
ZPX1,
INDX2: AB = { ZEROPAGE, ADD };
ZP0,
INDY0: AB = { ZEROPAGE, DIMUX };
REG,
READ,
WRITE: AB = { ABH, ABL };
default: AB = PC;
endcase
/*
* ABH/ABL pair is used for registering previous address bus state.
* This can be used to keep the current address, freeing up the original
* source of the address, such as the ALU or DI.
*/
always @(posedge clk)
if( state != PUSH0 && state != PUSH1 && RDY &&
state != PULL0 && state != PULL1 && state != PULL2 )
begin
ABL <= AB[7:0];
ABH <= AB[15:8];
end
/*
* Data Out MUX
*/
always @*
case( state )
WRITE: DO = ADD;
JSR0,
BRK0: DO = PCH;
JSR1,
BRK1: DO = PCL;
PUSH1: DO = php ? P : ADD;
BRK2: DO = (IRQ | NMI_edge) ? (P & 8'b1110_1111) : P;
default: DO = regfile;
endcase
/*
* Write Enable Generator
*/
always @*
case( state )
BRK0, // writing to stack or memory
BRK1,
BRK2,
JSR0,
JSR1,
PUSH1,
WRITE: WE = 1;
INDX3, // only if doing a STA, STX or STY
INDY3,
ABSX2,
ABS1,
ZPX1,
ZP0: WE = store;
default: WE = 0;
endcase
/*
* register file, contains A, X, Y and S (stack pointer) registers. At each
* cycle only 1 of those registers needs to be accessed, so they combined
* in a small memory, saving resources.
*/
reg write_register; // set when register file is written
always @*
case( state )
DECODE: write_register = load_reg & ~plp;
PULL1,
RTS2,
RTI3,
BRK3,
JSR0,
JSR2 : write_register = 1;
default: write_register = 0;
endcase
/*
* BCD adjust logic
*/
always @(posedge clk)
adj_bcd <= adc_sbc & D; // '1' when doing a BCD instruction
reg [3:0] ADJL;
reg [3:0] ADJH;
// adjustment term to be added to ADD[3:0] based on the following
// adj_bcd: '1' if doing ADC/SBC with D=1
// adc_bcd: '1' if doing ADC with D=1
// HC : half carry bit from ALU
always @* begin
casex( {adj_bcd, adc_bcd, HC} )
3'b0xx: ADJL = 4'd0; // no BCD instruction
3'b100: ADJL = 4'd10; // SBC, and digital borrow
3'b101: ADJL = 4'd0; // SBC, but no borrow
3'b110: ADJL = 4'd0; // ADC, but no carry
3'b111: ADJL = 4'd6; // ADC, and decimal/digital carry
endcase
end
// adjustment term to be added to ADD[7:4] based on the following
// adj_bcd: '1' if doing ADC/SBC with D=1
// adc_bcd: '1' if doing ADC with D=1
// CO : carry out bit from ALU
always @* begin
casex( {adj_bcd, adc_bcd, CO} )
3'b0xx: ADJH = 4'd0; // no BCD instruction
3'b100: ADJH = 4'd10; // SBC, and digital borrow
3'b101: ADJH = 4'd0; // SBC, but no borrow
3'b110: ADJH = 4'd0; // ADC, but no carry
3'b111: ADJH = 4'd6; // ADC, and decimal/digital carry
endcase
end
/*
* write to a register. Usually this is the (BCD corrected) output of the
* ALU, but in case of the JSR0 we use the S register to temporarily store
* the PCL. This is possible, because the S register itself is stored in
* the ALU during those cycles.
*/
always @(posedge clk)
if( write_register & RDY )
AXYS[regsel] <= (state == JSR0) ? DIMUX : { ADD[7:4] + ADJH, ADD[3:0] + ADJL };
/*
* register select logic. This determines which of the A, X, Y or
* S registers will be accessed.
*/
always @*
case( state )
INDY1,
INDX0,
ZPX0,
ABSX0 : regsel = index_y ? SEL_Y : SEL_X;
DECODE : regsel = dst_reg;
BRK0,
BRK3,
JSR0,
JSR2,
PULL0,
PULL1,
PUSH1,
RTI0,
RTI3,
RTS0,
RTS2 : regsel = SEL_S;
default: regsel = src_reg;
endcase
/*
* ALU
*/
ALU ALU( .clk(clk),
.op(alu_op),
.right(alu_shift_right),
.AI(AI),
.BI(BI),
.CI(CI),
.BCD(adc_bcd & (state == FETCH)),
.CO(CO),
.OUT(ADD),
.V(AV),
.Z(AZ),
.N(AN),
.HC(HC),
.RDY(RDY) );
/*
* Select current ALU operation
*/
always @*
case( state )
READ: alu_op = op;
BRA1: alu_op = backwards ? OP_SUB : OP_ADD;
FETCH,
REG : alu_op = op;
DECODE,
ABS1: alu_op = 4'bx;
PUSH1,
BRK0,
BRK1,
BRK2,
JSR0,
JSR1: alu_op = OP_SUB;
default: alu_op = OP_ADD;
endcase
/*
* Determine shift right signal to ALU
*/
always @*
if( state == FETCH || state == REG || state == READ )
alu_shift_right = shift_right;
else
alu_shift_right = 0;
/*
* Sign extend branch offset.
*/
always @(posedge clk)
if( RDY )
backwards <= DIMUX[7];
/*
* ALU A Input MUX
*/
always @*
case( state )
JSR1,
RTS1,
RTI1,
RTI2,
BRK1,
BRK2,
INDX1: AI = ADD;
REG,
ZPX0,
INDX0,
ABSX0,
RTI0,
RTS0,
JSR0,
JSR2,
BRK0,
PULL0,
INDY1,
PUSH0,
PUSH1: AI = regfile;
BRA0,
READ: AI = DIMUX;
BRA1: AI = ABH; // don't use PCH in case we're
FETCH: AI = load_only ? 0 : regfile;
DECODE,
ABS1: AI = 8'hxx; // don't care
default: AI = 0;
endcase
/*
* ALU B Input mux
*/
always @*
case( state )
BRA1,
RTS1,
RTI0,
RTI1,
RTI2,
INDX1,
READ,
REG,
JSR0,
JSR1,
JSR2,
BRK0,
BRK1,
BRK2,
PUSH0,
PUSH1,
PULL0,
RTS0: BI = 8'h00;
BRA0: BI = PCL;
DECODE,
ABS1: BI = 8'hxx;
default: BI = DIMUX;
endcase
/*
* ALU CI (carry in) mux
*/
always @*
case( state )
INDY2,
BRA1,
ABSX1: CI = CO;
DECODE,
ABS1: CI = 1'bx;
READ,
REG: CI = rotate ? C :
shift ? 0 : inc;
FETCH: CI = rotate ? C :
compare ? 1 :
(shift | load_only) ? 0 : C;
PULL0,
RTI0,
RTI1,
RTI2,
RTS0,
RTS1,
INDY0,
INDX1: CI = 1;
default: CI = 0;
endcase
/*
* Processor Status Register update
*
*/
/*
* Update C flag when doing ADC/SBC, shift/rotate, compare
*/
always @(posedge clk )
if( shift && state == WRITE )
C <= CO;
else if( state == RTI2 )
C <= DIMUX[0];
else if( ~write_back && state == DECODE ) begin
if( adc_sbc | shift | compare )
C <= CO;
else if( plp )
C <= ADD[0];
else begin
if( sec ) C <= 1;
if( clc ) C <= 0;
end
end
/*
* Update Z, N flags when writing A, X, Y, Memory, or when doing compare
*/
always @(posedge clk)
if( state == WRITE )
Z <= AZ;
else if( state == RTI2 )
Z <= DIMUX[1];
else if( state == DECODE ) begin
if( plp )
Z <= ADD[1];
else if( (load_reg & (regsel != SEL_S)) | compare | bit_ins )
Z <= AZ;
end
always @(posedge clk)
if( state == WRITE )
N <= AN;
else if( state == RTI2 )
N <= DIMUX[7];
else if( state == DECODE ) begin
if( plp )
N <= ADD[7];
else if( (load_reg & (regsel != SEL_S)) | compare )
N <= AN;
end else if( state == FETCH && bit_ins )
N <= DIMUX[7];
/*
* Update I flag
*/
always @(posedge clk)
if( state == BRK3 )
I <= 1;
else if( state == RTI2 )
I <= DIMUX[2];
else if( state == REG ) begin
if( sei ) I <= 1;
if( cli ) I <= 0;
end else if( state == DECODE )
if( plp ) I <= ADD[2];
/*
* Update D flag
*/
always @(posedge clk )
if( state == RTI2 )
D <= DIMUX[3];
else if( state == DECODE ) begin
if( sed ) D <= 1;
if( cld ) D <= 0;
if( plp ) D <= ADD[3];
end
/*
* Update V flag
*/
always @(posedge clk )
if( state == RTI2 )
V <= DIMUX[6];
else if( state == DECODE ) begin
if( adc_sbc ) V <= AV;
if( clv ) V <= 0;
if( plp ) V <= ADD[6];
end else if( state == FETCH && bit_ins )
V <= DIMUX[6];
/*
* Instruction decoder
*/
/*
* IR register/mux. Hold previous DI value in IRHOLD in PULL0 and PUSH0
* states. In these states, the IR has been prefetched, and there is no
* time to read the IR again before the next decode.
*/
always @(posedge clk )
if( reset )
IRHOLD_valid <= 0;
else if( RDY ) begin
if( state == PULL0 || state == PUSH0 ) begin
IRHOLD <= DIMUX;
IRHOLD_valid <= 1;
end else if( state == DECODE )
IRHOLD_valid <= 0;
end
assign IR = (IRQ & ~I) | NMI_edge ? 8'h00 :
IRHOLD_valid ? IRHOLD : DIMUX;
always @(posedge clk )
if( RDY )
DIHOLD <= DI;
assign DIMUX = ~RDY ? DIHOLD : DI;
/*
* Microcode state machine
*/
always @(posedge clk or posedge reset)
if( reset )
state <= BRK0;
else if( RDY ) case( state )
DECODE :
casex ( IR )
8'b0000_0000: state <= BRK0;
8'b0010_0000: state <= JSR0;
8'b0010_1100: state <= ABS0; // BIT abs
8'b0100_0000: state <= RTI0; //
8'b0100_1100: state <= JMP0;
8'b0110_0000: state <= RTS0;
8'b0110_1100: state <= JMPI0;
8'b0x00_1000: state <= PUSH0;
8'b0x10_1000: state <= PULL0;
8'b0xx1_1000: state <= REG; // CLC, SEC, CLI, SEI
8'b1xx0_00x0: state <= FETCH; // IMM
8'b1xx0_1100: state <= ABS0; // X/Y abs
8'b1xxx_1000: state <= REG; // DEY, TYA, ...
8'bxxx0_0001: state <= INDX0;
8'bxxx0_01xx: state <= ZP0;
8'bxxx0_1001: state <= FETCH; // IMM
8'bxxx0_1101: state <= ABS0; // even E column
8'bxxx0_1110: state <= ABS0; // even E column
8'bxxx1_0000: state <= BRA0; // odd 0 column
8'bxxx1_0001: state <= INDY0; // odd 1 column
8'bxxx1_01xx: state <= ZPX0; // odd 4,5,6,7 columns
8'bxxx1_1001: state <= ABSX0; // odd 9 column
8'bxxx1_11xx: state <= ABSX0; // odd C, D, E, F columns
8'bxxxx_1010: state <= REG; // <shift> A, TXA, ... NOP
endcase
ZP0 : state <= write_back ? READ : FETCH;
ZPX0 : state <= ZPX1;
ZPX1 : state <= write_back ? READ : FETCH;
ABS0 : state <= ABS1;
ABS1 : state <= write_back ? READ : FETCH;
ABSX0 : state <= ABSX1;
ABSX1 : state <= (CO | store | write_back) ? ABSX2 : FETCH;
ABSX2 : state <= write_back ? READ : FETCH;
INDX0 : state <= INDX1;
INDX1 : state <= INDX2;
INDX2 : state <= INDX3;
INDX3 : state <= FETCH;
INDY0 : state <= INDY1;
INDY1 : state <= INDY2;
INDY2 : state <= (CO | store) ? INDY3 : FETCH;
INDY3 : state <= FETCH;
READ : state <= WRITE;
WRITE : state <= FETCH;
FETCH : state <= DECODE;
REG : state <= DECODE;
PUSH0 : state <= PUSH1;
PUSH1 : state <= DECODE;
PULL0 : state <= PULL1;
PULL1 : state <= PULL2;
PULL2 : state <= DECODE;
JSR0 : state <= JSR1;
JSR1 : state <= JSR2;
JSR2 : state <= JSR3;
JSR3 : state <= FETCH;
RTI0 : state <= RTI1;
RTI1 : state <= RTI2;
RTI2 : state <= RTI3;
RTI3 : state <= RTI4;
RTI4 : state <= DECODE;
RTS0 : state <= RTS1;
RTS1 : state <= RTS2;
RTS2 : state <= RTS3;
RTS3 : state <= FETCH;
BRA0 : state <= cond_true ? BRA1 : DECODE;
BRA1 : state <= (CO ^ backwards) ? BRA2 : DECODE;
BRA2 : state <= DECODE;
JMP0 : state <= JMP1;
JMP1 : state <= DECODE;
JMPI0 : state <= JMPI1;
JMPI1 : state <= JMP0;
BRK0 : state <= BRK1;
BRK1 : state <= BRK2;
BRK2 : state <= BRK3;
BRK3 : state <= JMP0;
endcase
/*
* Additional control signals
*/
always @(posedge clk)
if( reset )
res <= 1;
else if( state == DECODE )
res <= 0;
always @(posedge clk)
if( state == DECODE && RDY )
casex( IR )
8'b0xx01010, // ASLA, ROLA, LSRA, RORA
8'b0xxxxx01, // ORA, AND, EOR, ADC
8'b100x10x0, // DEY, TYA, TXA, TXS
8'b1010xxx0, // LDA/LDX/LDY
8'b10111010, // TSX
8'b1011x1x0, // LDX/LDY
8'b11001010, // DEX
8'b1x1xxx01, // LDA, SBC
8'bxxx01000: // DEY, TAY, INY, INX
load_reg <= 1;
default: load_reg <= 0;
endcase
always @(posedge clk)
if( state == DECODE && RDY )
casex( IR )
8'b1110_1000, // INX
8'b1100_1010, // DEX
8'b101x_xx10: // LDX, TAX, TSX
dst_reg <= SEL_X;
8'b0x00_1000, // PHP, PHA
8'b1001_1010: // TXS
dst_reg <= SEL_S;
8'b1x00_1000, // DEY, DEX
8'b101x_x100, // LDY
8'b1010_x000: // LDY #imm, TAY
dst_reg <= SEL_Y;
default: dst_reg <= SEL_A;
endcase
always @(posedge clk)
if( state == DECODE && RDY )
casex( IR )
8'b1011_1010: // TSX
src_reg <= SEL_S;
8'b100x_x110, // STX
8'b100x_1x10, // TXA, TXS
8'b1110_xx00, // INX, CPX
8'b1100_1010: // DEX
src_reg <= SEL_X;
8'b100x_x100, // STY
8'b1001_1000, // TYA
8'b1100_xx00, // CPY
8'b1x00_1000: // DEY, INY
src_reg <= SEL_Y;
default: src_reg <= SEL_A;
endcase
always @(posedge clk)
if( state == DECODE && RDY )
casex( IR )
8'bxxx1_0001, // INDY
8'b10x1_x110, // LDX/STX zpg/abs, Y
8'bxxxx_1001: // abs, Y
index_y <= 1;
default: index_y <= 0;
endcase
always @(posedge clk)
if( state == DECODE && RDY )
casex( IR )
8'b100x_x1x0, // STX, STY
8'b100x_xx01: // STA
store <= 1;
default: store <= 0;
endcase
always @(posedge clk )
if( state == DECODE && RDY )
casex( IR )
8'b0xxx_x110, // ASL, ROL, LSR, ROR
8'b11xx_x110: // DEC/INC
write_back <= 1;
default: write_back <= 0;
endcase
always @(posedge clk )
if( state == DECODE && RDY )
casex( IR )
8'b101x_xxxx: // LDA, LDX, LDY
load_only <= 1;
default: load_only <= 0;
endcase
always @(posedge clk )
if( state == DECODE && RDY )
casex( IR )
8'b111x_x110, // INC
8'b11x0_1000: // INX, INY
inc <= 1;
default: inc <= 0;
endcase
always @(posedge clk )
if( (state == DECODE || state == BRK0) && RDY )
casex( IR )
8'bx11x_xx01: // SBC, ADC
adc_sbc <= 1;
default: adc_sbc <= 0;
endcase
always @(posedge clk )
if( (state == DECODE || state == BRK0) && RDY )
casex( IR )
8'b011x_xx01: // ADC
adc_bcd <= D;
default: adc_bcd <= 0;
endcase
always @(posedge clk )
if( state == DECODE && RDY )
casex( IR )
8'b0xxx_x110, // ASL, ROL, LSR, ROR (abs, absx, zpg, zpgx)
8'b0xxx_1010: // ASL, ROL, LSR, ROR (acc)
shift <= 1;
default: shift <= 0;
endcase
always @(posedge clk )
if( state == DECODE && RDY )
casex( IR )
8'b11x0_0x00, // CPX, CPY (imm/zp)
8'b11x0_1100, // CPX, CPY (abs)
8'b110x_xx01: // CMP
compare <= 1;
default: compare <= 0;
endcase
always @(posedge clk )
if( state == DECODE && RDY )
casex( IR )
8'b01xx_xx10: // ROR, LSR
shift_right <= 1;
default: shift_right <= 0;
endcase
always @(posedge clk )
if( state == DECODE && RDY )
casex( IR )
8'b0x1x_1010, // ROL A, ROR A
8'b0x1x_x110: // ROR, ROL
rotate <= 1;
default: rotate <= 0;
endcase
always @(posedge clk )
if( state == DECODE && RDY )
casex( IR )
8'b00xx_xx10: // ROL, ASL
op <= OP_ROL;
8'b0010_x100: // BIT zp/abs
op <= OP_AND;
8'b01xx_xx10: // ROR, LSR
op <= OP_A;
8'b1000_1000, // DEY
8'b1100_1010, // DEX
8'b110x_x110, // DEC
8'b11xx_xx01, // CMP, SBC
8'b11x0_0x00, // CPX, CPY (imm, zpg)
8'b11x0_1100: op <= OP_SUB;
8'b010x_xx01, // EOR
8'b00xx_xx01: // ORA, AND
op <= { 2'b11, IR[6:5] };
default: op <= OP_ADD;
endcase
always @(posedge clk )
if( state == DECODE && RDY )
casex( IR )
8'b0010_x100: // BIT zp/abs
bit_ins <= 1;
default: bit_ins <= 0;
endcase
/*
* special instructions
*/
always @(posedge clk )
if( state == DECODE && RDY ) begin
php <= (IR == 8'h08);
clc <= (IR == 8'h18);
plp <= (IR == 8'h28);
sec <= (IR == 8'h38);
cli <= (IR == 8'h58);
sei <= (IR == 8'h78);
clv <= (IR == 8'hb8);
cld <= (IR == 8'hd8);
sed <= (IR == 8'hf8);
brk <= (IR == 8'h00);
end
always @(posedge clk)
if( RDY )
cond_code <= IR[7:5];
always @*
case( cond_code )
3'b000: cond_true = ~N;
3'b001: cond_true = N;
3'b010: cond_true = ~V;
3'b011: cond_true = V;
3'b100: cond_true = ~C;
3'b101: cond_true = C;
3'b110: cond_true = ~Z;
3'b111: cond_true = Z;
endcase
reg NMI_1 = 0; // delayed NMI signal
always @(posedge clk)
NMI_1 <= NMI;
always @(posedge clk )
if( NMI_edge && state == BRK3 )
NMI_edge <= 0;
else if( NMI & ~NMI_1 )
NMI_edge <= 1;
endmodule
/*
* ALU.
*
* AI and BI are 8 bit inputs. Result in OUT.
* CI is Carry In.
* CO is Carry Out.
*
* op[3:0] is defined as follows:
*
* 0011 AI + BI
* 0111 AI - BI
* 1011 AI + AI
* 1100 AI | BI
* 1101 AI & BI
* 1110 AI ^ BI
* 1111 AI
*
*/
module ALU( clk, op, right, AI, BI, CI, CO, BCD, OUT, V, Z, N, HC, RDY );
input clk;
input right;
input [3:0] op; // operation
input [7:0] AI;
input [7:0] BI;
input CI;
input BCD; // BCD style carry
output [7:0] OUT;
output CO;
output V;
output Z;
output N;
output HC;
input RDY;
reg [7:0] OUT;
reg CO;
wire V;
wire Z;
reg N;
reg HC;
reg AI7;
reg BI7;
reg [8:0] temp_logic;
reg [7:0] temp_BI;
reg [4:0] temp_l;
reg [4:0] temp_h;
wire [8:0] temp = { temp_h, temp_l[3:0] };
wire adder_CI = (right | (op[3:2] == 2'b11)) ? 0 : CI;
// calculate the logic operations. The 'case' can be done in 1 LUT per
// bit. The 'right' shift is a simple mux that can be implemented by
// F5MUX.
always @* begin
case( op[1:0] )
2'b00: temp_logic = {1'b0, AI | BI};
2'b01: temp_logic = {1'b0, AI & BI};
2'b10: temp_logic = {1'b0, AI ^ BI};
2'b11: temp_logic = {1'b0, AI};
endcase
if( right )
temp_logic = { AI[0], CI, AI[7:1] };
end
// Add logic result to BI input. This only makes sense when logic = AI.
// This stage can be done in 1 LUT per bit, using carry chain logic.
always @* begin
case( op[3:2] )
2'b00: temp_BI = BI; // A+B
2'b01: temp_BI = ~BI; // A-B
2'b10: temp_BI = temp_logic[7:0]; // A+A
2'b11: temp_BI = 0; // A+0
endcase
end
// HC9 is the half carry bit when doing BCD add
wire HC9 = BCD & (temp_l[3:1] >= 3'd5);
// CO9 is the carry-out bit when doing BCD add
wire CO9 = BCD & (temp_h[3:1] >= 3'd5);
// combined half carry bit
wire temp_HC = temp_l[4] | HC9;
// perform the addition as 2 separate nibble, so we get
// access to the half carry flag
always @* begin
temp_l = temp_logic[3:0] + temp_BI[3:0] + {4'b0,adder_CI};
temp_h = temp_logic[8:4] + temp_BI[7:4] + {4'b0,temp_HC};
end
// calculate the flags
always @(posedge clk)
if( RDY ) begin
AI7 <= AI[7];
BI7 <= temp_BI[7];
OUT <= temp[7:0];
CO <= temp[8] | CO9;
N <= temp[7];
HC <= temp_HC;
end
assign V = AI7 ^ BI7 ^ CO ^ N;
assign Z = ~|OUT;
endmodule
// test module
module cpu6502_test_top(clk, reset, AB, DI, DO, WE);
input clk,reset;
output reg [15:0] AB; // address bus
output reg [7:0] DI; // data in, read bus
output wire [7:0] DO; // data out, write bus
output wire WE; // write enable
wire IRQ=0; // interrupt request
wire NMI=0; // non-maskable interrupt request
wire RDY=1; // Ready signal. Pauses CPU when RDY=0
cpu6502 cpu( clk, reset, AB, DI, DO, WE, IRQ, NMI, RDY );
always @(posedge clk)
begin
DI <= rom[AB[3:0]];
end
reg [7:0] rom[0:15];
// LDY #$13
// .loop: DEY
// BNE .loop
// BRK
initial begin
rom[0] = 8'ha0;
rom[1] = 8'h13;
rom[2] = 8'h88;
rom[3] = 8'hd0;
rom[4] = 8'hfd;
rom[5] = 8'h00;
end
endmodule
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