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[/] [cpu16/] [trunk/] [cpu16.v] - Blame information for rev 4

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/*
 * None pipelined, 3-state, 16-bit CPU
 *
 * Copyright (C) 2019, Yvo Zoer
 *
 * 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 2
 * 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, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 */
 
// comment this line out to use LE's (warning: size goes up dramatically to around 800LE's)
`define USE_RAM_FOR_REGFILE
 
 
module cpu16 (
        clk,
        reset_n,
        clk_en,
        address,
        din,
        dout,
        cs_n,
        oe_n,
        we_n,
        ub_n,
        lb_n,
        irq,            // combined interrupt request from intc
        busreq,
        busack
        );
 
        input clk;
        input reset_n;
        output [15:0] address;
        input clk_en;
        input [15:0] din;
        output [15:0] dout;
        output cs_n;
        output oe_n;
        output we_n;
        output ub_n;
        output lb_n;
        input irq;
        input busreq;
        output busack;
 
        // alu
        parameter OP_SUB                = 5'd0;         // ok
        parameter OP_SBC                = 5'd1;         // ok
        parameter OP_ADD                = 5'd2;
        parameter OP_ADC                = 5'd3;
        parameter OP_CMP                = 5'd4;
        parameter OP_AND                = 5'd5;         // ok
        parameter OP_OR                 = 5'd6;         // ok
        parameter OP_XOR                = 5'd7;         // ok
 
        // alu immediate
        parameter OP_SUBI               = 5'd8;         // ok
        parameter OP_SBCI               = 5'd9;         // ok
        parameter OP_ADDI               = 5'd10;        // ok
        parameter OP_ADCI               = 5'd11;        // ok
        parameter OP_CMPI               = 5'd12;
        parameter OP_ANDI               = 5'd13;        // ok
        parameter OP_ORI                = 5'd14;        // ok
        parameter OP_XORI               = 5'd15;        // ok
 
        // shifter + load
        parameter OP_MOV                = 5'd16;
        parameter OP_SRAV               = 5'd17;
        parameter OP_SLLV               = 5'd18;
        parameter OP_LD                 = 5'd19;
 
        // shifter immediate + store
        parameter OP_LUI                = 5'd20;
        parameter OP_SRA                = 5'd21;
        parameter OP_SLL                = 5'd22;
        parameter OP_ST                 = 5'd23;
 
        // jump
        parameter OP_J                  = 5'd24;
        parameter OP_JAL                = 5'd25;
        parameter OP_JR                 = 5'd26;
        parameter OP_JALR               = 5'd27;
 
        // conditional branch
        parameter OP_BXX                = 5'd28;
        parameter OP_MSR                = 5'd29;
        parameter OP_RFE                = 5'd30;
        parameter OP_SWI                = 5'd31;
 
        // states
        parameter STATE_FETCH   = 4'h0;
        parameter STATE_DECODE  = 4'h1;
        parameter STATE_ALU             = 4'h2;
        parameter STATE_SHIFT   = 4'h3;
        parameter STATE_LOAD    = 4'h4;
        parameter STATE_STORE   = 4'h5;
        parameter STATE_J               = 4'h6;
        parameter STATE_JAL             = 4'h7;
        parameter STATE_BRANCH  = 4'h8;
        parameter STATE_MOVSR   = 4'h9;
        parameter STATE_RFE             = 4'ha;
        parameter STATE_SWI             = 4'hb;
        parameter STATE_INTREQ  = 4'hc;
        parameter STATE_BUSREQ  = 4'hd;
 
        // instruction register wires
        wire [4:0] ir_opcode = ir[15:11];
        wire [2:0] ir_dst = ir[10:8];
        wire [2:0] ir_src = ir[7:5];
        wire ir_size = ir[4];   // byte/word flag
        wire [3:0] ir_imm4 = ir[3:0];     // load/store offset
        wire [7:0] ir_imm8 = ir[7:0];     // immediate value / relative conditional branch
        wire [10:0] ir_imm11 = ir[10:0];  // unconditional branch 
 
        // busack valid once we're in the busreq state
        reg busack;
        always @(*)
                if ( state == STATE_BUSREQ )
                        busack = 1'b1;
                else
                        busack = 1'b0;
 
// chip-select
        reg cs_n;
        always @(*)
                case (state)
                        STATE_FETCH,
                        STATE_LOAD,
                        STATE_STORE : cs_n = 1'b0;
                        default : cs_n = 1'b1;
                endcase
 
        // output enable
        reg oe_n;
        always @(*)
                case (state)
                        STATE_FETCH,
                        STATE_LOAD : oe_n = 1'b0;
                        default : oe_n = 1'b1;
                endcase
        // memory write signal
        reg we_n;
        always @(posedge clk)
                if ( state == STATE_STORE )
                        we_n <= clk_en;
                else
                        we_n <= 1'b1;
 
        // upper / lower byte write/read enable
        // NOTE: could integrate dout here as well..
        reg ub_n;
        reg lb_n;
        always @(*)
                if ( state == STATE_STORE )
                        begin
                                if ( ir_size )
                                        begin   // word
                                                ub_n = 1'b0;
                                                lb_n = 1'b0;
                                        end
                                else
                                        begin   // byte
                                                if ( address[0] )
                                                        begin
                                                                ub_n = 1'b1;
                                                                lb_n = 1'b0;
                                                        end
                                                else
                                                        begin
                                                                ub_n = 1'b0;
                                                                lb_n = 1'b1;
                                                        end
                                        end
                        end
                else
                        begin   // word access for everything else
                                ub_n = 1'b0;
                                lb_n = 1'b0;
                        end
 
        // data output logic
        reg [15:0] dout;
        always @(*)
                if ( ir_size )
                        dout = qa;      // word
                else
                        begin
                                if ( address[0] )
                                        dout = { 8'h00, qa[7:0] };
                                else
                                        dout = { qa[7:0], 8'h00 };
                        end
 
        // address mux 
        reg [15:0] address;
        always @(*)
                case (state)
                        STATE_LOAD,
                        STATE_STORE : address = qb + ir_imm4;
                        default : address = pc; // { pc[15:1], 1'b0 };
                endcase
 
        // main state machine
        reg [3:0] state, next_state;
        always @(*)
                case (state)
                        STATE_FETCH : next_state = STATE_DECODE;
                        STATE_DECODE : begin
                                casex (ir_opcode)
                                        5'b0xxxx : next_state = STATE_ALU;              // sub/sbc/add/adc/and/or/xor/cmp
                                        5'b10x0x,
                                        5'b10x10 : next_state = STATE_SHIFT;
                                        5'b10011 : next_state = STATE_LOAD;
                                        5'b10111 : next_state = STATE_STORE;
                                        5'b110x0 : next_state = STATE_J;        // 24/26
                                        5'b110x1 : next_state = STATE_JAL;      // 25/27
                                        5'b11100 : next_state = STATE_BRANCH;
                                        5'b11101 : next_state = STATE_MOVSR;
                                        5'b11110 : next_state = STATE_RFE;
                                        5'b11111 : next_state = STATE_SWI;
                                        default : next_state = STATE_FETCH;
                                endcase
                                end
                        STATE_INTREQ : next_state = STATE_FETCH;
                        STATE_BUSREQ : begin
                                if ( busreq )
                                        next_state = STATE_BUSREQ;
                                else
                                        next_state = STATE_FETCH;       // will miss interrupt when coming out of busreq (STATE_NOP?)
                                end
                        default : begin
                                if ( busreq )
                                        next_state = STATE_BUSREQ;
                                else if ( irq & i )     // only fires when interrupt enable is set (i = 1)
                                        next_state = STATE_INTREQ;
                                else
                                        next_state = STATE_FETCH;
                                end
                endcase
 
        // instruction register
        reg [15:0] ir, next_ir;
        always @(*)
                if ( state == STATE_FETCH )
                        next_ir = din;
                else
                        next_ir = ir;
 
        // exception pc- and status register
        reg [15:0] epc, next_epc;
        reg [4:0] esr, next_esr;
        always @(*)
                case (state)
                        STATE_INTREQ : begin
                                next_epc = pc;
                                next_esr = { i,n,v,z,c };
                                end
                        default : begin
                                next_epc = epc;
                                next_esr = esr;
                                end
                endcase
 
        // source data for program counter ( register / immediate )
        reg [15:0] pc_data;
        always @(*)
                if ( ir_opcode[1] )     // immediate for PC
                        pc_data = qb;   //{ qb[15:1], 1'b0 };   // TEST: jr/jalr
                else
                        pc_data = pc + { { 4 { ir_imm11[10] } }, ir_imm11[10:0], 1'b0 }; // TEST: j/jal doubled for jumps
 
        // condition check
        reg condition_true;
        always @(*)
                case (ir_dst)
                        3'd0 : condition_true = z;
                        3'd1 : condition_true = ~z;
                        3'd2 : condition_true = ~n;
                        3'd3 : condition_true = n;
                        3'd4 : condition_true = ~c;
                        3'd5 : condition_true = c;
                        3'd6 : condition_true = ~v;
                        3'd7 : condition_true = v;
                endcase
 
        // program counter
        reg [15:0] pc, next_pc;
        always @(*)
                case (state)
                        STATE_FETCH : next_pc = pc + 16'd2;
                        STATE_SWI : next_pc = { ir_imm8, 1'b0 };
                        STATE_J,
                        STATE_JAL : next_pc = pc_data;
                        STATE_BRANCH : begin
                                if ( condition_true )
                                        next_pc = pc + { {  7 { ir_imm8[7] } }, ir_imm8[7:0], 1'b0 };
                                else
                                        next_pc = pc;
                                end
                        STATE_INTREQ : next_pc = 16'h0002;      // interrupt vector appears at 0x0002
                        STATE_RFE : next_pc = epc;
                        default : next_pc = pc;
                endcase
 
        // alu data mux
        reg [15:0] alu_data;
        always @(*)
                if (ir_opcode[3])       // immediate
                        alu_data = { 8'h00, ir_imm8 };
                else
                        alu_data = qb;  // register
 
        // alu
        wire [15:0] alu_result;
        wire alu_n;
        wire alu_v;
        wire alu_z;
        wire alu_c;
        alu i_alu (
                .a(qa),
                .b(alu_data),
                .c_in(c),
                .func(ir_opcode[2:0]),
                .result(alu_result),
                .n(alu_n),
                .v(alu_v),
                .z(alu_z),
                .c(alu_c)
                );
 
        // shifter data mux
        reg [15:0] shifter_data;
        always @(*)
                if ( ir_opcode[2] )     // immediate
                        shifter_data = { ir_imm8, 8'h00 };      // lui from instruction word
                else
                        shifter_data = qb;      // mov
 
        // shifter distance -- roll into b argument inside shifter?
        reg [3:0] shift_distance;
        always @(*)
                if ( ir_opcode[2] )
                        shift_distance = ir_imm4;
                else
                        shift_distance = qb[3:0];
 
        // shifter
        wire [15:0] shifter_result;
        wire shift_n;
        wire shift_z;
        wire shift_c;
        wire shift_v;
        shifter i_shifter (
                .a(qa),
                .b(shifter_data),
                .n_in(n),
                .v_in(v),
                .z_in(z),
                .c_in(c),
                .distance(shift_distance),
                .func(ir_opcode[1:0]),
                .result(shifter_result),
                .n(shift_n),
                .v(shift_v),
                .z(shift_z),
                .c(shift_c)
                );
 
        // memory data mux for lw/lb
        reg [15:0] mem_data;
        always @(*)
                if ( ir_size )
                        mem_data = din;
                else
                        begin
                                if ( address[0] )
                                        mem_data = { 8'h00, din[7:0] };
                                else
                                        mem_data = { 8'h00, din[15:8] };
                        end
 
        // register data
        reg [15:0] reg_data;
        always @(*)
                case (state)
                        STATE_ALU : reg_data = alu_result;
                        STATE_SHIFT : reg_data = shifter_result;
                        STATE_JAL,
                        STATE_SWI : reg_data = pc;
                        default : reg_data = mem_data;
                endcase
 
        // register writeback
        reg reg_write;
        always @(*)
                case (state)
                        STATE_ALU,
                        STATE_SHIFT,
                        STATE_LOAD,
                        STATE_JAL,
                        STATE_SWI : reg_write = 1'b1;
                        default : reg_write = 1'b0;
                endcase
 
        // override dst address for JAL to save 3 bits
        reg [2:0] ra_dst;
        always @(*)
                if ( state == STATE_JAL )
                        ra_dst = 3'd6;  // hard linked register
                else
                        ra_dst = ir_dst;
 
        // register file        
        `ifdef USE_RAM_FOR_REGFILE
                wire [15:0] qa, qb;
                regfile8x16 i_regfile (
                        .clock(clk),
                        .data(reg_data),
                        .rdaddress_a(ir_dst),
                        .rdaddress_b(ir_src),
                        .wraddress(ra_dst),
                        .wren(reg_write&clk_en),
                        .qa(qa),
                        .qb(qb)
                        );
        `else
                reg [15:0] regs[7:0];
                always @(posedge clk)
                        if (reg_write&clk_en)
                                regs[ir_dst] <= reg_data;
                wire [15:0] qa = regs[ir_dst];
                wire [15:0] qb = regs[ir_src];
        `endif
 
        // flags
        reg i, next_i;
        reg n, next_n;
        reg v, next_v;
        reg z, next_z;
        reg c, next_c;
        always @(*)
                case (state)
                        STATE_ALU : begin
                                next_i = i;
                                next_n = alu_n;
                                next_v = alu_v;
                                next_z = alu_z;
                                next_c = alu_c;
                                end
                        STATE_SHIFT : begin
                                next_i = i;
                                next_n = shift_n;
                                next_v = shift_v;
                                next_z = shift_z;
                                next_c = shift_c;
                                end
                        STATE_MOVSR : begin
                                next_i = (i & ~ir_imm8[4]) | ( ir_imm8[7] & ir_imm8[4]);
                                next_n = (n & ~ir_imm8[3]) | ( ir_imm8[7] & ir_imm8[3]);
                                next_v = (v & ~ir_imm8[2]) | ( ir_imm8[7] & ir_imm8[2]);
                                next_z = (z & ~ir_imm8[1]) | ( ir_imm8[7] & ir_imm8[1]);
                                next_c = (c & ~ir_imm8[0]) | ( ir_imm8[7] & ir_imm8[0]);
                                end
                        STATE_INTREQ : begin
                                next_i = 1'b0;  // disable interrupts
                                next_n = n;
                                next_v = v;
                                next_z = z;
                                next_c = c;
                                end
                        STATE_RFE : begin
                                next_i = esr[4];
                                next_n = esr[3];
                                next_v = esr[2];
                                next_z = esr[1];
                                next_c = esr[0];
                                end
                        default : begin
                                next_i = i;
                                next_n = n;
                                next_v = v;
                                next_z = z;
                                next_c = c;
                                end
                endcase
 
        // synchronous writeback
        always @(posedge clk or negedge reset_n)
                if (!reset_n)
                        begin
                                state <= STATE_FETCH;
                                ir <= 16'd0;
                                pc <= 16'd0;
                                i <= 1'b1;      // TEST: enable interrupts
                                n <= 1'b0;
                                v <= 1'b0;
                                z <= 1'b0;
                                c <= 1'b0;
                                epc <= 16'd0;
                                esr <= 5'd0;
                        end
                else if ( clk_en )
                        begin
                                state <= next_state;
                                ir <= next_ir;
                                pc <= next_pc;
                                i <= next_i;
                                n <= next_n;
                                v <= next_v;
                                z <= next_z;
                                c <= next_c;
                                epc <= next_epc;
                                esr <= next_esr;
                        end
endmodule

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Subversion Repositories cpu16

[/] [cpu16/] [trunk/] [cpu16.v] - Blame information for rev 4

Line No.	Rev	Author	Line
1	2	yzoer	`/*`
2			`* None pipelined, 3-state, 16-bit CPU`
3			`*`
4			`* Copyright (C) 2019, Yvo Zoer`
5			`*`
6			`* This program is free software; you can redistribute it and/or`
7			`* modify it under the terms of the GNU General Public License`
8			`* as published by the Free Software Foundation; either version 2`
9			`* of the License, or (at your option) any later version.`
10			`*`
11			`* This program is distributed in the hope that it will be useful,`
12			`* but WITHOUT ANY WARRANTY; without even the implied warranty of`
13			`* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the`
14			`* GNU General Public License for more details.`
15			`*`
16			`* You should have received a copy of the GNU General Public License`
17			`* along with this program; if not, write to the Free Software`
18			`* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.`
19			`*/`
20	3	yzoer
21			`// comment this line out to use LE's (warning: size goes up dramatically to around 800LE's)`
22			`define USE_RAM_FOR_REGFILE
23
24
25	2	yzoer	`module cpu16 (`
26			`clk,`
27			`reset_n,`
28			`clk_en,`
29			`address,`
30			`din,`
31			`dout,`
32			`cs_n,`
33			`oe_n,`
34			`we_n,`
35			`ub_n,`
36			`lb_n,`
37			`irq, // combined interrupt request from intc`
38			`busreq,`
39			`busack`
40			`);`
41
42			`input clk;`
43			`input reset_n;`
44			`output [15:0] address;`
45			`input clk_en;`
46			`input [15:0] din;`
47			`output [15:0] dout;`
48			`output cs_n;`
49			`output oe_n;`
50			`output we_n;`
51			`output ub_n;`
52			`output lb_n;`
53			`input irq;`
54			`input busreq;`
55			`output busack;`
56
57			`// alu`
58			`parameter OP_SUB = 5'd0; // ok`
59			`parameter OP_SBC = 5'd1; // ok`
60			`parameter OP_ADD = 5'd2;`
61			`parameter OP_ADC = 5'd3;`
62			`parameter OP_CMP = 5'd4;`
63			`parameter OP_AND = 5'd5; // ok`
64			`parameter OP_OR = 5'd6; // ok`
65			`parameter OP_XOR = 5'd7; // ok`
66
67			`// alu immediate`
68			`parameter OP_SUBI = 5'd8; // ok`
69			`parameter OP_SBCI = 5'd9; // ok`
70			`parameter OP_ADDI = 5'd10; // ok`
71			`parameter OP_ADCI = 5'd11; // ok`
72			`parameter OP_CMPI = 5'd12;`
73			`parameter OP_ANDI = 5'd13; // ok`
74			`parameter OP_ORI = 5'd14; // ok`
75			`parameter OP_XORI = 5'd15; // ok`
76
77			`// shifter + load`
78			`parameter OP_MOV = 5'd16;`
79			`parameter OP_SRAV = 5'd17;`
80			`parameter OP_SLLV = 5'd18;`
81			`parameter OP_LD = 5'd19;`
82
83			`// shifter immediate + store`
84			`parameter OP_LUI = 5'd20;`
85			`parameter OP_SRA = 5'd21;`
86			`parameter OP_SLL = 5'd22;`
87			`parameter OP_ST = 5'd23;`
88
89			`// jump`
90			`parameter OP_J = 5'd24;`
91			`parameter OP_JAL = 5'd25;`
92			`parameter OP_JR = 5'd26;`
93			`parameter OP_JALR = 5'd27;`
94
95			`// conditional branch`
96			`parameter OP_BXX = 5'd28;`
97			`parameter OP_MSR = 5'd29;`
98			`parameter OP_RFE = 5'd30;`
99			`parameter OP_SWI = 5'd31;`
100
101			`// states`
102			`parameter STATE_FETCH = 4'h0;`
103			`parameter STATE_DECODE = 4'h1;`
104			`parameter STATE_ALU = 4'h2;`
105			`parameter STATE_SHIFT = 4'h3;`
106			`parameter STATE_LOAD = 4'h4;`
107			`parameter STATE_STORE = 4'h5;`
108			`parameter STATE_J = 4'h6;`
109			`parameter STATE_JAL = 4'h7;`
110			`parameter STATE_BRANCH = 4'h8;`
111			`parameter STATE_MOVSR = 4'h9;`
112			`parameter STATE_RFE = 4'ha;`
113			`parameter STATE_SWI = 4'hb;`
114			`parameter STATE_INTREQ = 4'hc;`
115			`parameter STATE_BUSREQ = 4'hd;`
116
117			`// instruction register wires`
118			`wire [4:0] ir_opcode = ir[15:11];`
119			`wire [2:0] ir_dst = ir[10:8];`
120			`wire [2:0] ir_src = ir[7:5];`
121			`wire ir_size = ir[4]; // byte/word flag`
122			`wire [3:0] ir_imm4 = ir[3:0]; // load/store offset`
123			`wire [7:0] ir_imm8 = ir[7:0]; // immediate value / relative conditional branch`
124			`wire [10:0] ir_imm11 = ir[10:0]; // unconditional branch`
125
126			`// busack valid once we're in the busreq state`
127			`reg busack;`
128			`always @(*)`
129			`if ( state == STATE_BUSREQ )`
130			`busack = 1'b1;`
131			`else`
132			`busack = 1'b0;`
133
134			`// chip-select`
135			`reg cs_n;`
136			`always @(*)`
137			`case (state)`
138			`STATE_FETCH,`
139			`STATE_LOAD,`
140			`STATE_STORE : cs_n = 1'b0;`
141			`default : cs_n = 1'b1;`
142			`endcase`
143
144			`// output enable`
145			`reg oe_n;`
146			`always @(*)`
147			`case (state)`
148			`STATE_FETCH,`
149			`STATE_LOAD : oe_n = 1'b0;`
150			`default : oe_n = 1'b1;`
151			`endcase`
152			`// memory write signal`
153			`reg we_n;`
154			`always @(posedge clk)`
155			`if ( state == STATE_STORE )`
156			`we_n <= clk_en;`
157			`else`
158			`we_n <= 1'b1;`
159
160			`// upper / lower byte write/read enable`
161			`// NOTE: could integrate dout here as well..`
162			`reg ub_n;`
163			`reg lb_n;`
164			`always @(*)`
165			`if ( state == STATE_STORE )`
166			`begin`
167			`if ( ir_size )`
168			`begin // word`
169			`ub_n = 1'b0;`
170			`lb_n = 1'b0;`
171			`end`
172			`else`
173			`begin // byte`
174			`if ( address[0] )`
175			`begin`
176			`ub_n = 1'b1;`
177			`lb_n = 1'b0;`
178			`end`
179			`else`
180			`begin`
181			`ub_n = 1'b0;`
182			`lb_n = 1'b1;`
183			`end`
184			`end`
185			`end`
186			`else`
187			`begin // word access for everything else`
188			`ub_n = 1'b0;`
189			`lb_n = 1'b0;`
190			`end`
191
192			`// data output logic`
193			`reg [15:0] dout;`
194			`always @(*)`
195			`if ( ir_size )`
196			`dout = qa; // word`
197			`else`
198			`begin`
199			`if ( address[0] )`
200			`dout = { 8'h00, qa[7:0] };`
201			`else`
202			`dout = { qa[7:0], 8'h00 };`
203			`end`
204
205			`// address mux`
206			`reg [15:0] address;`
207			`always @(*)`
208			`case (state)`
209			`STATE_LOAD,`
210			`STATE_STORE : address = qb + ir_imm4;`
211			`default : address = pc; // { pc[15:1], 1'b0 };`
212			`endcase`
213
214			`// main state machine`
215			`reg [3:0] state, next_state;`
216			`always @(*)`
217			`case (state)`
218			`STATE_FETCH : next_state = STATE_DECODE;`
219			`STATE_DECODE : begin`
220			`casex (ir_opcode)`
221			`5'b0xxxx : next_state = STATE_ALU; // sub/sbc/add/adc/and/or/xor/cmp`
222			`5'b10x0x,`
223			`5'b10x10 : next_state = STATE_SHIFT;`
224			`5'b10011 : next_state = STATE_LOAD;`
225			`5'b10111 : next_state = STATE_STORE;`
226			`5'b110x0 : next_state = STATE_J; // 24/26`
227			`5'b110x1 : next_state = STATE_JAL; // 25/27`
228			`5'b11100 : next_state = STATE_BRANCH;`
229			`5'b11101 : next_state = STATE_MOVSR;`
230			`5'b11110 : next_state = STATE_RFE;`
231			`5'b11111 : next_state = STATE_SWI;`
232			`default : next_state = STATE_FETCH;`
233			`endcase`
234			`end`
235			`STATE_INTREQ : next_state = STATE_FETCH;`
236			`STATE_BUSREQ : begin`
237			`if ( busreq )`
238			`next_state = STATE_BUSREQ;`
239			`else`
240			`next_state = STATE_FETCH; // will miss interrupt when coming out of busreq (STATE_NOP?)`
241			`end`
242			`default : begin`
243			`if ( busreq )`
244			`next_state = STATE_BUSREQ;`
245			`else if ( irq & i ) // only fires when interrupt enable is set (i = 1)`
246			`next_state = STATE_INTREQ;`
247			`else`
248			`next_state = STATE_FETCH;`
249			`end`
250			`endcase`
251
252			`// instruction register`
253			`reg [15:0] ir, next_ir;`
254			`always @(*)`
255			`if ( state == STATE_FETCH )`
256			`next_ir = din;`
257			`else`
258			`next_ir = ir;`
259
260			`// exception pc- and status register`
261			`reg [15:0] epc, next_epc;`
262			`reg [4:0] esr, next_esr;`
263			`always @(*)`
264			`case (state)`
265			`STATE_INTREQ : begin`
266			`next_epc = pc;`
267			`next_esr = { i,n,v,z,c };`
268			`end`
269			`default : begin`
270			`next_epc = epc;`
271			`next_esr = esr;`
272			`end`
273			`endcase`
274
275			`// source data for program counter ( register / immediate )`
276			`reg [15:0] pc_data;`
277			`always @(*)`
278			`if ( ir_opcode[1] ) // immediate for PC`
279			`pc_data = qb; //{ qb[15:1], 1'b0 }; // TEST: jr/jalr`
280			`else`
281			`pc_data = pc + { { 4 { ir_imm11[10] } }, ir_imm11[10:0], 1'b0 }; // TEST: j/jal doubled for jumps`
282
283			`// condition check`
284			`reg condition_true;`
285			`always @(*)`
286			`case (ir_dst)`
287			`3'd0 : condition_true = z;`
288			`3'd1 : condition_true = ~z;`
289			`3'd2 : condition_true = ~n;`
290			`3'd3 : condition_true = n;`
291			`3'd4 : condition_true = ~c;`
292			`3'd5 : condition_true = c;`
293			`3'd6 : condition_true = ~v;`
294			`3'd7 : condition_true = v;`
295			`endcase`
296
297			`// program counter`
298			`reg [15:0] pc, next_pc;`
299			`always @(*)`
300			`case (state)`
301			`STATE_FETCH : next_pc = pc + 16'd2;`
302			`STATE_SWI : next_pc = { ir_imm8, 1'b0 };`
303			`STATE_J,`
304			`STATE_JAL : next_pc = pc_data;`
305			`STATE_BRANCH : begin`
306			`if ( condition_true )`
307			`next_pc = pc + { { 7 { ir_imm8[7] } }, ir_imm8[7:0], 1'b0 };`
308			`else`
309			`next_pc = pc;`
310			`end`
311			`STATE_INTREQ : next_pc = 16'h0002; // interrupt vector appears at 0x0002`
312			`STATE_RFE : next_pc = epc;`
313			`default : next_pc = pc;`
314			`endcase`
315
316			`// alu data mux`
317			`reg [15:0] alu_data;`
318			`always @(*)`
319			`if (ir_opcode[3]) // immediate`
320			`alu_data = { 8'h00, ir_imm8 };`
321			`else`
322			`alu_data = qb; // register`
323
324			`// alu`
325			`wire [15:0] alu_result;`
326			`wire alu_n;`
327			`wire alu_v;`
328			`wire alu_z;`
329			`wire alu_c;`
330			`alu i_alu (`
331			`.a(qa),`
332			`.b(alu_data),`
333			`.c_in(c),`
334			`.func(ir_opcode[2:0]),`
335			`.result(alu_result),`
336			`.n(alu_n),`
337			`.v(alu_v),`
338			`.z(alu_z),`
339			`.c(alu_c)`
340			`);`
341
342			`// shifter data mux`
343			`reg [15:0] shifter_data;`
344			`always @(*)`
345			`if ( ir_opcode[2] ) // immediate`
346			`shifter_data = { ir_imm8, 8'h00 }; // lui from instruction word`
347			`else`
348			`shifter_data = qb; // mov`
349
350			`// shifter distance -- roll into b argument inside shifter?`
351			`reg [3:0] shift_distance;`
352			`always @(*)`
353			`if ( ir_opcode[2] )`
354			`shift_distance = ir_imm4;`
355			`else`
356			`shift_distance = qb[3:0];`
357
358			`// shifter`
359			`wire [15:0] shifter_result;`
360			`wire shift_n;`
361			`wire shift_z;`
362			`wire shift_c;`
363			`wire shift_v;`
364			`shifter i_shifter (`
365			`.a(qa),`
366			`.b(shifter_data),`
367			`.n_in(n),`
368			`.v_in(v),`
369			`.z_in(z),`
370			`.c_in(c),`
371			`.distance(shift_distance),`
372			`.func(ir_opcode[1:0]),`
373			`.result(shifter_result),`
374			`.n(shift_n),`
375			`.v(shift_v),`
376			`.z(shift_z),`
377			`.c(shift_c)`
378			`);`
379
380			`// memory data mux for lw/lb`
381			`reg [15:0] mem_data;`
382			`always @(*)`
383			`if ( ir_size )`
384			`mem_data = din;`
385			`else`
386			`begin`
387			`if ( address[0] )`
388			`mem_data = { 8'h00, din[7:0] };`
389			`else`
390			`mem_data = { 8'h00, din[15:8] };`
391			`end`
392
393			`// register data`
394			`reg [15:0] reg_data;`
395			`always @(*)`
396			`case (state)`
397			`STATE_ALU : reg_data = alu_result;`
398			`STATE_SHIFT : reg_data = shifter_result;`
399			`STATE_JAL,`
400			`STATE_SWI : reg_data = pc;`
401			`default : reg_data = mem_data;`
402			`endcase`
403
404			`// register writeback`
405			`reg reg_write;`
406			`always @(*)`
407			`case (state)`
408			`STATE_ALU,`
409			`STATE_SHIFT,`
410			`STATE_LOAD,`
411			`STATE_JAL,`
412			`STATE_SWI : reg_write = 1'b1;`
413			`default : reg_write = 1'b0;`
414			`endcase`
415
416			`// override dst address for JAL to save 3 bits`
417			`reg [2:0] ra_dst;`
418			`always @(*)`
419			`if ( state == STATE_JAL )`
420			`ra_dst = 3'd6; // hard linked register`
421			`else`
422			`ra_dst = ir_dst;`
423	3	yzoer
424	2	yzoer	`// register file`
425	3	yzoer	`ifdef USE_RAM_FOR_REGFILE
426			`wire [15:0] qa, qb;`
427			`regfile8x16 i_regfile (`
428			`.clock(clk),`
429			`.data(reg_data),`
430			`.rdaddress_a(ir_dst),`
431			`.rdaddress_b(ir_src),`
432			`.wraddress(ra_dst),`
433			`.wren(reg_write&clk_en),`
434			`.qa(qa),`
435			`.qb(qb)`
436			`);`
437			`else
438			`reg [15:0] regs[7:0];`
439			`always @(posedge clk)`
440			`if (reg_write&clk_en)`
441			`regs[ir_dst] <= reg_data;`
442			`wire [15:0] qa = regs[ir_dst];`
443			`wire [15:0] qb = regs[ir_src];`
444			`endif
445	2	yzoer
446			`// flags`
447			`reg i, next_i;`
448			`reg n, next_n;`
449			`reg v, next_v;`
450			`reg z, next_z;`
451			`reg c, next_c;`
452			`always @(*)`
453			`case (state)`
454			`STATE_ALU : begin`
455			`next_i = i;`
456			`next_n = alu_n;`
457			`next_v = alu_v;`
458			`next_z = alu_z;`
459			`next_c = alu_c;`
460			`end`
461			`STATE_SHIFT : begin`
462			`next_i = i;`
463			`next_n = shift_n;`
464			`next_v = shift_v;`
465			`next_z = shift_z;`
466			`next_c = shift_c;`
467			`end`
468			`STATE_MOVSR : begin`
469			`next_i = (i & ~ir_imm8[4]) \| ( ir_imm8[7] & ir_imm8[4]);`
470			`next_n = (n & ~ir_imm8[3]) \| ( ir_imm8[7] & ir_imm8[3]);`
471			`next_v = (v & ~ir_imm8[2]) \| ( ir_imm8[7] & ir_imm8[2]);`
472			`next_z = (z & ~ir_imm8[1]) \| ( ir_imm8[7] & ir_imm8[1]);`
473			`next_c = (c & ~ir_imm8[0]) \| ( ir_imm8[7] & ir_imm8[0]);`
474			`end`
475			`STATE_INTREQ : begin`
476			`next_i = 1'b0; // disable interrupts`
477			`next_n = n;`
478			`next_v = v;`
479			`next_z = z;`
480			`next_c = c;`
481			`end`
482			`STATE_RFE : begin`
483			`next_i = esr[4];`
484			`next_n = esr[3];`
485			`next_v = esr[2];`
486			`next_z = esr[1];`
487			`next_c = esr[0];`
488			`end`
489			`default : begin`
490			`next_i = i;`
491			`next_n = n;`
492			`next_v = v;`
493			`next_z = z;`
494			`next_c = c;`
495			`end`
496			`endcase`
497
498			`// synchronous writeback`
499			`always @(posedge clk or negedge reset_n)`
500			`if (!reset_n)`
501			`begin`
502			`state <= STATE_FETCH;`
503			`ir <= 16'd0;`
504			`pc <= 16'd0;`
505			`i <= 1'b1; // TEST: enable interrupts`
506			`n <= 1'b0;`
507			`v <= 1'b0;`
508			`z <= 1'b0;`
509			`c <= 1'b0;`
510			`epc <= 16'd0;`
511			`esr <= 5'd0;`
512			`end`
513			`else if ( clk_en )`
514			`begin`
515			`state <= next_state;`
516			`ir <= next_ir;`
517			`pc <= next_pc;`
518			`i <= next_i;`
519			`n <= next_n;`
520			`v <= next_v;`
521			`z <= next_z;`
522			`c <= next_c;`
523			`epc <= next_epc;`
524			`esr <= next_esr;`
525			`end`
526			`endmodule`