cpu.py

   1 """
   2 /*
   3  * Copyright 2018 Jacob Lifshay
   4  *
   5  * Permission is hereby granted, free of charge, to any person obtaining a copy
   6  * of this software and associated documentation files (the "Software"), to deal
   7  * in the Software without restriction, including without limitation the rights
   8  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   9  * copies of the Software, and to permit persons to whom the Software is
  10  * furnished to do so, subject to the following conditions:
  11  *
  12  * The above copyright notice and this permission notice shall be included in all
  13  * copies or substantial portions of the Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  16  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  18  * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  19  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  20  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  21  * SOFTWARE.
  22  *
  23  */
  24 `timescale 1ns / 1ps
  25 `include "riscv.vh"
  26 `include "cpu.vh"
  27 """
  28
  29 from migen import *
  30 from migen.fhdl import verilog
  31 from migen.fhdl.structure import _Operator
  32
  33 from riscvdefs import *
  34 from cpudefs import *
  35
  36 class MemoryInterface:
  37     fetch_address = Signal(32, name="memory_interface_fetch_address") # XXX [2:]
  38     fetch_data = Signal(32, name="memory_interface_fetch_data")
  39     fetch_valid = Signal(name="memory_interface_fetch_valid")
  40     rw_address= Signal(32, name="memory_interface_rw_address") # XXX [2:]
  41     rw_byte_mask = Signal(4, name="memory_interface_rw_byte_mask")
  42     rw_read_not_write = Signal(name="memory_interface_rw_read_not_write")
  43     rw_active = Signal(name="memory_interface_rw_active")
  44     rw_data_in = Signal(32, name="memory_interface_rw_data_in")
  45     rw_data_out = Signal(32, name="memory_interface_rw_data_out")
  46     rw_address_valid = Signal(name="memory_interface_rw_address_valid")
  47     rw_wait = Signal(name="memory_interface_rw_wait")
  48
  49
  50 class Decoder:
  51     funct7 = Signal(7, name="decoder_funct7")
  52     funct3 = Signal(3, name="decoder_funct3")
  53     rd = Signal(5, name="decoder_rd")
  54     rs1 = Signal(5, name="decoder_rs1")
  55     rs2 = Signal(5, name="decoder_rs2")
  56     immediate = Signal(32, name="decoder_immediate")
  57     opcode = Signal(7, name="decoder_opcode")
  58     act = Signal(decode_action, name="decoder_action")
  59
  60
  61 class CPU(Module):
  62     """
  63     """
  64
  65     def get_ls_misaligned(self, ls, funct3, load_store_address_low_2):
  66         return Case(funct3[:2],
  67                 { F3.sb: ls.eq(Constant(0)),
  68                   F3.sh: ls.eq(load_store_address_low_2[0] != 0),
  69                   F3.sw: ls.eq(load_store_address_low_2[0:2] != Constant(0, 2)),
  70                   "default": ls.eq(Constant(1))
  71                 })
  72
  73     def get_lsbm(self, dc):
  74         return Cat(Constant(1),
  75                    Mux((dc.funct3[1] | dc.funct3[0]),
  76                        Constant(1), Constant(0)),
  77                    Mux((dc.funct3[1]),
  78                        Constant(0b11, 2), Constant(0, 2)))
  79
  80     def __init__(self):
  81         self.clk = ClockSignal()
  82         self.reset = ResetSignal()
  83         self.tty_write = Signal()
  84         self.tty_write_data = Signal(8)
  85         self.tty_write_busy = Signal()
  86         self.switch_2 = Signal()
  87         self.switch_3 = Signal()
  88         self.led_1 = Signal()
  89         self.led_3 = Signal()
  90
  91         ram_size = Constant(0x8000)
  92         ram_start = Constant(0x10000, 32)
  93         reset_vector = Signal(32)
  94         mtvec = Signal(32)
  95
  96         reset_vector.eq(ram_start)
  97         mtvec.eq(ram_start + 0x40)
  98
  99         l = []
 100         for i in range(31):
 101             l.append(Signal(32, name="register%d" % i))
 102         registers = Array(l)
 103
 104         mi = MemoryInterface()
 105
 106         mii = Instance("cpu_memory_interface", name="memory_instance",
 107                     p_ram_size = ram_size,
 108                     p_ram_start = ram_start,
 109                     i_clk=ClockSignal(),
 110                     i_rst=ResetSignal(),
 111                     i_fetch_address = mi.fetch_address,
 112                     o_fetch_data = mi.fetch_data,
 113                     o_fetch_valid = mi.fetch_valid,
 114                     i_rw_address = mi.rw_address,
 115                     i_rw_byte_mask = mi.rw_byte_mask,
 116                     i_rw_read_not_write = mi.rw_read_not_write,
 117                     i_rw_active = mi.rw_active,
 118                     i_rw_data_in = mi.rw_data_in,
 119                     o_rw_data_out = mi.rw_data_out,
 120                     o_rw_address_valid = mi.rw_address_valid,
 121                     o_rw_wait = mi.rw_wait,
 122                     o_tty_write = self.tty_write,
 123                     o_tty_write_data = self.tty_write_data,
 124                     i_tty_write_busy = self.tty_write_busy,
 125                     i_switch_2 = self.switch_2,
 126                     i_switch_3 = self.switch_3,
 127                     o_led_1 = self.led_1,
 128                     o_led_3 = self.led_3
 129                   )
 130         self.specials += mii
 131
 132         fetch_act = Signal(fetch_action)
 133         fetch_target_pc = Signal(32)
 134         fetch_output_pc = Signal(32)
 135         fetch_output_instruction = Signal(32)
 136         fetch_output_st = Signal(fetch_output_state)
 137
 138         fs = Instance("CPUFetchStage", name="fetch_stage",
 139             i_clk=ClockSignal(),
 140             i_rst=ResetSignal(),
 141             o_memory_interface_fetch_address = mi.fetch_address,
 142             i_memory_interface_fetch_data = mi.fetch_data,
 143             i_memory_interface_fetch_valid = mi.fetch_valid,
 144             i_fetch_action = fetch_act,
 145             i_target_pc = fetch_target_pc,
 146             o_output_pc = fetch_output_pc,
 147             o_output_instruction = fetch_output_instruction,
 148             o_output_state = fetch_output_st,
 149             i_reset_vector = reset_vector,
 150             i_mtvec = mtvec,
 151         )
 152         self.specials += fs
 153
 154         dc = Decoder()
 155
 156         cd = Instance("CPUDecoder", name="decoder",
 157             i_instruction = fetch_output_instruction,
 158             o_funct7 = dc.funct7,
 159             o_funct3 = dc.funct3,
 160             o_rd = dc.rd,
 161             o_rs1 = dc.rs1,
 162             o_rs2 = dc.rs2,
 163             o_immediate = dc.immediate,
 164             o_opcode = dc.opcode,
 165             o_decode_action = dc.act
 166         )
 167         self.specials += cd
 168
 169         register_rs1 = Signal(32)
 170         register_rs2 = Signal(32)
 171         self.comb += If(dc.rs1 == 0,
 172                         register_rs1.eq(0)
 173                      ).Else(
 174                         register_rs1.eq(registers[dc.rs1-1]))
 175         self.comb += If(dc.rs2 == 0,
 176                         register_rs2.eq(0)
 177                      ).Else(
 178                         register_rs2.eq(registers[dc.rs2-1]))
 179
 180         load_store_address = Signal(32)
 181         load_store_address_low_2 = Signal(2)
 182
 183         self.comb += load_store_address.eq(dc.immediate + register_rs1)
 184         self.comb += load_store_address_low_2.eq(
 185                             dc.immediate[:2] + register_rs1[:2])
 186
 187         load_store_misaligned = Signal()
 188
 189         lsa = self.get_ls_misaligned(load_store_misaligned, dc.funct3,
 190                                      load_store_address_low_2)
 191         self.comb += lsa
 192
 193         # XXX rwaddr not 31:2 any more
 194         self.comb += mi.rw_address.eq(load_store_address[2:])
 195
 196         unshifted_load_store_byte_mask = Signal(4)
 197
 198         self.comb += unshifted_load_store_byte_mask.eq(self.get_lsbm(dc))
 199
 200         # XXX yuck.  this will cause migen simulation to fail
 201         # (however conversion to verilog works)
 202         self.comb += mi.rw_byte_mask.eq(
 203                 _Operator("<<", [unshifted_load_store_byte_mask,
 204                                         load_store_address_low_2]))
 205
 206         # XXX not obvious
 207         b3 = Mux(load_store_address_low_2[1],
 208                  Mux(load_store_address_low_2[0], register_rs2[0:8],
 209                                                   register_rs2[8:16]),
 210                  Mux(load_store_address_low_2[0], register_rs2[16:24],
 211                                                   register_rs2[24:32]))
 212         b2 = Mux(load_store_address_low_2[1], register_rs2[0:8],
 213                                               register_rs2[16:24])
 214         b1 = Mux(load_store_address_low_2[0], register_rs2[0:8],
 215                                               register_rs2[8:16])
 216         b0 = register_rs2[0:8]
 217
 218         self.comb += mi.rw_data_in.eq(Cat(b0, b1, b2, b3))
 219
 220         # XXX not obvious
 221         unmasked_loaded_value = Signal(32)
 222
 223         b0 = Mux(load_store_address_low_2[1],
 224                  Mux(load_store_address_low_2[0], mi.rw_data_out[24:32],
 225                                                   mi.rw_data_out[16:24]),
 226                  Mux(load_store_address_low_2[0], mi.rw_data_out[15:8],
 227                                                   mi.rw_data_out[0:8]))
 228         b1 = Mux(load_store_address_low_2[1], mi.rw_data_out[24:31],
 229                                               mi.rw_data_out[8:16])
 230         b23 = mi.rw_data_out[16:32]
 231
 232         self.comb += unmasked_loaded_value.eq(Cat(b0, b1, b23))
 233
 234         # XXX not obvious
 235         loaded_value = Signal(32)
 236
 237         b0 = unmasked_loaded_value[0:8]
 238         b1 = Mux(dc.funct3[0:2] == 0,
 239                 Replicate(~dc.funct3[2] & unmasked_loaded_value[7], 8),
 240                 unmasked_loaded_value[8:16])
 241         b2 = Mux(dc.funct3[1] == 0,
 242                 Replicate(~dc.funct3[2] &
 243                            Mux(dc.funct3[0], unmasked_loaded_value[15],
 244                                                   unmasked_loaded_value[7]),
 245                           16),
 246                 unmasked_loaded_value[16:32])
 247
 248         self.comb += loaded_value.eq(Cat(b0, b1, b2))
 249
 250         self.comb += mi.rw_active.eq(~self.reset
 251                         & (fetch_output_st == fetch_output_state_valid)
 252                         & ~load_store_misaligned
 253                         & ((dc.act & (DA.load | DA.store)) != 0))
 254
 255         self.comb += mi.rw_read_not_write.eq(~dc.opcode[5])
 256
 257         # alu
 258         alu_a = Signal(32)
 259         alu_b = Signal(32)
 260         alu_result = Signal(32)
 261
 262         self.comb += alu_a.eq(register_rs1)
 263         self.comb += alu_b.eq(Mux(dc.opcode[5],
 264                                   register_rs2,
 265                                   dc.immediate))
 266
 267         ali = Instance("cpu_alu", name="alu",
 268             i_funct7 = dc.funct7,
 269             i_funct3 = dc.funct3,
 270             i_opcode = dc.opcode,
 271             i_a = alu_a,
 272             i_b = alu_b,
 273             o_result = alu_result
 274         )
 275         self.specials += ali
 276
 277         lui_auipc_result = Signal(32)
 278         self.comb += lui_auipc_result.eq(Mux(dc.opcode[5],
 279                                              dc.immediate,
 280                                              dc.immediate + fetch_output_pc))
 281
 282 if __name__ == "__main__":
 283     example = CPU()
 284     print(verilog.convert(example,
 285          {
 286            example.tty_write,
 287            example.tty_write_data,
 288            example.tty_write_busy,
 289            example.switch_2,
 290            example.switch_3,
 291            example.led_1,
 292            example.led_3,
 293            }))
 294
 295 """
 296
 297     wire [31:0] lui_auipc_result = decoder_opcode[5] ? decoder_immediate : decoder_immediate + fetch_output_pc;
 298
 299     assign fetch_target_pc[31:1] = ((decoder_opcode != `opcode_jalr ? fetch_output_pc[31:1] : register_rs1[31:1]) + decoder_immediate[31:1]);
 300     assign fetch_target_pc[0] = 0;
 301
 302     wire misaligned_jump_target = fetch_target_pc[1];
 303
 304     wire [31:0] branch_arg_a = {register_rs1[31] ^ ~decoder_funct3[1], register_rs1[30:0]};
 305     wire [31:0] branch_arg_b = {register_rs2[31] ^ ~decoder_funct3[1], register_rs2[30:0]};
 306
 307     wire branch_taken = decoder_funct3[0] ^ (decoder_funct3[2] ? branch_arg_a < branch_arg_b : branch_arg_a == branch_arg_b);
 308
 309     reg [31:0] mcause = 0;
 310     reg [31:0] mepc = 32'hXXXXXXXX;
 311     reg [31:0] mscratch = 32'hXXXXXXXX;
 312
 313     reg mstatus_mpie = 1'bX;
 314     reg mstatus_mie = 0;
 315     parameter mstatus_mprv = 0;
 316     parameter mstatus_tsr = 0;
 317     parameter mstatus_tw = 0;
 318     parameter mstatus_tvm = 0;
 319     parameter mstatus_mxr = 0;
 320     parameter mstatus_sum = 0;
 321     parameter mstatus_xs = 0;
 322     parameter mstatus_fs = 0;
 323     parameter mstatus_mpp = 2'b11;
 324     parameter mstatus_spp = 0;
 325     parameter mstatus_spie = 0;
 326     parameter mstatus_upie = 0;
 327     parameter mstatus_sie = 0;
 328     parameter mstatus_uie = 0;
 329
 330     reg mie_meie = 1'bX;
 331     reg mie_mtie = 1'bX;
 332     reg mie_msie = 1'bX;
 333     parameter mie_seie = 0;
 334     parameter mie_ueie = 0;
 335     parameter mie_stie = 0;
 336     parameter mie_utie = 0;
 337     parameter mie_ssie = 0;
 338     parameter mie_usie = 0;
 339
 340     task reset_to_initial;
 341     begin
 342         mcause = 0;
 343         mepc = 32'hXXXXXXXX;
 344         mscratch = 32'hXXXXXXXX;
 345         mstatus_mie = 0;
 346         mstatus_mpie = 1'bX;
 347         mie_meie = 1'bX;
 348         mie_mtie = 1'bX;
 349         mie_msie = 1'bX;
 350         registers['h01] <= 32'hXXXXXXXX;
 351         registers['h02] <= 32'hXXXXXXXX;
 352         registers['h03] <= 32'hXXXXXXXX;
 353         registers['h04] <= 32'hXXXXXXXX;
 354         registers['h05] <= 32'hXXXXXXXX;
 355         registers['h06] <= 32'hXXXXXXXX;
 356         registers['h07] <= 32'hXXXXXXXX;
 357         registers['h08] <= 32'hXXXXXXXX;
 358         registers['h09] <= 32'hXXXXXXXX;
 359         registers['h0A] <= 32'hXXXXXXXX;
 360         registers['h0B] <= 32'hXXXXXXXX;
 361         registers['h0C] <= 32'hXXXXXXXX;
 362         registers['h0D] <= 32'hXXXXXXXX;
 363         registers['h0E] <= 32'hXXXXXXXX;
 364         registers['h0F] <= 32'hXXXXXXXX;
 365         registers['h10] <= 32'hXXXXXXXX;
 366         registers['h11] <= 32'hXXXXXXXX;
 367         registers['h12] <= 32'hXXXXXXXX;
 368         registers['h13] <= 32'hXXXXXXXX;
 369         registers['h14] <= 32'hXXXXXXXX;
 370         registers['h15] <= 32'hXXXXXXXX;
 371         registers['h16] <= 32'hXXXXXXXX;
 372         registers['h17] <= 32'hXXXXXXXX;
 373         registers['h18] <= 32'hXXXXXXXX;
 374         registers['h19] <= 32'hXXXXXXXX;
 375         registers['h1A] <= 32'hXXXXXXXX;
 376         registers['h1B] <= 32'hXXXXXXXX;
 377         registers['h1C] <= 32'hXXXXXXXX;
 378         registers['h1D] <= 32'hXXXXXXXX;
 379         registers['h1E] <= 32'hXXXXXXXX;
 380         registers['h1F] <= 32'hXXXXXXXX;
 381     end
 382     endtask
 383
 384     task write_register(input [4:0] register_number, input [31:0] value);
 385     begin
 386         if(register_number != 0)
 387             registers[register_number] <= value;
 388     end
 389     endtask
 390
 391     function [31:0] evaluate_csr_funct3_operation(input [2:0] funct3, input [31:0] previous_value, input [31:0] written_value);
 392     begin
 393         case(funct3)
 394         `funct3_csrrw, `funct3_csrrwi:
 395             evaluate_csr_funct3_operation = written_value;
 396         `funct3_csrrs, `funct3_csrrsi:
 397             evaluate_csr_funct3_operation = written_value | previous_value;
 398         `funct3_csrrc, `funct3_csrrci:
 399             evaluate_csr_funct3_operation = ~written_value & previous_value;
 400         default:
 401             evaluate_csr_funct3_operation = 32'hXXXXXXXX;
 402         endcase
 403     end
 404     endfunction
 405
 406     parameter misa_a = 1'b0;
 407     parameter misa_b = 1'b0;
 408     parameter misa_c = 1'b0;
 409     parameter misa_d = 1'b0;
 410     parameter misa_e = 1'b0;
 411     parameter misa_f = 1'b0;
 412     parameter misa_g = 1'b0;
 413     parameter misa_h = 1'b0;
 414     parameter misa_i = 1'b1;
 415     parameter misa_j = 1'b0;
 416     parameter misa_k = 1'b0;
 417     parameter misa_l = 1'b0;
 418     parameter misa_m = 1'b0;
 419     parameter misa_n = 1'b0;
 420     parameter misa_o = 1'b0;
 421     parameter misa_p = 1'b0;
 422     parameter misa_q = 1'b0;
 423     parameter misa_r = 1'b0;
 424     parameter misa_s = 1'b0;
 425     parameter misa_t = 1'b0;
 426     parameter misa_u = 1'b0;
 427     parameter misa_v = 1'b0;
 428     parameter misa_w = 1'b0;
 429     parameter misa_x = 1'b0;
 430     parameter misa_y = 1'b0;
 431     parameter misa_z = 1'b0;
 432     parameter misa = {
 433         2'b01,
 434         4'b0,
 435         misa_z,
 436         misa_y,
 437         misa_x,
 438         misa_w,
 439         misa_v,
 440         misa_u,
 441         misa_t,
 442         misa_s,
 443         misa_r,
 444         misa_q,
 445         misa_p,
 446         misa_o,
 447         misa_n,
 448         misa_m,
 449         misa_l,
 450         misa_k,
 451         misa_j,
 452         misa_i,
 453         misa_h,
 454         misa_g,
 455         misa_f,
 456         misa_e,
 457         misa_d,
 458         misa_c,
 459         misa_b,
 460         misa_a};
 461
 462     parameter mvendorid = 32'b0;
 463     parameter marchid = 32'b0;
 464     parameter mimpid = 32'b0;
 465     parameter mhartid = 32'b0;
 466
 467     function [31:0] make_mstatus(input mstatus_tsr,
 468         input mstatus_tw,
 469         input mstatus_tvm,
 470         input mstatus_mxr,
 471         input mstatus_sum,
 472         input mstatus_mprv,
 473         input [1:0] mstatus_xs,
 474         input [1:0] mstatus_fs,
 475         input [1:0] mstatus_mpp,
 476         input mstatus_spp,
 477         input mstatus_mpie,
 478         input mstatus_spie,
 479         input mstatus_upie,
 480         input mstatus_mie,
 481         input mstatus_sie,
 482         input mstatus_uie);
 483     begin
 484         make_mstatus = {(mstatus_xs == 2'b11) | (mstatus_fs == 2'b11),
 485             8'b0,
 486             mstatus_tsr,
 487             mstatus_tw,
 488             mstatus_tvm,
 489             mstatus_mxr,
 490             mstatus_sum,
 491             mstatus_mprv,
 492             mstatus_xs,
 493             mstatus_fs,
 494             mstatus_mpp,
 495             2'b0,
 496             mstatus_spp,
 497             mstatus_mpie,
 498             1'b0,
 499             mstatus_spie,
 500             mstatus_upie,
 501             mstatus_mie,
 502             1'b0,
 503             mstatus_sie,
 504             mstatus_uie};
 505     end
 506     endfunction
 507
 508     wire mip_meip = 0; // TODO: implement external interrupts
 509     parameter mip_seip = 0;
 510     parameter mip_ueip = 0;
 511     wire mip_mtip = 0; // TODO: implement timer interrupts
 512     parameter mip_stip = 0;
 513     parameter mip_utip = 0;
 514     parameter mip_msip = 0;
 515     parameter mip_ssip = 0;
 516     parameter mip_usip = 0;
 517
 518     wire csr_op_is_valid;
 519
 520     function `fetch_action get_fetch_action(
 521         input `fetch_output_state fetch_output_state,
 522         input `decode_action decode_action,
 523         input load_store_misaligned,
 524         input memory_interface_rw_address_valid,
 525         input memory_interface_rw_wait,
 526         input branch_taken,
 527         input misaligned_jump_target,
 528         input csr_op_is_valid
 529         );
 530     begin
 531         case(fetch_output_state)
 532         `fetch_output_state_empty:
 533             get_fetch_action = `fetch_action_default;
 534         `fetch_output_state_trap:
 535             get_fetch_action = `fetch_action_ack_trap;
 536         `fetch_output_state_valid: begin
 537             if((decode_action & `decode_action_trap_illegal_instruction) != 0) begin
 538                 get_fetch_action = `fetch_action_error_trap;
 539             end
 540             else if((decode_action & `decode_action_trap_ecall_ebreak) != 0) begin
 541                 get_fetch_action = `fetch_action_noerror_trap;
 542             end
 543             else if((decode_action & (`decode_action_load | `decode_action_store)) != 0) begin
 544                 if(load_store_misaligned | ~memory_interface_rw_address_valid) begin
 545                     get_fetch_action = `fetch_action_error_trap;
 546                 end
 547                 else if(memory_interface_rw_wait) begin
 548                     get_fetch_action = `fetch_action_wait;
 549                 end
 550                 else begin
 551                     get_fetch_action = `fetch_action_default;
 552                 end
 553             end
 554             else if((decode_action & `decode_action_fence_i) != 0) begin
 555                 get_fetch_action = `fetch_action_fence;
 556             end
 557             else if((decode_action & `decode_action_branch) != 0) begin
 558                 if(branch_taken) begin
 559                     if(misaligned_jump_target) begin
 560                         get_fetch_action = `fetch_action_error_trap;
 561                     end
 562                     else begin
 563                         get_fetch_action = `fetch_action_jump;
 564                     end
 565                 end
 566                 else
 567                 begin
 568                     get_fetch_action = `fetch_action_default;
 569                 end
 570             end
 571             else if((decode_action & (`decode_action_jal | `decode_action_jalr)) != 0) begin
 572                 if(misaligned_jump_target) begin
 573                     get_fetch_action = `fetch_action_error_trap;
 574                 end
 575                 else begin
 576                     get_fetch_action = `fetch_action_jump;
 577                 end
 578             end
 579             else if((decode_action & `decode_action_csr) != 0) begin
 580                 if(csr_op_is_valid)
 581                     get_fetch_action = `fetch_action_default;
 582                 else
 583                     get_fetch_action = `fetch_action_error_trap;
 584             end
 585             else begin
 586                 get_fetch_action = `fetch_action_default;
 587             end
 588         end
 589         default:
 590             get_fetch_action = 32'hXXXXXXXX;
 591         endcase
 592     end
 593     endfunction
 594
 595     assign fetch_action = get_fetch_action(
 596         fetch_output_state,
 597         decode_action,
 598         load_store_misaligned,
 599         memory_interface_rw_address_valid,
 600         memory_interface_rw_wait,
 601         branch_taken,
 602         misaligned_jump_target,
 603         csr_op_is_valid
 604         );
 605
 606     task handle_trap;
 607     begin
 608         mstatus_mpie = mstatus_mie;
 609         mstatus_mie = 0;
 610         mepc = (fetch_action == `fetch_action_noerror_trap) ? fetch_output_pc + 4 : fetch_output_pc;
 611         if(fetch_action == `fetch_action_ack_trap) begin
 612             mcause = `cause_instruction_access_fault;
 613         end
 614         else if((decode_action & `decode_action_trap_illegal_instruction) != 0) begin
 615             mcause = `cause_illegal_instruction;
 616         end
 617         else if((decode_action & `decode_action_trap_ecall_ebreak) != 0) begin
 618             mcause = decoder_immediate[0] ? `cause_machine_environment_call : `cause_breakpoint;
 619         end
 620         else if((decode_action & `decode_action_load) != 0) begin
 621             if(load_store_misaligned)
 622                 mcause = `cause_load_address_misaligned;
 623             else
 624                 mcause = `cause_load_access_fault;
 625         end
 626         else if((decode_action & `decode_action_store) != 0) begin
 627             if(load_store_misaligned)
 628                 mcause = `cause_store_amo_address_misaligned;
 629             else
 630                 mcause = `cause_store_amo_access_fault;
 631         end
 632         else if((decode_action & (`decode_action_branch | `decode_action_jal | `decode_action_jalr)) != 0) begin
 633             mcause = `cause_instruction_address_misaligned;
 634         end
 635         else begin
 636             mcause = `cause_illegal_instruction;
 637         end
 638     end
 639     endtask
 640
 641     wire [11:0] csr_number = decoder_immediate;
 642     wire [31:0] csr_input_value = decoder_funct3[2] ? decoder_rs1 : register_rs1;
 643     wire csr_reads = decoder_funct3[1] | (decoder_rd != 0);
 644     wire csr_writes = ~decoder_funct3[1] | (decoder_rs1 != 0);
 645
 646     function get_csr_op_is_valid(input [11:0] csr_number, input csr_reads, input csr_writes);
 647     begin
 648         case(csr_number)
 649         `csr_ustatus,
 650         `csr_fflags,
 651         `csr_frm,
 652         `csr_fcsr,
 653         `csr_uie,
 654         `csr_utvec,
 655         `csr_uscratch,
 656         `csr_uepc,
 657         `csr_ucause,
 658         `csr_utval,
 659         `csr_uip,
 660         `csr_sstatus,
 661         `csr_sedeleg,
 662         `csr_sideleg,
 663         `csr_sie,
 664         `csr_stvec,
 665         `csr_scounteren,
 666         `csr_sscratch,
 667         `csr_sepc,
 668         `csr_scause,
 669         `csr_stval,
 670         `csr_sip,
 671         `csr_satp,
 672         `csr_medeleg,
 673         `csr_mideleg,
 674         `csr_dcsr,
 675         `csr_dpc,
 676         `csr_dscratch:
 677             get_csr_op_is_valid = 0;
 678         `csr_cycle,
 679         `csr_time,
 680         `csr_instret,
 681         `csr_cycleh,
 682         `csr_timeh,
 683         `csr_instreth,
 684         `csr_mvendorid,
 685         `csr_marchid,
 686         `csr_mimpid,
 687         `csr_mhartid:
 688             get_csr_op_is_valid = ~csr_writes;
 689         `csr_misa,
 690         `csr_mstatus,
 691         `csr_mie,
 692         `csr_mtvec,
 693         `csr_mscratch,
 694         `csr_mepc,
 695         `csr_mcause,
 696         `csr_mip:
 697             get_csr_op_is_valid = 1;
 698         `csr_mcounteren,
 699         `csr_mtval,
 700         `csr_mcycle,
 701         `csr_minstret,
 702         `csr_mcycleh,
 703         `csr_minstreth:
 704             // TODO: CSRs not implemented yet
 705             get_csr_op_is_valid = 0;
 706         endcase
 707     end
 708     endfunction
 709
 710     assign csr_op_is_valid = get_csr_op_is_valid(csr_number, csr_reads, csr_writes);
 711
 712     wire [63:0] cycle_counter = 0; // TODO: implement cycle_counter
 713     wire [63:0] time_counter = 0; // TODO: implement time_counter
 714     wire [63:0] instret_counter = 0; // TODO: implement instret_counter
 715
 716     always @(posedge clk) begin:main_block
 717         if(reset) begin
 718             reset_to_initial();
 719             disable main_block;
 720         end
 721         case(fetch_output_state)
 722         `fetch_output_state_empty: begin
 723         end
 724         `fetch_output_state_trap: begin
 725             handle_trap();
 726         end
 727         `fetch_output_state_valid: begin:valid
 728             if((fetch_action == `fetch_action_error_trap) | (fetch_action == `fetch_action_noerror_trap)) begin
 729                 handle_trap();
 730             end
 731             else if((decode_action & `decode_action_load) != 0) begin
 732                 if(~memory_interface_rw_wait)
 733                     write_register(decoder_rd, loaded_value);
 734             end
 735             else if((decode_action & `decode_action_op_op_imm) != 0) begin
 736                 write_register(decoder_rd, alu_result);
 737             end
 738             else if((decode_action & `decode_action_lui_auipc) != 0) begin
 739                 write_register(decoder_rd, lui_auipc_result);
 740             end
 741             else if((decode_action & (`decode_action_jal | `decode_action_jalr)) != 0) begin
 742                 write_register(decoder_rd, fetch_output_pc + 4);
 743             end
 744             else if((decode_action & `decode_action_csr) != 0) begin:csr
 745                 reg [31:0] csr_output_value;
 746                 reg [31:0] csr_written_value;
 747                 csr_output_value = 32'hXXXXXXXX;
 748                 csr_written_value = 32'hXXXXXXXX;
 749                 case(csr_number)
 750                 `csr_cycle: begin
 751                     csr_output_value = cycle_counter[31:0];
 752                 end
 753                 `csr_time: begin
 754                     csr_output_value = time_counter[31:0];
 755                 end
 756                 `csr_instret: begin
 757                     csr_output_value = instret_counter[31:0];
 758                 end
 759                 `csr_cycleh: begin
 760                     csr_output_value = cycle_counter[63:32];
 761                 end
 762                 `csr_timeh: begin
 763                     csr_output_value = time_counter[63:32];
 764                 end
 765                 `csr_instreth: begin
 766                     csr_output_value = instret_counter[63:32];
 767                 end
 768                 `csr_mvendorid: begin
 769                     csr_output_value = mvendorid;
 770                 end
 771                 `csr_marchid: begin
 772                     csr_output_value = marchid;
 773                 end
 774                 `csr_mimpid: begin
 775                     csr_output_value = mimpid;
 776                 end
 777                 `csr_mhartid: begin
 778                     csr_output_value = mhartid;
 779                 end
 780                 `csr_misa: begin
 781                     csr_output_value = misa;
 782                 end
 783                 `csr_mstatus: begin
 784                     csr_output_value = make_mstatus(mstatus_tsr,
 785                                                     mstatus_tw,
 786                                                     mstatus_tvm,
 787                                                     mstatus_mxr,
 788                                                     mstatus_sum,
 789                                                     mstatus_mprv,
 790                                                     mstatus_xs,
 791                                                     mstatus_fs,
 792                                                     mstatus_mpp,
 793                                                     mstatus_spp,
 794                                                     mstatus_mpie,
 795                                                     mstatus_spie,
 796                                                     mstatus_upie,
 797                                                     mstatus_mie,
 798                                                     mstatus_sie,
 799                                                     mstatus_uie);
 800                     csr_written_value = evaluate_csr_funct3_operation(decoder_funct3, csr_output_value, csr_input_value);
 801                     if(csr_writes) begin
 802                         mstatus_mpie = csr_written_value[7];
 803                         mstatus_mie = csr_written_value[3];
 804                     end
 805                 end
 806                 `csr_mie: begin
 807                     csr_output_value = 0;
 808                     csr_output_value[11] = mie_meie;
 809                     csr_output_value[9] = mie_seie;
 810                     csr_output_value[8] = mie_ueie;
 811                     csr_output_value[7] = mie_mtie;
 812                     csr_output_value[5] = mie_stie;
 813                     csr_output_value[4] = mie_utie;
 814                     csr_output_value[3] = mie_msie;
 815                     csr_output_value[1] = mie_ssie;
 816                     csr_output_value[0] = mie_usie;
 817                     csr_written_value = evaluate_csr_funct3_operation(decoder_funct3, csr_output_value, csr_input_value);
 818                     if(csr_writes) begin
 819                         mie_meie = csr_written_value[11];
 820                         mie_mtie = csr_written_value[7];
 821                         mie_msie = csr_written_value[3];
 822                     end
 823                 end
 824                 `csr_mtvec: begin
 825                     csr_output_value = mtvec;
 826                 end
 827                 `csr_mscratch: begin
 828                     csr_output_value = mscratch;
 829                     csr_written_value = evaluate_csr_funct3_operation(decoder_funct3, csr_output_value, csr_input_value);
 830                     if(csr_writes)
 831                         mscratch = csr_written_value;
 832                 end
 833                 `csr_mepc: begin
 834                     csr_output_value = mepc;
 835                     csr_written_value = evaluate_csr_funct3_operation(decoder_funct3, csr_output_value, csr_input_value);
 836                     if(csr_writes)
 837                         mepc = csr_written_value;
 838                 end
 839                 `csr_mcause: begin
 840                     csr_output_value = mcause;
 841                     csr_written_value = evaluate_csr_funct3_operation(decoder_funct3, csr_output_value, csr_input_value);
 842                     if(csr_writes)
 843                         mcause = csr_written_value;
 844                 end
 845                 `csr_mip: begin
 846                     csr_output_value = 0;
 847                     csr_output_value[11] = mip_meip;
 848                     csr_output_value[9] = mip_seip;
 849                     csr_output_value[8] = mip_ueip;
 850                     csr_output_value[7] = mip_mtip;
 851                     csr_output_value[5] = mip_stip;
 852                     csr_output_value[4] = mip_utip;
 853                     csr_output_value[3] = mip_msip;
 854                     csr_output_value[1] = mip_ssip;
 855                     csr_output_value[0] = mip_usip;
 856                 end
 857                 endcase
 858                 if(csr_reads)
 859                     write_register(decoder_rd, csr_output_value);
 860             end
 861             else if((decode_action & (`decode_action_fence | `decode_action_fence_i | `decode_action_store | `decode_action_branch)) != 0) begin
 862                 // do nothing
 863             end
 864         end
 865         endcase
 866     end
 867
 868 endmodule
 869 """
 870