GBZ80Core.v

   1 `define REG_A 0
   2 `define REG_B 1
   3 `define REG_C 2
   4 `define REG_D 3
   5 `define REG_E 4
   6 `define REG_F 5
   7 `define REG_H 6
   8 `define REG_L 7
   9 `define REG_SPH 8
  10 `define REG_SPL 9
  11 `define REG_PCH 10
  12 `define REG_PCL 11
  13
  14 `define FLAG_Z 8'b10000000
  15 `define FLAG_N 8'b01000000
  16 `define FLAG_H 8'b00100000
  17 `define FLAG_C 8'b00010000
  18
  19 `define STATE_FETCH                     2'h0
  20 `define STATE_DECODE                    2'h1
  21 `define STATE_EXECUTE           2'h2
  22 `define STATE_WRITEBACK         2'h3
  23
  24 `define INSN_LD_reg_imm8        8'b00xxx110
  25 `define INSN_HALT                               8'b01110110
  26 `define INSN_LD_HL_reg          8'b01110xxx
  27 `define INSN_LD_reg_HL          8'b01xxx110
  28 `define INSN_LD_reg_reg         8'b01xxxxxx
  29 `define INSN_LD_reg_imm16       8'b00xx0001
  30 `define INSN_LD_SP_HL           8'b11111001
  31 `define INSN_PUSH_reg           8'b11xx0101
  32 `define INSN_POP_reg                    8'b11xx0001
  33 `define INSN_LDH_AC                     8'b111x0010     // Either LDH A,(C) or LDH (C),A
  34 `define INSN_LDx_AHL                    8'b001xx010     // LDD/LDI A,(HL) / (HL),A
  35 `define INSN_ALU8                               8'b10xxxxxx     // 10 xxx yyy
  36 `define INSN_NOP                                8'b00000000
  37 `define INSN_RST                                8'b11xxx111
  38 `define INSN_RET                                8'b110x1001     // 1 = RETI, 0 = RET
  39 `define INSN_RETCC                      8'b110xx000
  40 `define INSN_CALL                               8'b11001101
  41 `define INSN_CALLCC                     8'b110xx100     // Not that call/cc.
  42 `define INSN_JP_imm                     8'b11000011
  43 `define INSN_JPCC_imm           8'b110xx010
  44 `define INSN_ALU_A              8'b00xxx111
  45 `define INSN_JP_HL                      8'b11101001
  46 `define INSN_JR_imm                     8'b00011000
  47 `define INSN_JRCC_imm           8'b001xx000
  48
  49 `define INSN_cc_NZ                      2'b00
  50 `define INSN_cc_Z                               2'b01
  51 `define INSN_cc_NC                      2'b10
  52 `define INSN_cc_C                               2'b11
  53
  54 `define INSN_reg_A              3'b111
  55 `define INSN_reg_B              3'b000
  56 `define INSN_reg_C              3'b001
  57 `define INSN_reg_D              3'b010
  58 `define INSN_reg_E              3'b011
  59 `define INSN_reg_H              3'b100
  60 `define INSN_reg_L              3'b101
  61 `define INSN_reg_dHL    3'b110
  62 `define INSN_reg16_BC   2'b00
  63 `define INSN_reg16_DE   2'b01
  64 `define INSN_reg16_HL   2'b10
  65 `define INSN_reg16_SP   2'b11
  66 `define INSN_stack_AF   2'b11
  67 `define INSN_stack_BC   2'b00
  68 `define INSN_stack_DE   2'b01
  69 `define INSN_stack_HL   2'b10
  70 `define INSN_alu_ADD            3'b000
  71 `define INSN_alu_ADC            3'b001
  72 `define INSN_alu_SUB            3'b010
  73 `define INSN_alu_SBC            3'b011
  74 `define INSN_alu_AND            3'b100
  75 `define INSN_alu_XOR            3'b101
  76 `define INSN_alu_OR             3'b110
  77 `define INSN_alu_CP             3'b111          // Oh lawd, is dat some CP?
  78 `define INSN_alu_RLCA           3'b000
  79 `define INSN_alu_RRCA           3'b001
  80 `define INSN_alu_RLA            3'b010
  81 `define INSN_alu_RRA            3'b011
  82 `define INSN_alu_DAA            3'b100
  83 `define INSN_alu_CPL            3'b101
  84 `define INSN_alu_SCF            3'b110
  85 `define INSN_alu_CCF            3'b111
  86
  87 module GBZ80Core(
  88         input clk,
  89         output reg [15:0] busaddress,   /* BUS_* is latched on STATE_FETCH. */
  90         inout [7:0] busdata,
  91         output reg buswr, output reg busrd);
  92
  93         reg [1:0] state = 0;                                    /* State within this bus cycle (see STATE_*). */
  94         reg [2:0] cycle = 0;                                    /* Cycle for instructions. */
  95
  96         reg [7:0] registers[11:0];
  97
  98         reg [15:0] address;                             /* Address for the next bus operation. */
  99
 100         reg [7:0] opcode;                               /* Opcode from the current machine cycle. */
 101
 102         reg [7:0] rdata, wdata;         /* Read data from this bus cycle, or write data for the next. */
 103         reg rd = 1, wr = 0, newcycle = 1;
 104
 105         reg [7:0] tmp, tmp2;                    /* Generic temporary regs. */
 106
 107         reg [7:0] buswdata;
 108         assign busdata = buswr ? buswdata : 8'bzzzzzzzz;
 109
 110         reg ie = 0;
 111
 112         initial begin
 113                 registers[ 0] <= 0;
 114                 registers[ 1] <= 0;
 115                 registers[ 2] <= 0;
 116                 registers[ 3] <= 0;
 117                 registers[ 4] <= 0;
 118                 registers[ 5] <= 0;
 119                 registers[ 6] <= 0;
 120                 registers[ 7] <= 0;
 121                 registers[ 8] <= 0;
 122                 registers[ 9] <= 0;
 123                 registers[10] <= 0;
 124                 registers[11] <= 0;
 125                 ie <= 0;
 126                 rd <= 1;
 127                 wr <= 0;
 128                 newcycle <= 1;
 129                 state <= 0;
 130                 cycle <= 0;
 131         end
 132
 133         always @(posedge clk)
 134                 case (state)
 135                 `STATE_FETCH: begin
 136                         if (newcycle) begin
 137                                 busaddress <= {registers[`REG_PCH], registers[`REG_PCL]};
 138                                 buswr <= 0;
 139                                 busrd <= 1;
 140                         end else begin
 141                                 busaddress <= address;
 142                                 buswr <= wr;
 143                                 busrd <= rd;
 144                                 if (wr)
 145                                         buswdata <= wdata;
 146                         end
 147                         state <= `STATE_DECODE;
 148                 end
 149                 `STATE_DECODE: begin
 150                         if (newcycle) begin
 151                                 opcode <= busdata;
 152                                 rdata <= busdata;
 153                                 newcycle <= 0;
 154                                 cycle <= 0;
 155                         end else begin
 156                                 if (rd) rdata <= busdata;
 157                                 cycle <= cycle + 1;
 158                         end
 159                         buswr <= 0;
 160                         busrd <= 0;
 161                         wr <= 0;
 162                         rd <= 0;
 163                         address <= 16'bxxxxxxxxxxxxxxxx;        // Make it obvious if something of type has happened.
 164                         wdata <= 8'bxxxxxxxx;
 165                         state <= `STATE_EXECUTE;
 166                 end
 167                 `STATE_EXECUTE: begin
 168 `define EXEC_INC_PC \
 169         {registers[`REG_PCH], registers[`REG_PCL]} <= {registers[`REG_PCH], registers[`REG_PCL]} + 1
 170 `define EXEC_NEXTADDR_PCINC \
 171         address <= {registers[`REG_PCH], registers[`REG_PCL]} + 1
 172 `define EXEC_NEWCYCLE \
 173         newcycle <= 1; rd <= 1; wr <= 0
 174                         casex (opcode)
 175                         `INSN_LD_reg_imm8: begin
 176                                 case (cycle)
 177                                 0:      begin
 178                                                 `EXEC_INC_PC;
 179                                                 `EXEC_NEXTADDR_PCINC;
 180                                                 rd <= 1;
 181                                         end
 182                                 1:      begin
 183                                                 `EXEC_INC_PC;
 184                                                 if (opcode[5:3] == `INSN_reg_dHL) begin
 185                                                         address <= {registers[`REG_H], registers[`REG_L]};
 186                                                         wdata <= rdata;
 187                                                         rd <= 0;
 188                                                         wr <= 1;
 189                                                 end else begin
 190                                                         `EXEC_NEWCYCLE;
 191                                                 end
 192                                         end
 193                                 2:      begin
 194                                                 `EXEC_NEWCYCLE;
 195                                         end
 196                                 endcase
 197                         end
 198                         `INSN_HALT: begin
 199                                 `EXEC_NEWCYCLE;
 200                                 /* XXX Interrupts needed for HALT. */
 201                         end
 202                         `INSN_LD_HL_reg: begin
 203                                 case (cycle)
 204                                 0:      begin
 205                                                 case (opcode[2:0])
 206                                                 `INSN_reg_A:    wdata <= registers[`REG_A];
 207                                                 `INSN_reg_B:    wdata <= registers[`REG_B];
 208                                                 `INSN_reg_C:    wdata <= registers[`REG_C];
 209                                                 `INSN_reg_D:    wdata <= registers[`REG_D];
 210                                                 `INSN_reg_E:    wdata <= registers[`REG_E];
 211                                                 `INSN_reg_H:    wdata <= registers[`REG_H];
 212                                                 `INSN_reg_L:    wdata <= registers[`REG_L];
 213                                                 endcase
 214                                                 address <= {registers[`REG_H], registers[`REG_L]};
 215                                                 wr <= 1; rd <= 0;
 216                                         end
 217                                 1:      begin
 218                                                 `EXEC_INC_PC;
 219                                                 `EXEC_NEWCYCLE;
 220                                         end
 221                                 endcase
 222                         end
 223                         `INSN_LD_reg_HL: begin
 224                                 case(cycle)
 225                                 0:      begin
 226                                                 address <= {registers[`REG_H], registers[`REG_L]};
 227                                                 rd <= 1;
 228                                         end
 229                                 1:      begin
 230                                                 tmp <= rdata;
 231                                                 `EXEC_INC_PC;
 232                                                 `EXEC_NEWCYCLE;
 233                                         end
 234                                 endcase
 235                         end
 236                         `INSN_LD_reg_reg: begin
 237                                 `EXEC_INC_PC;
 238                                 `EXEC_NEWCYCLE;
 239                                 case (opcode[2:0])
 240                                 `INSN_reg_A:    tmp <= registers[`REG_A];
 241                                 `INSN_reg_B:    tmp <= registers[`REG_B];
 242                                 `INSN_reg_C:    tmp <= registers[`REG_C];
 243                                 `INSN_reg_D:    tmp <= registers[`REG_D];
 244                                 `INSN_reg_E:    tmp <= registers[`REG_E];
 245                                 `INSN_reg_H:    tmp <= registers[`REG_H];
 246                                 `INSN_reg_L:    tmp <= registers[`REG_L];
 247                                 endcase
 248                         end
 249                         `INSN_LD_reg_imm16: begin
 250                                 `EXEC_INC_PC;
 251                                 case (cycle)
 252                                 0:      begin
 253                                                 `EXEC_NEXTADDR_PCINC;
 254                                                 rd <= 1;
 255                                         end
 256                                 1:      begin
 257                                                 `EXEC_NEXTADDR_PCINC;
 258                                                 rd <= 1;
 259                                         end
 260                                 2: begin `EXEC_NEWCYCLE; end
 261                                 endcase
 262                         end
 263                         `INSN_LD_SP_HL: begin
 264                                 case (cycle)
 265                                 0:      begin
 266                                                 tmp <= registers[`REG_H];
 267                                         end
 268                                 1:      begin
 269                                                 `EXEC_NEWCYCLE;
 270                                                 `EXEC_INC_PC;
 271                                                 tmp <= registers[`REG_L];
 272                                         end
 273                                 endcase
 274                         end
 275                         `INSN_PUSH_reg: begin   /* PUSH is 16 cycles! */
 276                                 case (cycle)
 277                                 0:      begin
 278                                                 wr <= 1;
 279                                                 address <= {registers[`REG_SPH],registers[`REG_SPL]}-1;
 280                                                 case (opcode[5:4])
 281                                                 `INSN_stack_AF: wdata <= registers[`REG_A];
 282                                                 `INSN_stack_BC: wdata <= registers[`REG_B];
 283                                                 `INSN_stack_DE: wdata <= registers[`REG_D];
 284                                                 `INSN_stack_HL: wdata <= registers[`REG_H];
 285                                                 endcase
 286                                         end
 287                                 1:      begin
 288                                                 wr <= 1;
 289                                                 address <= {registers[`REG_SPH],registers[`REG_SPL]}-1;
 290                                                 case (opcode[5:4])
 291                                                 `INSN_stack_AF: wdata <= registers[`REG_F];
 292                                                 `INSN_stack_BC: wdata <= registers[`REG_C];
 293                                                 `INSN_stack_DE: wdata <= registers[`REG_E];
 294                                                 `INSN_stack_HL: wdata <= registers[`REG_L];
 295                                                 endcase
 296                                         end
 297                                 2:      begin /* Twiddle thumbs. */ end
 298                                 3:      begin
 299                                                 `EXEC_NEWCYCLE;
 300                                                 `EXEC_INC_PC;
 301                                         end
 302                                 endcase
 303                         end
 304                         `INSN_POP_reg: begin    /* POP is 12 cycles! */
 305                                 case (cycle)
 306                                 0:      begin
 307                                                 rd <= 1;
 308                                                 address <= {registers[`REG_SPH],registers[`REG_SPL]};
 309                                         end
 310                                 1:      begin
 311                                                 rd <= 1;
 312                                                 address <= {registers[`REG_SPH],registers[`REG_SPL]};
 313                                         end
 314                                 2:      begin
 315                                                 `EXEC_NEWCYCLE;
 316                                                 `EXEC_INC_PC;
 317                                         end
 318                                 endcase
 319                         end
 320                         `INSN_LDH_AC: begin
 321                                 case (cycle)
 322                                 0:      begin
 323                                                 address <= {8'hFF,registers[`REG_C]};
 324                                                 if (opcode[4]) begin    // LD A,(C)
 325                                                         rd <= 1;
 326                                                 end else begin
 327                                                         wr <= 1;
 328                                                         wdata <= registers[`REG_A];
 329                                                 end
 330                                         end
 331                                 1:      begin
 332                                                 `EXEC_NEWCYCLE;
 333                                                 `EXEC_INC_PC;
 334                                         end
 335                                 endcase
 336                         end
 337                         `INSN_LDx_AHL: begin
 338                                 case (cycle)
 339                                 0:      begin
 340                                                 address <= {registers[`REG_H],registers[`REG_L]};
 341                                                 if (opcode[3]) begin    // LDx A, (HL)
 342                                                         rd <= 1;
 343                                                 end else begin
 344                                                         wr <= 1;
 345                                                         wdata <= registers[`REG_A];
 346                                                 end
 347                                         end
 348                                 1:      begin
 349                                                 `EXEC_NEWCYCLE;
 350                                                 `EXEC_INC_PC;
 351                                         end
 352                                 endcase
 353                         end
 354                         `INSN_ALU8: begin
 355                                 if ((opcode[2:0] == `INSN_reg_dHL) && (cycle == 0)) begin
 356                                         // fffffffff fuck your shit, read from (HL) :(
 357                                         rd <= 1;
 358                                         address <= {registers[`REG_H], registers[`REG_L]};
 359                                 end else begin
 360                                         `EXEC_NEWCYCLE;
 361                                         `EXEC_INC_PC;
 362                                         case (opcode[2:0])
 363                                         `INSN_reg_A:    tmp <= registers[`REG_A];
 364                                         `INSN_reg_B:    tmp <= registers[`REG_B];
 365                                         `INSN_reg_C:    tmp <= registers[`REG_C];
 366                                         `INSN_reg_D:    tmp <= registers[`REG_D];
 367                                         `INSN_reg_E:    tmp <= registers[`REG_E];
 368                                         `INSN_reg_H:    tmp <= registers[`REG_H];
 369                                         `INSN_reg_L:    tmp <= registers[`REG_L];
 370                                         `INSN_reg_dHL:  tmp <= rdata;
 371                                         endcase
 372                                 end
 373                         end
 374                         `INSN_ALU_A: begin
 375                                 `EXEC_NEWCYCLE;
 376                                 `EXEC_INC_PC;
 377                         end
 378                         `INSN_NOP: begin
 379                                 `EXEC_NEWCYCLE;
 380                                 `EXEC_INC_PC;
 381                         end
 382                         `INSN_RST: begin
 383                                 case (cycle)
 384                                 0:      begin
 385                                                 `EXEC_INC_PC;           // This goes FIRST in RST
 386                                         end
 387                                 1:      begin
 388                                                 wr <= 1;
 389                                                 address <= {registers[`REG_SPH],registers[`REG_SPL]}-1;
 390                                                 wdata <= registers[`REG_PCH];
 391                                         end
 392                                 2:      begin
 393                                                 wr <= 1;
 394                                                 address <= {registers[`REG_SPH],registers[`REG_SPL]}-2;
 395                                                 wdata <= registers[`REG_PCL];
 396                                         end
 397                                 3:      begin
 398                                                 `EXEC_NEWCYCLE;
 399                                                 {registers[`REG_PCH],registers[`REG_PCL]} <=
 400                                                         {10'b0,opcode[5:3],3'b0};
 401                                         end
 402                                 endcase
 403                         end
 404                         `INSN_RET: begin
 405                                 case (cycle)
 406                                 0:      begin
 407                                                 rd <= 1;
 408                                                 address <= {registers[`REG_SPH],registers[`REG_SPL]};
 409                                         end
 410                                 1:      begin   // SPECIAL CASE: cycle does NOT increase linearly with ret!
 411                                                 `EXEC_INC_PC;
 412                                                 if (opcode != `INSN_RETCC)
 413                                                         $stop;
 414                                                 case (opcode[4:3])      // cycle 1 is skipped if we are not retcc
 415                                                 `INSN_cc_NZ:    if (registers[`REG_F][7]) begin `EXEC_NEWCYCLE; end
 416                                                 `INSN_cc_Z:             if (~registers[`REG_F][7]) begin `EXEC_NEWCYCLE; end
 417                                                 `INSN_cc_NC:    if (registers[`REG_F][4]) begin `EXEC_NEWCYCLE; end
 418                                                 `INSN_cc_C:             if (~registers[`REG_F][4]) begin `EXEC_NEWCYCLE; end
 419                                                 endcase
 420                                                 rd <= 1;
 421                                                 address <= {registers[`REG_SPH],registers[`REG_SPL]};
 422                                         end
 423                                 2:      begin
 424                                                 rd <= 1;
 425                                                 address <= {registers[`REG_SPH],registers[`REG_SPL]} + 1;
 426                                         end
 427                                 3:      begin /* twiddle thumbs */ end
 428                                 4:      begin
 429                                                 `EXEC_NEWCYCLE;
 430                                                 // do NOT increment PC!
 431                                         end
 432                                 endcase
 433                         end
 434                         `INSN_CALL,`INSN_CALLCC: begin
 435                                 case (cycle)
 436                                 0:      begin
 437                                                 `EXEC_INC_PC;
 438                                                 `EXEC_NEXTADDR_PCINC;
 439                                                 rd <= 1;
 440                                         end
 441                                 1:      begin
 442                                                 `EXEC_INC_PC;
 443                                                 `EXEC_NEXTADDR_PCINC;
 444                                                 rd <= 1;
 445                                         end
 446                                 2:      begin
 447                                                 `EXEC_INC_PC;
 448                                                 if (!opcode[0]) // i.e., is callcc
 449                                                         /* We need to check the condition code to bail out. */
 450                                                         case (opcode[4:3])
 451                                                         `INSN_cc_NZ:    if (registers[`REG_F][7]) begin `EXEC_NEWCYCLE; end
 452                                                         `INSN_cc_Z:             if (~registers[`REG_F][7]) begin `EXEC_NEWCYCLE; end
 453                                                         `INSN_cc_NC:    if (registers[`REG_F][4]) begin `EXEC_NEWCYCLE; end
 454                                                         `INSN_cc_C:             if (~registers[`REG_F][4]) begin `EXEC_NEWCYCLE; end
 455                                                         endcase
 456                                         end
 457                                 3:      begin
 458                                                 address <= {registers[`REG_SPH],registers[`REG_SPL]} - 1;
 459                                                 wdata <= registers[`REG_PCH];
 460                                                 wr <= 1;
 461                                         end
 462                                 4:      begin
 463                                                 address <= {registers[`REG_SPH],registers[`REG_SPL]} - 2;
 464                                                 wdata <= registers[`REG_PCL];
 465                                                 wr <= 1;
 466                                         end
 467                                 5:      begin
 468                                                 `EXEC_NEWCYCLE; /* do NOT increment the PC */
 469                                         end
 470                                 endcase
 471                         end
 472                         `INSN_JP_imm,`INSN_JPCC_imm: begin
 473                                 case (cycle)
 474                                 0:      begin
 475                                                 `EXEC_INC_PC;
 476                                                 `EXEC_NEXTADDR_PCINC;
 477                                                 rd <= 1;
 478                                         end
 479                                 1:      begin
 480                                                 `EXEC_INC_PC;
 481                                                 `EXEC_NEXTADDR_PCINC;
 482                                                 rd <= 1;
 483                                         end
 484                                 2:      begin
 485                                                 `EXEC_INC_PC;
 486                                                 if (!opcode[0]) begin   // i.e., JP cc,nn
 487                                                         /* We need to check the condition code to bail out. */
 488                                                         case (opcode[4:3])
 489                                                         `INSN_cc_NZ:    if (registers[`REG_F][7]) begin `EXEC_NEWCYCLE; end
 490                                                         `INSN_cc_Z:             if (~registers[`REG_F][7]) begin `EXEC_NEWCYCLE; end
 491                                                         `INSN_cc_NC:    if (registers[`REG_F][4]) begin `EXEC_NEWCYCLE; end
 492                                                         `INSN_cc_C:             if (~registers[`REG_F][4]) begin `EXEC_NEWCYCLE; end
 493                                                         endcase
 494                                                 end
 495                                         end
 496                                 3:      begin
 497                                                 `EXEC_NEWCYCLE;
 498                                         end
 499                                 endcase
 500                         end
 501                         `INSN_JP_HL: begin
 502                                 `EXEC_NEWCYCLE;
 503                         end
 504                         `INSN_JR_imm,`INSN_JRCC_imm: begin
 505                                 case (cycle)
 506                                 0:      begin
 507                                                 `EXEC_INC_PC;
 508                                                 `EXEC_NEXTADDR_PCINC;
 509                                                 rd <= 1;
 510                                         end
 511                                 1: begin
 512                                                 `EXEC_INC_PC;
 513                                                 if (opcode[5]) begin    // i.e., JP cc,nn
 514                                                         /* We need to check the condition code to bail out. */
 515                                                         case (opcode[4:3])
 516                                                         `INSN_cc_NZ:    if (registers[`REG_F][7]) begin `EXEC_NEWCYCLE; end
 517                                                         `INSN_cc_Z:             if (~registers[`REG_F][7]) begin `EXEC_NEWCYCLE; end
 518                                                         `INSN_cc_NC:    if (registers[`REG_F][4]) begin `EXEC_NEWCYCLE; end
 519                                                         `INSN_cc_C:             if (~registers[`REG_F][4]) begin `EXEC_NEWCYCLE; end
 520                                                         endcase
 521                                                 end
 522                                         end
 523                                 2:      begin
 524                                                 `EXEC_NEWCYCLE;
 525                                         end
 526                                 endcase
 527                         end
 528                         default:
 529                                 $stop;
 530                         endcase
 531                         state <= `STATE_WRITEBACK;
 532                 end
 533                 `STATE_WRITEBACK: begin
 534                         casex (opcode)
 535                         `INSN_LD_reg_imm8:
 536                                 case (cycle)
 537                                 0:      begin end
 538                                 1:      case (opcode[5:3])
 539                                         `INSN_reg_A:    begin registers[`REG_A] <= rdata; end
 540                                         `INSN_reg_B:    begin registers[`REG_B] <= rdata; end
 541                                         `INSN_reg_C:    begin registers[`REG_C] <= rdata; end
 542                                         `INSN_reg_D:    begin registers[`REG_D] <= rdata; end
 543                                         `INSN_reg_E:    begin registers[`REG_E] <= rdata; end
 544                                         `INSN_reg_H:    begin registers[`REG_H] <= rdata; end
 545                                         `INSN_reg_L:    begin registers[`REG_L] <= rdata; end
 546                                         `INSN_reg_dHL:  begin /* Go off to cycle 2 */ end
 547                                         endcase
 548                                 2:      begin end
 549                                 endcase
 550                         `INSN_HALT: begin
 551                                 /* Nothing needs happen here. */
 552                                 /* XXX Interrupts needed for HALT. */
 553                         end
 554                         `INSN_LD_HL_reg: begin
 555                                 /* Nothing of interest here */
 556                         end
 557                         `INSN_LD_reg_HL: begin
 558                                 case (cycle)
 559                                 0:      begin end
 560                                 1:      begin
 561                                                 case (opcode[5:3])
 562                                                 `INSN_reg_A:    registers[`REG_A] <= tmp;
 563                                                 `INSN_reg_B:    registers[`REG_B] <= tmp;
 564                                                 `INSN_reg_C:    registers[`REG_C] <= tmp;
 565                                                 `INSN_reg_D:    registers[`REG_D] <= tmp;
 566                                                 `INSN_reg_E:    registers[`REG_E] <= tmp;
 567                                                 `INSN_reg_H:    registers[`REG_H] <= tmp;
 568                                                 `INSN_reg_L:    registers[`REG_L] <= tmp;
 569                                                 endcase
 570                                         end
 571                                 endcase
 572                         end
 573                         `INSN_LD_reg_reg: begin
 574                                 case (opcode[5:3])
 575                                 `INSN_reg_A:    registers[`REG_A] <= tmp;
 576                                 `INSN_reg_B:    registers[`REG_B] <= tmp;
 577                                 `INSN_reg_C:    registers[`REG_C] <= tmp;
 578                                 `INSN_reg_D:    registers[`REG_D] <= tmp;
 579                                 `INSN_reg_E:    registers[`REG_E] <= tmp;
 580                                 `INSN_reg_H:    registers[`REG_H] <= tmp;
 581                                 `INSN_reg_L:    registers[`REG_L] <= tmp;
 582                                 endcase
 583                         end
 584                         `INSN_LD_reg_imm16: begin
 585                                 case (cycle)
 586                                 0:      begin /* */ end
 587                                 1:      begin
 588                                                 case (opcode[5:4])
 589                                                 `INSN_reg16_BC: registers[`REG_C] <= rdata;
 590                                                 `INSN_reg16_DE: registers[`REG_E] <= rdata;
 591                                                 `INSN_reg16_HL: registers[`REG_L] <= rdata;
 592                                                 `INSN_reg16_SP: registers[`REG_SPL] <= rdata;
 593                                                 endcase
 594                                         end
 595                                 2: begin
 596                                                 case (opcode[5:4])
 597                                                 `INSN_reg16_BC: registers[`REG_B] <= rdata;
 598                                                 `INSN_reg16_DE: registers[`REG_D] <= rdata;
 599                                                 `INSN_reg16_HL: registers[`REG_H] <= rdata;
 600                                                 `INSN_reg16_SP: registers[`REG_SPH] <= rdata;
 601                                                 endcase
 602                                         end
 603                                 endcase
 604                         end
 605                         `INSN_LD_SP_HL: begin
 606                                 case (cycle)
 607                                 0:      registers[`REG_SPH] <= tmp;
 608                                 1: registers[`REG_SPL] <= tmp;
 609                                 endcase
 610                         end
 611                         `INSN_PUSH_reg: begin   /* PUSH is 16 cycles! */
 612                                 case (cycle)
 613                                 0:      {registers[`REG_SPH],registers[`REG_SPL]} <=
 614                                                 {registers[`REG_SPH],registers[`REG_SPL]} - 1;
 615                                 1:      {registers[`REG_SPH],registers[`REG_SPL]} <=
 616                                                 {registers[`REG_SPH],registers[`REG_SPL]} - 1;
 617                                 2:      begin /* type F */ end
 618                                 3:      begin /* type F */ end
 619                                 endcase
 620                         end
 621                         `INSN_POP_reg: begin    /* POP is 12 cycles! */
 622                                 case (cycle)
 623                                 0:      {registers[`REG_SPH],registers[`REG_SPL]} <=
 624                                                 {registers[`REG_SPH],registers[`REG_SPL]} + 1;
 625                                 1:      begin
 626                                                 case (opcode[5:4])
 627                                                 `INSN_stack_AF: registers[`REG_F] <= rdata;
 628                                                 `INSN_stack_BC: registers[`REG_C] <= rdata;
 629                                                 `INSN_stack_DE: registers[`REG_E] <= rdata;
 630                                                 `INSN_stack_HL: registers[`REG_L] <= rdata;
 631                                                 endcase
 632                                                 {registers[`REG_SPH],registers[`REG_SPL]} <=
 633                                                         {registers[`REG_SPH],registers[`REG_SPL]} + 1;
 634                                         end
 635                                 2:      begin
 636                                                 case (opcode[5:4])
 637                                                 `INSN_stack_AF: registers[`REG_A] <= rdata;
 638                                                 `INSN_stack_BC: registers[`REG_B] <= rdata;
 639                                                 `INSN_stack_DE: registers[`REG_D] <= rdata;
 640                                                 `INSN_stack_HL: registers[`REG_H] <= rdata;
 641                                                 endcase
 642                                         end
 643                                 endcase
 644                         end
 645                         `INSN_LDH_AC: begin
 646                                 case (cycle)
 647                                 0:      begin /* Type F */ end
 648                                 1:      if (opcode[4])
 649                                                 registers[`REG_A] <= rdata;
 650                                 endcase
 651                         end
 652                         `INSN_LDx_AHL: begin
 653                                 case (cycle)
 654                                 0:      begin /* Type F */ end
 655                                 1:      begin
 656                                                 if (opcode[3])
 657                                                         registers[`REG_A] <= rdata;
 658                                                 {registers[`REG_H],registers[`REG_L]} <=
 659                                                         opcode[4] ? // if set, LDD, else LDI
 660                                                         ({registers[`REG_H],registers[`REG_L]} - 1) :
 661                                                         ({registers[`REG_H],registers[`REG_L]} + 1);
 662                                         end
 663                                 endcase
 664                         end
 665                         `INSN_ALU8: begin
 666                                 if ((opcode[2:0] == `INSN_reg_dHL) && (cycle == 0)) begin
 667                                         /* Sit on our asses. */
 668                                 end else begin          /* Actually do the computation! */
 669                                         case (opcode[5:3])
 670                                         `INSN_alu_ADD: begin
 671                                                 registers[`REG_A] <=
 672                                                         registers[`REG_A] + tmp;
 673                                                 registers[`REG_F] <=
 674                                                         { /* Z */ ((registers[`REG_A] + tmp) == 0) ? 1'b1 : 1'b0,
 675                                                           /* N */ 1'b0,
 676                                                           /* H */ (({1'b0,registers[`REG_A][3:0]} + {1'b0,tmp[3:0]}) >> 4 == 1) ? 1'b1 : 1'b0,
 677                                                           /* C */ (({1'b0,registers[`REG_A]} + {1'b0,tmp}) >> 8 == 1) ? 1'b1 : 1'b0,
 678                                                           registers[`REG_F][3:0]
 679                                                         };
 680                                         end
 681                                         `INSN_alu_ADC: begin
 682                                                 registers[`REG_A] <=
 683                                                         registers[`REG_A] + tmp + {7'b0,registers[`REG_F][4]};
 684                                                 registers[`REG_F] <=
 685                                                         { /* Z */ ((registers[`REG_A] + tmp + {7'b0,registers[`REG_F][4]}) == 0) ? 1'b1 : 1'b0,
 686                                                           /* N */ 1'b0,
 687                                                           /* H */ (({1'b0,registers[`REG_A][3:0]} + {1'b0,tmp[3:0]} + {4'b0,registers[`REG_F][4]}) >> 4 == 1) ? 1'b1 : 1'b0,
 688                                                           /* C */ (({1'b0,registers[`REG_A]} + {1'b0,tmp} + {8'b0,registers[`REG_F][4]}) >> 8 == 1) ? 1'b1 : 1'b0,
 689                                                           registers[`REG_F][3:0]
 690                                                         };
 691                                         end
 692                                         `INSN_alu_SUB: begin
 693                                                 registers[`REG_A] <=
 694                                                         registers[`REG_A] - tmp;
 695                                                 registers[`REG_F] <=
 696                                                         { /* Z */ ((registers[`REG_A] - tmp) == 0) ? 1'b1 : 1'b0,
 697                                                           /* N */ 1'b1,
 698                                                           /* H */ (({1'b0,registers[`REG_A][3:0]} - {1'b0,tmp[3:0]}) >> 4 == 1) ? 1'b1 : 1'b0,
 699                                                           /* C */ (({1'b0,registers[`REG_A]} - {1'b0,tmp}) >> 8 == 1) ? 1'b1 : 1'b0,
 700                                                           registers[`REG_F][3:0]
 701                                                         };
 702                                         end
 703                                         `INSN_alu_SBC: begin
 704                                                 registers[`REG_A] <=
 705                                                         registers[`REG_A] - (tmp + {7'b0,registers[`REG_F][4]});
 706                                                 registers[`REG_F] <=
 707                                                         { /* Z */ ((registers[`REG_A] - (tmp + {7'b0,registers[`REG_F][4]})) == 0) ? 1'b1 : 1'b0,
 708                                                           /* N */ 1'b1,
 709                                                           /* H */ (({1'b0,registers[`REG_A][3:0]} - ({1'b0,tmp[3:0]} + {4'b0,registers[`REG_F][4]})) >> 4 == 1) ? 1'b1 : 1'b0,
 710                                                           /* C */ (({1'b0,registers[`REG_A]} - ({1'b0,tmp} + {8'b0,registers[`REG_F][4]})) >> 8 == 1) ? 1'b1 : 1'b0,
 711                                                           registers[`REG_F][3:0]
 712                                                         };
 713                                         end
 714                                         `INSN_alu_AND: begin
 715                                                 registers[`REG_A] <=
 716                                                         registers[`REG_A] & tmp;
 717                                                 registers[`REG_F] <=
 718                                                         { /* Z */ ((registers[`REG_A] & tmp) == 0) ? 1'b1 : 1'b0,
 719                                                           3'b010,
 720                                                           registers[`REG_F][3:0]
 721                                                         };
 722                                         end
 723                                         `INSN_alu_OR: begin
 724                                                 registers[`REG_A] <=
 725                                                         registers[`REG_A] | tmp;
 726                                                 registers[`REG_F] <=
 727                                                         { /* Z */ ((registers[`REG_A] | tmp) == 0) ? 1'b1 : 1'b0,
 728                                                           3'b000,
 729                                                           registers[`REG_F][3:0]
 730                                                         };
 731                                         end
 732                                         `INSN_alu_XOR: begin
 733                                                 registers[`REG_A] <=
 734                                                         registers[`REG_A] ^ tmp;
 735                                                 registers[`REG_F] <=
 736                                                         { /* Z */ ((registers[`REG_A] ^ tmp) == 0) ? 1'b1 : 1'b0,
 737                                                           3'b000,
 738                                                           registers[`REG_F][3:0]
 739                                                         };
 740                                         end
 741                                         `INSN_alu_CP: begin
 742                                                 registers[`REG_F] <=
 743                                                         { /* Z */ ((registers[`REG_A] - tmp) == 0) ? 1'b1 : 1'b0,
 744                                                           /* N */ 1'b1,
 745                                                           /* H */ (({1'b0,registers[`REG_A][3:0]} - {1'b0,tmp[3:0]}) >> 4 == 1) ? 1'b1 : 1'b0,
 746                                                           /* C */ (({1'b0,registers[`REG_A]} - {1'b0,tmp}) >> 8 == 1) ? 1'b1 : 1'b0,
 747                                                           registers[`REG_F][3:0]
 748                                                         };
 749                                         end
 750                                         default:
 751                                                 $stop;
 752                                         endcase
 753                                 end
 754                         end
 755                         `INSN_ALU_A: begin
 756                                 case(opcode[5:3])
 757                                 `INSN_alu_RLCA: begin
 758                                         registers[`REG_A] <= {registers[`REG_A][6:0],registers[`REG_A][7]};
 759                                         registers[`REG_F] <= {registers[`REG_F][7:5],registers[`REG_A][7],registers[`REG_F][3:0]};
 760                                 end
 761                                 `INSN_alu_RRCA: begin
 762                                         registers[`REG_A] <= {registers[`REG_A][0],registers[`REG_A][7:1]};
 763                                         registers[`REG_F] <= {registers[`REG_F][7:5],registers[`REG_A][0],registers[`REG_F][3:0]};
 764                                 end
 765                                 `INSN_alu_RLA: begin
 766                                         registers[`REG_A] <= {registers[`REG_A][6:0],registers[`REG_F][4]};
 767                                         registers[`REG_F] <= {registers[`REG_F][7:5],registers[`REG_A][7],registers[`REG_F][3:0]};
 768                                 end
 769                                 `INSN_alu_RRA: begin
 770                                         registers[`REG_A] <= {registers[`REG_A][4],registers[`REG_A][7:1]};
 771                                         registers[`REG_F] <= {registers[`REG_F][7:5],registers[`REG_A][0],registers[`REG_F][3:0]};
 772                                 end
 773                                 `INSN_alu_CPL: begin
 774                                         registers[`REG_A] <= ~registers[`REG_A];
 775                                         registers[`REG_F] <= {registers[`REG_F][7],1'b1,1'b1,registers[`REG_F][4:0]};
 776                                 end
 777                                 `INSN_alu_SCF: begin
 778                                         registers[`REG_F] <= {registers[`REG_F][7:5],1,registers[`REG_F][3:0]};
 779                                 end
 780                                 `INSN_alu_CCF: begin
 781                                         registers[`REG_F] <= {registers[`REG_F][7:5],~registers[`REG_F][4],registers[`REG_F][3:0]};
 782                                 end
 783                                 endcase
 784                         end
 785                         `INSN_NOP: begin /* NOP! */ end
 786                         `INSN_RST: begin
 787                                 case (cycle)
 788                                 0:      begin /* type F */ end
 789                                 1:      begin /* type F */ end
 790                                 2:      begin /* type F */ end
 791                                 3:      {registers[`REG_SPH],registers[`REG_SPL]} <=
 792                                                 {registers[`REG_SPH],registers[`REG_SPL]}-2;
 793                                 endcase
 794                         end
 795                         `INSN_RET,`INSN_RETCC: begin
 796                                 case (cycle)
 797                                 0:      if (opcode != `INSN_RETCC)
 798                                                 cycle <= 1;     // Skip cycle 1; it gets incremented on the next round.
 799                                 1: begin /* Nothing need happen here. */ end
 800                                 2:      registers[`REG_PCL] <= rdata;
 801                                 3:      registers[`REG_PCH] <= rdata;
 802                                 4:      begin
 803                                                 {registers[`REG_SPH],registers[`REG_SPL]} <=
 804                                                         {registers[`REG_SPH],registers[`REG_SPL]} + 2;
 805                                                 if (opcode[4] && (opcode != `INSN_RETCC))       /* RETI */
 806                                                         ie <= 1;
 807                                         end
 808                                 endcase
 809                         end
 810                         `INSN_CALL,`INSN_CALLCC: begin
 811                                 case (cycle)
 812                                 0:      begin /* type F */ end
 813                                 1:      tmp <= rdata;   // tmp contains newpcl
 814                                 2:      tmp2 <= rdata;  // tmp2 contains newpch
 815                                 3:      begin /* type F */ end
 816                                 4:      registers[`REG_PCH] <= tmp2;
 817                                 5: begin
 818                                                 {registers[`REG_SPH],registers[`REG_SPL]} <=
 819                                                         {registers[`REG_SPH],registers[`REG_SPL]} - 2;
 820                                                 registers[`REG_PCL] <= tmp;
 821                                         end
 822                                 endcase
 823                         end
 824                         `INSN_JP_imm,`INSN_JPCC_imm: begin
 825                                 case (cycle)
 826                                 0:      begin /* type F */ end
 827                                 1:      tmp <= rdata;   // tmp contains newpcl
 828                                 2:      tmp2 <= rdata;  // tmp2 contains newpch
 829                                 3:      {registers[`REG_PCH],registers[`REG_PCL]} <=
 830                                                 {tmp2,tmp};
 831                                 endcase
 832                         end
 833                         `INSN_JP_HL: begin
 834                                 {registers[`REG_PCH],registers[`REG_PCL]} <=
 835                                         {registers[`REG_H],registers[`REG_L]};
 836                         end
 837                         `INSN_JR_imm,`INSN_JRCC_imm: begin
 838                                 case (cycle)
 839                                 0:      begin /* type F */ end
 840                                 1:      tmp <= rdata;
 841                                 2: {registers[`REG_PCH],registers[`REG_PCL]} <=
 842                                                 {registers[`REG_PCH],registers[`REG_PCL]} +
 843                                                 {tmp[7]?8'hFF:8'h00,tmp};
 844                                 endcase
 845                         end
 846                         default:
 847                                 $stop;
 848                         endcase
 849                         state <= `STATE_FETCH;
 850                 end
 851                 endcase
 852 endmodule