all repos — nand2tetris @ 8c016de00acf4ccf622c4e2b1af6d218011fed0c

my nand2tetris progress

projects/06/src/n2tasm1.c

 1
 2
 3
 4
 5
 6
 7
 8
 9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
 100
 101
 102
 103
 104
 105
 106
 107
 108
 109
 110
 111
 112
 113
 114
 115
 116
 117
 118
 119
 120
 121
 122
 123
 124
 125
 126
 127
 128
 129
 130
 131
 132
 133
 134
 135
 136
 137
 138
 139
 140
 141
 142
 143
 144
 145
 146
 147
 148
 149
 150
 151
 152
 153
 154
 155
 156
 157
 158
 159
 160
 161
 162
 163
 164
 165
 166
 167
 168
 169
 170
 171
 172
 173
 174
 175
 176
 177
 178
 179
 180
 181
 182
 183
 184
 185
 186
 187
 188
 189
 190
 191
 192
 193
 194
 195
 196
 197
 198
 199
 200
 201
 202
 203
 204
 205
 206
 207
 208
 209
 210
 211
 212
 213
 214
 215
 216
 217
 218
 219
 220
 221
 222
 223
 224
 225
 226
 227
 228
 229
 230
 231
 232
 233
 234
 235
 236
 237
 238
 239
 240
 241
 242
 243
 244
 245
 246
 247
 248
 249
 250
 251
 252
 253
 254
 255
 256
 257
 258
 259
 260
 261
 262
 263
 264
 265
 266
 267
 268
 269
 270
 271
 272
 273
 274
 275
 276
 277
 278
 279
 280
 281
 282
 283
 284
 285
 286
 287
 288
 289
 290
 291
 292
 293
 294
 295
 296
 297
 298
 299
 300
 301
 302
 303
 304
 305
 306
 307
 308
 309
 310
 311
 312
 313
 314
 315
 316
 317
 318
 319
 320
 321
 322
 323
 324
 325
 326
 327
 328
 329
 330
 331
 332
 333
 334
 335
 336
 337
 338
 339
 340
 341
 342
 343
 344
 345
 346
 347
 348
 349
 350
 351
 352
 353
 354
 355
 356
 357
 358
 359
 360
 361
 362
 363
 364
 365
 366
 367
 368
 369
 370
 371
 372
 373
 374
 375
 376
 377
 378
 379
 380
 381
 382
 383
 384
 385
 386
 387
 388
 389
 390
 391
 392
 393
 394
 395
 396
 397
 398
 399
 400
 401
 402
 403
 404
 405
 406
 407
 408
 409
 410
 411
 412
 413
 414
 415
 416
 417
 418
 419
 420
 421
 422
 423
 424
 425
 426
 427
 428
 429
 430
 431
 432
 433
 434
 435
 436
 437
 438
 439
 440
 441
 442
 443
 444
 445
 446
 447
 448
 449
 450
 451
 452
 453
 454
 455
 456
 457
 458
 459
 460
 461
 462
 463
 464
 465
 466
 467
 468
 469
 470
 471
 472
 473
 474
 475
 476
 477
 478
 479
 480
 481
 482
 483
 484
 485
 486
 487
 488
 489
 490
 491
 492
 493
 494
 495
 496
 497
 498
 499
 500
 501
 502
 503
 504
 505
 506
 507
 508
 509
 510
 511
 512
 513
 514
 515
 516
 517
 518
 519
 520
 521
 522
 523
 524
 525
 526
 527
 528
 529
 530
 531
 532
 533
 534
 535
 536
 537
 538
 539
 540
 541
 542
 543
 544
 545
 546
 547
 548
 549
 550
 551
 552
 553
 554
 555
 556
 557
 558
 559
 560
 561
 562
 563
 564
 565
 566
 567
 568
 569
 570
 571
 572
 573
 574
 575
 576
 577
 578
 579
 580
 581
 582
 583
 584
 585
 586
 587
 588
 589
 590
 591
 592
 593
 594
 595
 596
 597
 598
 599
 600
 601
 602
 603
 604
 605
 606
 607
 608
 609
 610
 611
 612
 613
 614
 615
 616
 617
 618
 619
 620
 621
 622
 623
 624
 625
 626
 627
 628
 629
 630
 631
 632
 633
 634
 635
 636
 637
 638
 639
 640
 641
 642
 643
 644
 645
 646
 647
 648
 649
 650
 651
 652
 653
 654
 655
 656
 657
 658
 659
 660
 661
 662
 663
 664
 665
 666
 667
 668
 669
 670
 671
 672
 673
 674
 675
 676
 677
 678
 679
 680
 681
 682
 683
 684
 685
 686
 687
 688
 689
 690
 691
 692
 693
 694
 695
 696
 697
 698
 699
 700
 701
 702
 703
 704
 705
 706
 707
 708
 709
 710
 711
 712
 713
 714
 715
 716
 717
 718
 719
 720
 721
 722
 723
 724
 725
 726
 727
 728
 729
 730
 731
 732
 733
 734
 735
 736
 737
 738
 739
 740
 741
 742
 743
 744
 745
 746
 747
 748
 749
 750
 751
 752
 753
 754
 755
 756
 757
 758
 759
 760
 761
 762
 763
 764
 765
 766
 767
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#ifdef _DEBUG
#define DBGLOG                  printf
#else
#define DBGLOG(...)
#endif

#define err(...)              fprintf(stderr, __VA_ARGS__); \
                                fprintf(stderr, "%lu | %s\n", \
                                        g_asm_line_number, g_asm_line); \
				g_error_encountered = true;

#define valid_symbol_char(c) (('0' <= c && c <= '9') || ('A' <= c && c <= 'Z') \
                           || ('a' <= c && c <= 'z') || c == '_' || c == '.'   \
		                                     || c == '$' || c == ':')

#define MAX_LINE_LEN            256

#define MAX_SYMBOL_STR_LEN      MAX_LINE_LEN - 2
#define RESERVED_LABEL_NUM      23
#define MAX_SYMBOLS             32768

bool g_error_encountered = false;

char *g_asm_line;                 // currently-read line for convenience
size_t g_asm_line_number;         // current line number
size_t g_instruction_number = 0;  // instruction offset

struct symbol_t {
	char *symbolstr;
	uint16_t value;
};

char *g_reserved_symbol_strs[RESERVED_LABEL_NUM] = {"R0", "R1", "R2", "R3",
                                                    "R4", "R5", "R6", "R7",
                                                    "R8", "R9", "R10", "R11",
                                                    "R12", "R13", "R14", "R15",
                                                    "SP", "LCL", "ARG", "THIS",
                                                    "THAT", "SCREEN", "KBD"};
uint16_t g_reserved_symbol_values[RESERVED_LABEL_NUM] = {0, 1, 2, 3,
                                                         4, 5, 6, 7,
                                                         8, 9, 10, 11,
                                                         12, 13, 14, 15,
                                                         0, 1, 2, 3,
                                                         4, 0x4000, 0x6000};

// label and variable symbols will effectively be treated the same; the only
// difference is in their use (in a hack program)
struct symbol_t *g_symbol_list[MAX_SYMBOLS + RESERVED_LABEL_NUM];
size_t g_symbol_list_len = 0;
size_t g_current_variable_address = 16; // address for next variable symbol


void write_binary_word16(FILE *fp, uint16_t w)
{
        unsigned char i, msb;
        char binary_string[17];
        binary_string[16] = '\0';

        for (i = 0; i < 16; ++i) {
                msb = (char)(w >> 15);

                if (msb != 0)
                        binary_string[i] = '1';
                else
                        binary_string[i] = '0';
                w <<= 1;
        }
        fprintf(fp, "%s\n", binary_string);
}

static uint32_t myatoi(const char *str)
{
	size_t i;
	uint32_t ret = 0;

	for (i = 0; i < 5 && '0' <= str[i] && str[i] <= '9'; ++i) {
		ret = (ret * 10) + (str[i] - 0x30);
	}

	return ret;
}

void debug_dump_all_symbols()
{
	size_t i;
	struct symbol_t *symbol;

	DBGLOG("-------- DEBUG SYMBOL DUMP --------\n");
	for (i = 0; i < g_symbol_list_len; ++i) {
		symbol = g_symbol_list[i];
		DBGLOG("symbol (%s, %hu)\n", symbol->symbolstr, symbol->value);
		(void)symbol; // "use" to shut up compiler warning
	}
	DBGLOG("-------- END SYMBOL DUMP --------\n");

	g_symbol_list_len = 0;
}

void free_all_symbols()
{
	size_t i;
	struct symbol_t *symbol;

	for (i = 0; i < g_symbol_list_len; ++i) {
		symbol = g_symbol_list[i];
		free(symbol->symbolstr);    // not checking NULL
		free(symbol);
	}

	g_symbol_list_len = 0;
}

struct symbol_t *create_symbol(const char *str, uint16_t value)
{
	size_t len, i;
	struct symbol_t *symbol;
	char *symbolstr;

	//DBGLOG("creating symbol {'%s', %hu} ...\n", str, value);

	len = strlen(str);
	if (len == 0) {
		err("error creating empty label\n");
		return NULL;
	}

	symbol = malloc(sizeof(struct symbol_t));
	symbolstr = malloc(len + 1);
	if (symbol == NULL || symbolstr == NULL) {
		err("error creating label: malloc() returned NULL\n");
		exit(-1);
	}

	for (i = 0; i < len; ++i) {
		symbolstr[i] = str[i];
	}
	symbolstr[len] = '\0';

	symbol->symbolstr = symbolstr;
	symbol->value = value;

	return symbol;
}

struct symbol_t *lookup_symbol(char *str)
{
	size_t i;
	int diff;
	struct symbol_t *symbol;

	for (i = 0; i < g_symbol_list_len; ++i) {
		symbol = g_symbol_list[i];
		diff = strncmp(str, symbol->symbolstr, MAX_SYMBOL_STR_LEN);
		if (!diff) {
			return symbol;
		}
	}

	return NULL;  // symbol not found
}

bool add_symbol(struct symbol_t *symbol)
{
	struct symbol_t *symbol_already_present;

	if (g_symbol_list_len > (MAX_SYMBOLS + RESERVED_LABEL_NUM)) {
		err("internal: symbol list full, over %d symbols\n",
		       (MAX_SYMBOLS + RESERVED_LABEL_NUM));
		return false;
	}

	symbol_already_present = lookup_symbol(symbol->symbolstr);
	if (symbol_already_present != NULL) { // if present
		err("error: failed to add symbol %s: already found in list\n",
		      symbol->symbolstr);
		return false;
	}

	g_symbol_list[g_symbol_list_len] = symbol;
	++g_symbol_list_len;
	return true;
}

// pre-fill list with 'reserved' symbols and values
bool init_symbol_list()
{
	size_t i;
	struct symbol_t *s;

	for (i = 0; i < RESERVED_LABEL_NUM; ++i) {
		if((s = create_symbol(g_reserved_symbol_strs[i],
		                      g_reserved_symbol_values[i])) == NULL) {
			return false;  // failed to create symbol
		}
		if (!add_symbol(s)) {
			return false;  // failed to add symbol
		}
	}

	return true;
}

// assumes line starts at symbol (_not_ at '(' character, if relevant)
bool parse_symbol_str(const char *line, char *symbol_str, size_t *symbol_len)
{
	char c;
	size_t i;

	//DBGLOG("Parsing symbol string: line %lu | %s\n", g_asm_line_number, line);

	*symbol_len = 0;
	for (i = 0; (c = line[i]) != '\0'; ++i) {
		if (i == 0 && ('0' <= c && c <= '9')) {
			err("syntax error: label starts with a number\n");
			return false;
		}
		// parse characters until closing ')'
		if (i > MAX_SYMBOL_STR_LEN) {
			err("syntax error: label is too long (> %d chars)\n",
			       MAX_SYMBOL_STR_LEN);
			return false;
		} else if (valid_symbol_char(c)) {
			symbol_str[i] = c;
		} else {
			if (i == 1) {
				err("syntax error: empty label '()'\n");
				return false;
			}

			break;
		}
	}

	symbol_str[i] = '\0';
	*symbol_len = i;

	return true;
}

// assumes line[0] == '('
bool parse_label(const char *line)
{
	bool ret;
	size_t label_len;
	uint16_t value;
	char label_str[MAX_SYMBOL_STR_LEN + 1];
	struct symbol_t *symbol;

	//DBGLOG("Parsing label: line %lu | %s\n", g_asm_line_number, line);

	if (line[0] != '(') {  // just to be safe
		return false;  // not a label; label's look like (THIS)
	}

	ret = parse_symbol_str(&line[1], label_str, &label_len);
	if (!ret) {
		return false;
	}

	if (line[label_len + 1] != ')') {
		err("syntax error: no matching ')' found for label, found "
		      "value '%c' in label\n", line[label_len + 1]);
		return false;
	}

	// create label, add to symbol list
	value = (uint16_t)g_instruction_number; // safe. won't
						// be > 65535
	symbol = create_symbol(label_str, value);
	if (symbol == NULL)
		return false;
	return add_symbol(symbol);
}

bool parse_line_for_label(const char *line)
{
	char c;
	size_t i;

	for (i = 0; (c = line[i]) != '\0'; ++i) {
		if (c == ' ' || c == '\t') {
			continue;
		} else if (i == 0 && c == '\0') {
			return false;
		} else if (c == '/') { // likely comment
			return false;
		} else if (c == '(') {
			return parse_label(&line[i]);
		} else if (('!' <= c && c <= '\'') || ('*' <= c && c <= '~')) {
			++g_instruction_number;  // likely instruction found
			return false;
		} else {
			err("syntax error: invalid character '%c' found in "
			      "line\n", c);
		}
	}

	return false;
}

static bool parse_a_type(const char *line, uint16_t *instruction)
{
	bool ret;
	char c, a_field_str[6], symbol_str[MAX_SYMBOL_STR_LEN + 1];
	uint32_t a_field = 0;
	size_t i, a = 0, symbol_len;
	struct symbol_t *symbol;

	//DBGLOG("line: %s\n", line);

	if (line[0] != '@') {
		err("syntax error: A-type instruction doesn't start with @\n");
		return false;
	}

	c = line[1];

	if (c == '\0') {
		err("syntax error: A-type instruction empty after @\n");
		return false;
	}

	if (valid_symbol_char(c) && !('0' <= c && c <= '9')) {
		ret = parse_symbol_str(&line[1], symbol_str, &symbol_len);
		if (!ret)
			return ret;

		symbol = lookup_symbol(symbol_str);
		if (symbol == NULL) {  // symbol doesn't exist in table, is new
			a_field = g_current_variable_address;
			symbol = create_symbol(symbol_str, a_field);
			if (symbol == NULL)
				return false; // create failed
			ret = add_symbol(symbol);
			if (!ret)
				return ret;
			++g_current_variable_address;
		} else {               // symbol exists in table, use its value
			a_field = symbol->value;  // symbol value from table
		}
	} else {
		for (i = 1; (c = line[i]) != '\0' && a < 6; ++i) {
			if ('0' <= c && c <= '9') {
				if (a > 4) {
					err("error: @<number> too long\n");
					return false;
				}
				a_field_str[a] = c; // get number
				a++;
			} else if ((c == ' ' || c == '\t' || c == '/') && i > 1) {
				break;
			} else {       // any other character encountered
				err("syntax error: invalid char '%c' found "
				      "after @\n", c);
				return false;
			}
		}

		if (a == 0) {
			err("syntax error: A-type instruction empty after "
			      "'@'\n");
			return false;
		}

		a_field_str[a] = '\0';

		a_field = myatoi(a_field_str);
	}

	if (a_field > 32767) {
		err("error: %u > 32767, too large\n", a_field);
		return false;
	}

	*instruction = 0x0000 | (uint16_t) a_field;
	return true;
}

/* returns dest bits 0b00000ddd
 * Note: order doesn't matter for multi-register dest (but officially it should)
 */
static bool parse_c_type_dest(const char *dest_line, uint8_t *dest)
{
	size_t len;

	//DBGLOG("dest_line: %s\n", dest_line);

	for (len = 0; dest_line[len] != '='; ++len) {  // read until '='
		if (dest_line[len] == 'A') {
			if (*dest & 0x04) { // if A register already set
				err("syntax error: A register set twice\n");
				return false;
			}
			*dest |= 0x04;
		} else if (dest_line[len] == 'D') {
			if (*dest & 0x02) { // if D register already set
				err("syntax error: D register set twice\n");
				return false;
			}
			*dest |= 0x02;
		} else if (dest_line[len] == 'M') {
			if (*dest & 0x01) { // if M register already set
				err("syntax error: M register set twice\n");
				return false;
			}
			*dest |= 0x01;
		} else {
			err("syntax error: invalid destination register %c\n",
			      dest_line[len]);
			return false;
		}

		if (len >= 3) {
			err("syntax error: dest field incorrect length %lu\n",
			      len);
			return false;
		}
	}
	if (len == 0) {
		err("syntax error: dest field empty\n");
		return false;
	}

	return true;
}

/* returns comp bits 0b0acccccc
 */
static bool parse_c_type_comp(const char *comp_line, uint8_t *comp)
{
	size_t len;

	//DBGLOG("comp_line: %s\n", comp_line);

	for (len = 0; comp_line[len] == '0' || comp_line[len] == '1' ||
	              comp_line[len] == '-' || comp_line[len] == 'D' ||
	              comp_line[len] == 'A' || comp_line[len] == 'M' ||
	              comp_line[len] == '!' || comp_line[len] == '+' ||
	              comp_line[len] == '&' || comp_line[len] == '|'; ++len) {}
	if (len == 1) {  // 0 1 D A M
		switch (comp_line[0]) {
		case '0': *comp = 0x2a; break; // 0 101010
		case '1': *comp = 0x3f; break; // 0 111111
		case 'D': *comp = 0x0c; break; // 0 001100
		case 'A': *comp = 0x30; break; // 0 110000
		case 'M': *comp = 0x70; break; // 1 110000
		default:
			err("syntax error: comp field incorrect value\n");
			return false;
		}
	} else if (len == 2) {  // -1 !D !A !M -D -A -M 
		if (comp_line[0] == '-') {
			switch (comp_line[1]) {
			case '1': *comp = 0x3a; break; // 0 111010
			case 'D': *comp = 0x0f; break; // 0 001111
			case 'A': *comp = 0x33; break; // 0 110011
			case 'M': *comp = 0x73; break; // 1 110011
			default:
				err("syntax error: comp field incorrect value\n");
				return false;
			}
		} else if (comp_line[0] == '!') {
			switch (comp_line[1]) {
			case 'D': *comp = 0x0d; break; // 0 001101
			case 'A': *comp = 0x31; break; // 0 110001
			case 'M': *comp = 0x71; break; // 1 110001
			default:
				err("syntax error: comp field incorrect value\n");
				return false;
			}
		} else {
			err("syntax error: comp field incorrect value\n");
			return false;
		}
	} else if (len == 3) {
		if (comp_line[0] == 'D') {
			if (comp_line[2] == '1') {  // D+1 D-1
				if (comp_line[1] == '+') {
					*comp = 0x1f; // 0 011111
				} else if (comp_line[1] == '-') {
					*comp = 0x0e; // 0 001110
				} else {
					err("syntax error: comp field "
					      "incorrect value\n");
					return false;
				}
			} else {             // D+A D+M D-A D-M D&A D&M D|A D|M
				if (comp_line[2] == 'M') {
					*comp = 0x40;              // _1_ 000000
				} else if (comp_line[2] == 'A') {
					*comp = 0x00;              // _0_ 000000
				} else {
					err("syntax error: comp field "
					      "incorrect value\n");
					return false;
				}

				switch (comp_line[1]) {
				case '+': *comp |= 0x02; break; // 000010
				case '-': *comp |= 0x13; break; // 010011
				case '&': *comp |= 0x00; break; // 000000
				case '|': *comp |= 0x15; break; // 010101
				default:
					err("syntax error: comp field "
					      "incorrect value\n");
					return false;
				}
			}
		} else {
			// A+1 M+1 A-1 M-1 A-D M-D
			if (comp_line[0] == 'M') {
				*comp = 0x40;              // _1_ 000000
			} else if (comp_line[0] == 'A') {
				*comp = 0x00;              // _0_ 000000
			} else {
				err("syntax error: comp field incorrect value\n");
				return false;
			}

			if (comp_line[1] == '+' && comp_line[2] == '1') {
				*comp |= 0x37;  // 1 110111
			} else if (comp_line[1] == '-' && comp_line[2] == '1') {
				*comp |= 0x32;  // 1 110010
			} else if (comp_line[1] == '-' && comp_line[2] == 'D') {
				*comp |= 0x07;  // 1 000111
			} else {
				err("syntax error: comp field incorrect value\n");
				return false;
			}
		}
	} else {
		err("syntax error: comp field incorrect length %lu\n", len);
		return false;
	}

	return true;
}

/* returns jump bits 0b00000jjj
 */
static bool parse_c_type_jump(const char *jump_line, uint8_t *jump)
{
	size_t len;
	char *err_3rd_char = "syntax error: 3rd letter in jump field incorrect\n";

	//DBGLOG("jump_line: %s\n", jump_line);

	for (len = 0; jump_line[len] == 'J' || jump_line[len] == 'G' ||
	              jump_line[len] == 'T' || jump_line[len] == 'E' ||
	              jump_line[len] == 'Q' || jump_line[len] == 'L' ||
	              jump_line[len] == 'N' || jump_line[len] == 'M' ||
	              jump_line[len] == 'P'; ++len) {}
	if (len != 3) {
		err("syntax error: jump field incorrect length %lu\n", len);
		return false;
	}

	if (jump_line[0] == 'J') {                         // if "J__"
		switch (jump_line[1]) {
		case 'G':                                  // if "JG_"
			if (jump_line[2] == 'T') {         // if "JGT"
				*jump = 0x1;
			} else if (jump_line[2] == 'E') {  // if "JGE"
				*jump = 0x3;
			} else {
				err(err_3rd_char); return false;
			}
			break;
		case 'E':                                  // if "JE_"
			if (jump_line[2] == 'Q') {         // if "JEQ"
				*jump = 0x2;
			} else {
				err(err_3rd_char); return false;
			}
			break;
		case 'L':                                  // if "JL_"
			if (jump_line[2] == 'T') {         // if "JLT"
				*jump = 0x4;
			} else if (jump_line[2] == 'E') {  // if "JLE"
				*jump = 0x6;
			} else {
				err(err_3rd_char); return false;
			}
			break;
		case 'N':                                  // if "JN_"
			if (jump_line[2] == 'E') {         // if "JNE"
				*jump = 0x5;
			} else {
				err(err_3rd_char); return false;
			}
			break;
		case 'M':                                  // if "JM_"
			if (jump_line[2] == 'P') {         // if "JMP"
				*jump = 0x7;
			} else {
				err(err_3rd_char); return false;
			}
			break;
		default:
			err("syntax error: 2nd letter in jump field incorrect\n");
			return false;
		}
	} else {
		err("syntax error: jump field doesn't start with 'J'\n");
		return false;
	}

	return true;
}

/* Instruction format: 0b111accccccdddjjj
 * Assumes line begins with actual instruction (prepended whitespace stripped)
 * TODO: eventually just replace all wasteful c-instruction parsing w/ strcmp()
 */
static bool parse_c_type(const char *line, uint16_t *instruction)
{
	bool ret = false;
	char c;
	const char *dest_start = NULL;
	const char *comp_start = NULL;
	const char *jump_start = NULL;
	size_t i = 0;
	uint8_t dest = 0;  // default value when not present
	uint8_t comp = 0;
	uint8_t jump = 0;  // default value when not present

	// set pointers to comp, dest, and/or jump field (if applicable)
	c = line[0];
	for (i = 0; c != ' ' && c != '\t' &&
	            c != '\n' && c != '\0'; ++i) {  // read until end of line
		c = line[i];

		if (c == '=') {                     // indicates dest field
			if (1 <= i && i <= 3) {
				dest_start = &line[0];   // start of line
				// this 'i+1' might be dangerous!
				comp_start = &line[i+1]; // after "[dest]="
			} else {
				err("syntax error: dest incorrect length %lu\n", i);
				return false;
			}
		} else if (c == ';') {              // indicates jump field
			if (1 <= i && i <= 7) {
				// this 'i+1' might be dangerous!
				jump_start = &line[i+1]; // after "[comp];"
			} else {
				err("syntax error: jump incorrect length %lu\n", i);
				return false;
			}
		}
	}

	// Only the comp field is mandatory for assembly instructions;
	// dest and jump fields are optional, and may/may not be present
	if (comp_start == NULL) {
		comp_start = &line[0];  // start of line (no dest field)
	}

	ret = parse_c_type_comp(comp_start, &comp);
	if (!ret) {
		return false;
	}

	if (dest_start != NULL) {
		ret = parse_c_type_dest(dest_start, &dest);
		if (!ret) {
			return false;
		}
	}

	if (jump_start != NULL) {
		ret = parse_c_type_jump(jump_start, &jump);
		if (!ret) {
			return false;
		}
	}

	*instruction = 0xe000 | ((uint16_t)comp << 6)
	                      | ((uint16_t)dest << 3)
	                      | ((uint16_t)jump);
	return true;
}

// does not care about line line length; exits at first newline or after
// relevant portion parsed (allows for syntactically-incorrect lines, I know)
static bool parse_next_instruction(const char *line, uint16_t *instruction)
{
	bool ret = false;
	char c;
	size_t i = 0;

	while ((c = line[i]) != '\0') {
		if (c == ' ' || c == '\t')
			;  // skip any whitespace at start of line
		else if (c == '@') {
			ret = parse_a_type(&line[i], instruction);
			++g_instruction_number;
			break;
		} else if (c >= '!' && c < '~') {
			ret = parse_c_type(&line[i], instruction);
			++g_instruction_number;
			break;
		} else {
			err("syntax error: line '%s' incorrectly formatted\n",
			      line);
		}

		++i;
	}

	return ret;
}

// return false for comment or invalid assembly instruction
bool parse_line(const char *line, uint16_t *instruction)
{
	char c;
	bool slash_found = false;
	size_t i;

	// filter out comment lines
	for (i = 0; (c = line[i]) != '\0'; ++i) {
		if (c == ' ' || c == '\t') {
			continue;
		} else if (i == 0 && c == '\0') {
			return false;         // originally bugfix
		} else if (c == '/') {
			if (slash_found) {    // second slash means comment
				return false;
			}
			slash_found = true;
			continue;
		} else if (slash_found) {
			// this char not slash, but previous was: invalid syntax
			err("syntax error: found '/', comments need '//'\n");
			return false;
		} else if (c == '(') {        // found label, ignore and return
			return false;
		} else {                      // non-whitespace/slash discovered
			break;
		}
	}

	// comment not found, so attempting to parse instruction
	return parse_next_instruction(line, instruction);
}


char *usage_msg = "Usage: n2tasm1 [input/file.asm] [assembled/output]\n"
                  "       n2tasm1 [input/file.asm] # will output to STDOUT\n";

int main(int argc, char *argv[])
{
	bool result = false; 
	uint16_t instruction;
	char in_line[MAX_LINE_LEN];
	size_t i, file_line, in_line_len;
	char *in_file_path;
	FILE *fp, *fp_out;

	if (argc == 2) {
		fp_out = stdout;
	} else if (argc == 3) {
		fp_out = fopen(argv[2], "wb");
		if (fp_out == NULL) {
			fprintf(stderr, "failed to open %s for writing\n",
			        argv[2]);
			exit(-1);
		}
	} else {
		fprintf(stderr, usage_msg);
		exit(-1);
	}
	/*
	if (argc != 2) {                            // requires 1 argument
		fprintf(stderr, usage_msg);
		exit(-1);
	}
	*/

	in_file_path = argv[1];
	fp = fopen(in_file_path, "r");
	if (fp == NULL) {
		fprintf(stderr, "failed to open file %s for reading\n",
		        in_file_path);
		exit(-1);
	}

	if(!init_symbol_list()) {
		fprintf(stderr, "internal: failed to initialize symbol list\n");
		exit(-1);
	}

	file_line = 1;
	// first pass to read labels and associate with values
	while (fgets(in_line, MAX_LINE_LEN, fp) != NULL) {  // parse loop
		for (i = 0; in_line[i] != '\0'; ++i) {      // remove newlines
			if (in_line[i] == '\n' || in_line[i] == '\r') {
				in_line[i] = '\0';
				break;
			}
			++in_line_len;
		}

		if (in_line_len == 0 || in_line_len == 1)
			continue;

		g_asm_line = in_line;
		g_asm_line_number = file_line;
		result = parse_line_for_label(in_line);
		//if (result)
		//	DBGLOG("DEBUG: label found in line %s\n", in_line);

		++file_line;
	}

	// skip second pass if error found in first pass
	if (g_error_encountered) {
		fprintf(stderr, "error during first pass label parsing, "
		                "aborting...\n");
		exit(-1);
	}

	if (fseek(fp, 0, SEEK_SET)) {
		fprintf(stderr, "failed to re-read file %s from beginning\n",
		        in_file_path);
		exit(-1);
	}

	file_line = 1;
	while (fgets(in_line, MAX_LINE_LEN, fp) != NULL) {  // parse loop
		DBGLOG("line %lu: %s", file_line, in_line);
		for (i = 0; in_line[i] != '\0'; ++i) {      // remove newlines
			if (in_line[i] == '\n' || in_line[i] == '\r') {
				in_line[i] = '\0';
				break;
			}
			++in_line_len;
		}

		if (in_line_len == 0 || in_line_len == 1)
			continue;

		g_asm_line = in_line;
		g_asm_line_number = file_line;
		result = parse_line(in_line, &instruction);

		if (result)
			write_binary_word16(fp_out, instruction);

		++file_line;
	}

	//debug_dump_all_symbols();

	free_all_symbols();

	if (fclose(fp)) {
		fprintf(stderr, "Failed to close file %s\n", in_file_path);
		exit(-1);
	}

	return 0;
}