├── .gitignore ├── 0asm.asm ├── BOOTOS-LICENSE ├── LICENSE ├── Makefile ├── README.md ├── copy-to-sector.sh ├── examples ├── all.asm └── hello_world.asm ├── gdb-16bit.xml ├── gdb-real-mode ├── os.img ├── osbase.img └── test.asm /.gitignore: -------------------------------------------------------------------------------- 1 | 0asm.bin 2 | 0asm.elf 3 | 0asm.lst 4 | 0asm.o 5 | -------------------------------------------------------------------------------- /0asm.asm: -------------------------------------------------------------------------------- 1 | ;;; # 0asm 2 | ;;; 3 | ;;; 0asm is a 16-bit x86 assembler written in 16-bit x86 assembly which fits 4 | ;;; in under 512 bytes of x86 machine code! It runs on 5 | ;;; [bootOS](https://github.com/nanochess/bootOS/). 6 | ;;; 7 | ;;; ## Assembling 8 | ;;; 9 | ;;; You can assemble and run 0asm using nasm and QEMU (respectively): 10 | ;;; 11 | ;;; make run 12 | ;;; 13 | ;;; This also copies the input file `test.asm` into the disk under the filename 14 | ;;; `0`. One can then run the assembler in bootOS: 15 | ;;; 16 | ;;; $0asm 17 | ;;; 18 | ;;; and then run the output program under `@`: 19 | ;;; 20 | ;;; $@ 21 | ;;; Hello, world! $ 22 | ;;; 23 | ;;; Further examples are available under the `examples/` directory. 24 | ;;; 25 | ;;; ## Anticipated FAQ 26 | ;;; 27 | ;;; ### What does it support? 28 | ;;; 29 | ;;; 0asm supports several common instructions, labels (absolute and relative 30 | ;;; relocations), and octal literals (but only octal literals). It also 31 | ;;; supports all 16-bit x86 general purpose registers, but does not support 32 | ;;; segment registers. 33 | ;;; 34 | ;;; Many jump instructions are supported (call, jmp, jb, jnb, jz, jnz, jbe, 35 | ;;; jnbe), along with several string instructions (stosb, stosw, lodsb, lodsw, 36 | ;;; movsw, cbw, scasw), several stack instructions (push, pop, ret), a few 37 | ;;; fundamental arithmetic instructions in both register/register and 38 | ;;; register/immediate form (add, or, adc, and, xor, cmp, mov), and some misc 39 | ;;; instructions (int, stc, inc, dec, the pseudo-instruction db). 40 | ;;; 41 | ;;; Errors are not always handled, but the assembler does usually exit cleanly 42 | ;;; instead of producing garbage. 43 | ;;; 44 | ;;; ### How does it work? 45 | ;;; 46 | ;;; 0asm is a simple 2-pass assembler. The first pass outputs the machine 47 | ;;; code for instructions which do not require any relocation; it also collects 48 | ;;; labels into a "symbol table", and the addresses of immediates which need 49 | ;;; to be fixed into a "fixup table". The first pass also outputs the machine 50 | ;;; code for instructions which do not require any relocation. 51 | ;;; 52 | ;;; ### Is it self-hosting? 53 | ;;; 54 | ;;; Unfortunately not yet, for several reasons. 55 | ;;; 56 | ;;; 1. The underlying bootOS only supports 512 byte files. 57 | ;;; 2. The instruction encoding produced is not optimal, so it will not fit 58 | ;;; in 512 bytes once assembled. 59 | ;;; 3. Shift and memory-addressing instructions are not yet supported. 60 | ;;; 61 | ;;; These problems are not insurmountable, although it seems difficult. 62 | ;;; We could easily move the goalpost by typing the entire program using db, 63 | ;;; but of course that would be no fun. 64 | ;;; 65 | ;;; ## Development 66 | ;;; 67 | ;;; The code is decently commented. Many of the files in this repo are 68 | ;;; generated from `0asm.asm`, including the README. You can also debug by 69 | ;;; running `make debug` (to start QEMU in one pane) and `make gdbdebug` 70 | ;;; (to start GDB along with some helpful default scripts). 71 | ;;; 72 | ;;; ## License 73 | ;;; 74 | ;;; Copyright (c) 2019 Keyhan Vakil 75 | ;;; 76 | ;;; This program is free software: you can redistribute it and/or modify 77 | ;;; it under the terms of the GNU General Public License as published by 78 | ;;; the Free Software Foundation, either version 3 of the License, or 79 | ;;; (at your option) any later version. 80 | ;;; 81 | ;;; This program is distributed in the hope that it will be useful, 82 | ;;; but WITHOUT ANY WARRANTY; without even the implied warranty of 83 | ;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 84 | ;;; GNU General Public License for more details. 85 | ;;; 86 | ;;; You should have received a copy of the GNU General Public License 87 | ;;; along with this program. If not, see . 88 | 89 | ;; Vocabulary 90 | 91 | ;; hash: a formula which maps identifiers to a 16-bit value (for shorter, but 92 | ;; lossy, string comparisons). See the hash procedure for its computation. 93 | 94 | ;; short table: an associative array of (key, value) pairs. Keys are typically 95 | ;; outputs of the hash function, while values are any 16-bit values. 96 | 97 | ;; long table: a short table with 16-bit values. Long tables are ONLY used for 98 | ;; the label fixup, otherwise we use short tables because they are more size 99 | ;; efficient. 100 | 101 | ;; clobbered: indicates that a register is changed by a procedure in some 102 | ;; (typically unspecified) fashion. 103 | 104 | ;; consumed: as we read characters from our input buffer, we "consume" 105 | ;; them. We generally keep the invariant that al is the next character in 106 | ;; the buffer, and si points to the rest of the buffer. Note that consuming 107 | ;; is different than clobbering, as consuming means that we maintain this 108 | ;; invariant! 109 | 110 | ;; ";!": this comment symbol indicates that some code or functionality is 111 | ;; easily added, but was removed so that the program fits in 512 bytes. 112 | ;; As more features are implemented, these comments are removed. 113 | 114 | cpu 186 115 | bits 16 116 | fixup_table: equ 0x4000 117 | symbol_table: equ 0x5000 118 | register_table: equ 0x6000 119 | input_start: equ 0x6b00 120 | output_start: equ 0x6d00 121 | int_exit: equ 0x20 122 | int_read_file: equ 0x23 123 | int_save_file: equ 0x24 124 | 125 | ; Macro which hashes the given string, and stores the result in the hash_result 126 | ; variable. This MUST be kept in sync with the implementation of the runtime 127 | ; hash function below. 128 | %macro hash_s 1.nolist 129 | %assign hash_result 0x23 130 | %strlen %%len %1 131 | %assign %%i 1 132 | %rep %%len 133 | %substr %%ch %1 %%i 134 | %assign hash_result (31 * hash_result + %%ch) 135 | %assign hash_result (hash_result & 0xffff) 136 | %assign %%i (%%i+1) 137 | %endrep 138 | %endmacro 139 | 140 | %macro dw_hash 1.nolist 141 | hash_s %1 142 | dw hash_result 143 | %endmacro 144 | 145 | global _start 146 | _start: 147 | 148 | ;; Zero out fixup, symbol table, and the stack. Many of the functions below 149 | ;; (like lookup and add_table) assume that tables are zero-terminated, and 150 | ;; this initialization ensures that this is the case. 151 | initialize_tables: 152 | ; This zeros out 0x4000 to 0x6f00. 153 | ; 0x4000 is the address of the fixup table. 154 | ; 0x5000 is the address of the symbol table. 155 | ; 0x6f00 is the start of the stack. 156 | mov di,fixup_table 157 | ; We set only ch here, which saves us a byte. This means that the lower 158 | ; cl might be garbage, but this is OK -- the maximum value is 0x2fff, 159 | ; which would have us write up to 0x7000, right below bootOS. The minimum 160 | ; value is 0x2f00, which still zeros out everything we need. 161 | ; 162 | ; In any case, error-checking is definitely not the priority of this 163 | ; assembler... 164 | mov ch,0x2f 165 | xor ax,ax 166 | rep stosb 167 | 168 | ;; Unlike other tables, the register table is generated at runtime, since the 169 | ;; lookup values are sequential, we can save bytes by only storing the keys. 170 | initialize_register_table: 171 | ; It's shorter to do this manually than to use add_table. 172 | mov si,partial_register_table 173 | mov di,register_table 174 | ; ax is zero from above. 175 | .initialize_register_table_loop: 176 | ; Copy over partial register table key. 177 | movsw 178 | ; Store the index of this register. 179 | stosb 180 | inc ax 181 | ; If al == 16, then we've finished copying partial_register_table. 182 | ; From initialize_tables above, we know that this table was zero. 183 | ; It is important that we don't overwrite this final zero entry, since 184 | ; we are using it as a sentinel value. 185 | cmp al,0x10 186 | jne .initialize_register_table_loop 187 | .initialize_register_table_end: 188 | 189 | ;; Initialize si to point to the input buffer and di to point to the output 190 | ;; buffer. Read in the input file into the input buffer. 191 | init: 192 | ; Start stack pointer right below input buffer. This leaves space for 193 | ; 0x200 input bytes, which is the size of files in bootOS. The output 194 | ; buffer will start at 0x6d00. 195 | mov sp,input_start 196 | mov di,sp 197 | ; Read in the file. 198 | mov bx,infile 199 | int int_read_file 200 | jc done_error 201 | ; All registers except sp could be clobbered at this point. 202 | mov si,sp 203 | ; Give space for 0x200 output bytes, until 0x6f00. 204 | mov di,output_start 205 | 206 | ;; Keep reading instructions. 207 | read_instruction_loop: 208 | ; This lets us use "ret" when we successfully parse an instruction, which 209 | ; ends up saving many bytes over "jmp read_instruction", because we need 210 | ; to jump back so often. 211 | call read_instruction 212 | jmp read_instruction_loop 213 | 214 | ;; Read and assemble a single instruction. 215 | read_instruction: 216 | lodsb 217 | 218 | ;; This block will be executed when we are finished parsing the entire file. 219 | done: 220 | ; The input file is NUL terminated. If this is not a NUL, then we need to 221 | ; keep parsing the file. 222 | and al,al 223 | jnz done_end 224 | .fixup_labels: 225 | ; Set lookup to "long mode" by modifying lodsb -> lodsw. 226 | ; This allows us to lookup into long tables like the symbol table. 227 | inc byte [lookup_lodsb] 228 | mov si,fixup_table 229 | .fixup_labels_loop: 230 | ; Load key from fixup table. 231 | lodsw 232 | and ax,ax 233 | jz .fixup_labels_done 234 | ; Lookup key in symbol table. 235 | mov bp,si 236 | mov si,symbol_table 237 | ; Basically mov cx,ax, but shorter and ax is getting clobbered anyway. 238 | xchg ax,cx 239 | call lookup 240 | ; Error: could not find a label for this address we need to fix. 241 | jnc done_error 242 | 243 | ; Currently there is a "fixup hint" (*i) stored at the address of the 244 | ; immediate we are fixing (i), and the label address is d. This code sets 245 | ; *i = *i + d - i. This allows us to support both relative and absolute 246 | ; relocations, by choosing the fixup hint appropriately. (See jmp_and_call 247 | ; as well as parse_2x.parse_2x_label for relative and absolute relocations 248 | ; respectively.) 249 | 250 | ; Store the label address. 251 | push ax 252 | ; Get the location to fixup. 253 | lodsw 254 | mov di,ax 255 | ; *i -= i 256 | sub [di],ax 257 | pop ax 258 | ; *i += d 259 | add [di],ax 260 | jmp .fixup_labels_loop 261 | .fixup_labels_done: 262 | ; Original start of output buffer. 263 | mov di,output_start 264 | ; Output to file. 265 | mov bx,outfile 266 | int int_save_file 267 | done_error: 268 | int int_exit 269 | done_end: 270 | 271 | ; Skip space or space-ish character 272 | cmp al,' ' 273 | jbe read_instruction 274 | 275 | ;; Parse comment by consuming until we see a newline. 276 | read_comment: 277 | cmp al,';' 278 | jnz .read_comment_end 279 | .read_comment_loop: 280 | lodsb 281 | cmp al,`\n` 282 | jnz .read_comment_loop 283 | ret 284 | .read_comment_end: 285 | 286 | call hash 287 | 288 | ; At this point, al equals the first non-identifier character, 289 | ; and si points to right after al. 290 | ; 291 | ; Some examples follow. The position of al is marked a, the position 292 | ; of si is marked s. $ is used to indicate a newline. 293 | ; 294 | ; mov ax,cx$ 295 | ; as 296 | ; 297 | ; hello:$ 298 | ; as 299 | ; 300 | ; ret$ 301 | ; as 302 | 303 | ;; If the next character in the buffer is a colon, this adds the label to our 304 | ;; symbol table. 305 | add_to_label: 306 | cmp al,':' 307 | jne add_to_label_end 308 | 309 | ; SELF-MODIFYING. 310 | symbol_table_addr: 311 | mov ax,symbol_table 312 | ; Load address of the immediate in the above instruction 313 | mov bx,symbol_table_addr+1 314 | 315 | ; Inlining the tail call to add_table here saves us a two bytes of a jmp. 316 | 317 | ;; Store (cx, di) at ax, and increment bx to point to the new end. 318 | ;; Inputs: 319 | ;; ax is the address of the end of the table. 320 | ;; bx is the address of the immediate in the instruction that loaded ax. 321 | ;; (Note: this code is self-modifying.) 322 | ;; cx is the key to write. 323 | ;; di is the value to write. 324 | ;; Outputs: 325 | ;; ax is clobbered to the initial value of di. 326 | ;; *bx is incremented by 4 to point to the new end. 327 | ;; cx is clobbered. 328 | ;; bx, di are preserved. 329 | add_table: 330 | ; Set ax = old cx, di = old ax, cx = old di. 331 | xchg ax,di 332 | xchg ax,cx 333 | ; Store old di at old ax. 334 | stosw 335 | ; Store old cx at old ax+2. 336 | ; This xchg could be mov ax,cx, but it's shorter this way. 337 | xchg ax,cx 338 | stosw 339 | ; Update the end of the table. 340 | mov [bx],di 341 | mov di,ax 342 | ret 343 | add_to_label_end: 344 | 345 | ; db 346 | parse_db: 347 | hash_s 'db' 348 | cmp cx,hash_result 349 | je store_odigit_byte 350 | .parse_db_end: 351 | 352 | ; int 353 | parse_int: 354 | hash_s 'int' 355 | cmp cx,hash_result 356 | jne parse_int_end 357 | mov al,0xcd 358 | stosb 359 | store_odigit_byte: 360 | call odigit 361 | ; This "jump chain" allows us to jump to a single statement (done_error), from 362 | ; multiple places, without requiring long jumps (signed displacement greater 363 | ; than one byte). Each individual jump in the chain is within 128 bytes of the 364 | ; previous jump. 365 | done_error_chain_0: 366 | jnc done_error 367 | stosb 368 | ret 369 | parse_int_end: 370 | 371 | ; All calls to lookup expect a saved value of si in bp, which is restored 372 | ; on success. 373 | mov bp,si 374 | 375 | ;; Deals with jmp and call instructions, encoded as: 376 | ;; - opcode (1 or 2 bytes) - from the table. 377 | ;; - immediate (2 bytes) - needs to be relocated by linker. 378 | jmp_and_call: 379 | mov si,jmp_and_call_table 380 | call lookup 381 | jnc jmp_and_call_end 382 | .jmp_and_call_match: 383 | cmp al,0x90 384 | jnb .single_byte 385 | ; If this is greater than 0x90, it's an extended instruction. 386 | ; The first byte should be 0xf, and the next byte is obtained from 387 | ; the table. 388 | mov byte [di],0xf 389 | inc di 390 | .single_byte: 391 | stosb 392 | ; Hash the label and add it to the fixup table. 393 | call add_fixup 394 | ; Add a fixup hint of -2. (This effectively creates a relative relocation, 395 | ; see done.fixup_labels for details.) 396 | mov ax,0xfffe 397 | stosw 398 | ret 399 | 400 | add_fixup: 401 | call hash_pre 402 | ; SELF-MODIFYING. 403 | fixup_table_addr: 404 | mov ax,fixup_table 405 | ; Load address of the immediate in the above instruction 406 | mov bx,fixup_table_addr+1 407 | jmp add_table 408 | jmp_and_call_end: 409 | 410 | ;; Deals with single-byte no argument instructions, encoded as just the opcode. 411 | parse_10: 412 | ; Note we don't need to set si or bp here. 413 | ; If the lookup above fails, then si points to one after jmp_and_call_table 414 | ; (i.e., table10), and bp is still correctly saved. 415 | call lookup 416 | jc stosb_ret 417 | .parse_10_end: 418 | 419 | ;; Deals with single-byte single-register instructions, encoded as just the 420 | ;; opcode plus a register number. 421 | parse_11: 422 | call lookup 423 | jnc parse_11_end 424 | push ax 425 | call accept_register 426 | done_error_chain_1: 427 | jnc done_error_chain_0 428 | pop dx 429 | add ax,dx 430 | stosb_ret: 431 | stosb 432 | ret 433 | parse_11_end: 434 | 435 | ;; Deals with instructions which take two arguments: either a 436 | ;; register-register, or a register-immediate. This also handles the case of 437 | ;; register-label (which is encoded as an absolute relocation of a 438 | ;; register-immediate). 439 | ;; 440 | ;; This is the trickiest case, consult an Intel manual for details. 441 | parse_2x: 442 | call lookup 443 | done_error_chain_2: 444 | jnc done_error_chain_1 445 | ; Save the opcode. 446 | push ax 447 | ; The first argument MUST be a register, regardless. 448 | call accept_register 449 | ; (Note accept_register clobbers the comma in ax.) 450 | done_error_chain_3: 451 | jnc done_error_chain_2 452 | 453 | ; Restore opcode in dx. 454 | pop dx 455 | 456 | ; Check if this is a 16-bit register or an 8-bit register. 457 | ; bl will be 1 if we should use a 16-bit immediate, and 0 if we should use 458 | ; an 8-bit immediate. It also encodes if this is operating on an 8-bit or 459 | ; 16-bit register. Technically x86 makes a distinction between these two, 460 | ; but we treat them the same (leading to longer but still valid instruction 461 | ; encodings). 462 | 463 | ; Sets carry flag if al < 0x8. 464 | cmp al,0x8 465 | sbb bx,bx 466 | inc bx 467 | and al,0x7 468 | 469 | ; Set LSB of opcode byte correctly. 470 | add dl,bl 471 | 472 | ; Try to get another register. 473 | push ax 474 | push si 475 | call accept_register 476 | ; (Note accept_register clobbers the newline in ax.) 477 | ; If it's not a register, it must be an immediate or a label. 478 | jnc .parse_2x_immediate 479 | 480 | ; In this case, we are parsing a register-register instruction. 481 | ; We store this as: 482 | ; - opcode (1 byte) 483 | ; - Mod R/M byte (1 byte) 484 | 485 | ; The parse was successful, so we need to clean the old si from the stack. 486 | ; We don't use cx from now on, so pop into it. 487 | pop cx 488 | 489 | ; Compute the Mod R/M byte: 490 | ; 11 src[0:3] dst[0:3] 491 | ; where src and dst are the register numbers. 492 | ; Right now ax=src, and dst is on the top of stack. 493 | 494 | ; Shifting up by three here can overflow the second MSB, but it's 495 | ; overwritten below anyway. 496 | shl al,0x3 497 | ; Note that we always set the top two bits, as we only support register 498 | ; addressing here. 499 | or al,0xc0 500 | 501 | ; Get the destination register off the stack. 502 | pop cx 503 | or cl,al 504 | ; Mod R/M byte 505 | mov dh,cl 506 | ; Opcode byte in dl 507 | ; Basically mov ax,dx, but dx doesn't matter. 508 | xchg ax,dx 509 | stosw 510 | ret 511 | 512 | .parse_2x_immediate: 513 | ; Backtrack to point to the immediate / label. 514 | pop si 515 | 516 | ; Check if this is a MOV instruction. 517 | cmp dl,0x88 518 | ; Opcode byte for all group 1 immediate instructions 519 | mov al,0x80 520 | je .parse_group1_immediate 521 | 522 | ; Opcode byte for MOV with immediate 523 | mov al,0xc6 524 | ; In this case it's a MOV instruction, we kill the old top bits of the 525 | ; opcode. This effectively allows us to reuse the opcode byte for Group 1 526 | ; instructions, and makes the MOV instruction a "raw" encoding which uses 527 | ; an opcode byte of 0. It also maintains whether this instruction is 528 | ; supposed to operate on 8-bit or 16-bit registers. 529 | mov dl,bl 530 | 531 | .parse_group1_immediate: 532 | 533 | ; In the immediate case, the opcode byte of the register-register 534 | ; instruction is USUALLY reusable as the Mod R/M byte. (The exception 535 | ; is the case when we have a MOV instruction.) First, we can remove the 536 | ; LSB for the opcode byte (which indicates if the instruction operates 537 | ; on 8-bit or 16-bit registers). The middle three bits of this opcode 538 | ; byte select the correct operation for all Group 1 instructions. 539 | 540 | add al,bl 541 | stosb 542 | ; Construct the Mod R/M byte using the old register. 543 | pop ax 544 | add al,dl 545 | ; Clear off the last bit of the opcode. 546 | sub al,bl 547 | ; Set the top two bits of Mod R/M to indicate registers. 548 | or al,0xc0 549 | stosb 550 | ; Parse off the octal number. 551 | push si 552 | call odigit 553 | ; If it's not an octal number, it must be a register. 554 | jnc .parse_2x_label 555 | ; The parse was successful, so we need to clean the old si from the stack. 556 | ; We don't use cx from now on, so pop into it. 557 | pop cx 558 | 559 | .parse_2x_append_immediate: 560 | ; Blindly store both bytes of the immediate. 561 | stosw 562 | ; If bx = 0, we want to decrement di, so that we effectively store only 563 | ; one byte of the immediate. 564 | ; If bx = 1, then we don't want to change di. 565 | ; Because we have bx available, this is shorter than the similar code in 566 | ; jmp_and_call.jmp_and_call_match. 567 | add di,bx 568 | dec di 569 | ret 570 | 571 | .parse_2x_label: 572 | ; Backtrack. 573 | pop si 574 | call add_fixup 575 | ; We want to add a value to the relocation such that, when di is added by 576 | ; the fixup, it will correspond to the absolute address of the loaded 577 | ; label. This is the start of the output buffer (output_start), minus the 578 | ; actual location the binary will start (0x7c00). 579 | add ah,0xf 580 | stosw 581 | ret 582 | 583 | ;; Accept a register pointed to by the buffer. 584 | ;; 585 | ;; Inputs: 586 | ;; al is the first character in the buffer. 587 | ;; si is a buffer pointing to the input after al. 588 | ;; Outputs: 589 | ;; ax is DESTROYED. It now indicates the register number and does not point 590 | ;; to the buffer, therefore ignoring the character after this register. 591 | ;; bx is clobbered. 592 | ;; bp is clobbered. 593 | ;; si is consumed. 594 | accept_register: 595 | call hash_pre 596 | mov bp,si 597 | mov si,register_table 598 | ; FALLTHROUGH to lookup (saves us two bytes) 599 | 600 | ;; Lookup the key in the given SHORT table, returning the value. 601 | ;; 602 | ;; Inputs: 603 | ;; cx is the key to lookup. 604 | ;; si points to the first key of the table. 605 | ;; bp is the old value of si. 606 | ;; Outputs: 607 | ;; al is the returned value, or 0 on failure. 608 | ;; si is modified. 609 | ;; if the lookup is successful, it is set to bp. 610 | ;; if the lookup fails, it points one word after the end of the table. 611 | ;; carry flag is set iff the lookup was successful. 612 | lookup: 613 | lodsw 614 | and ax,ax 615 | jz lookup_not_found 616 | lookup_keep_going: 617 | cmp ax,cx 618 | lookup_lodsb: 619 | ; SELF-MODIFYING. 620 | ; This can be changed to a lodsw to make lookup use long tables. 621 | lodsb 622 | jne lookup 623 | lookup_done: 624 | stc 625 | mov si,bp 626 | lookup_not_found: 627 | ret 628 | 629 | ;; Compute a hash for table indexing. Stops at the first non-identifier 630 | ;; character after al, consuming all characters and incrementing si. This MUST 631 | ;; be kept in sync with the hash_s macro above. 632 | ;; 633 | ;; Inputs: 634 | ;; al is the first character in the buffer. 635 | ;; si is a buffer pointing to the input after al. 636 | ;; Outputs: 637 | ;; al is consumed. 638 | ;; si is consumed. 639 | ;; cx is the returned hash value. 640 | hash_pre: 641 | lodsb 642 | hash: 643 | mov cx,0x23 644 | .hash_loop: 645 | ; Using cbw here lets us use ax as al, which we need (because we want to 646 | ; use the full 16-bit for the hash to reduce collisions). 647 | cbw 648 | ; cx = 31 * cx + (next character) 649 | sub ax,cx 650 | shl cx,0x5 651 | add cx,ax 652 | lodsb 653 | ; Stop if we see a non-identifier character. 654 | cmp al,'@' 655 | jg .hash_loop 656 | ret 657 | 658 | ;; Convert ASCII octal. 659 | ;; The representation must start with "0o". 660 | ;; Inputs: 661 | ;; al is the first character in the buffer. 662 | ;; si is a buffer pointing to the input after al. 663 | ;; Outputs: 664 | ;; ax is the returned value. It is clobbered if the parse fails. 665 | ;; cx is clobbered. 666 | ;; si is consumed. 667 | ;; the carry flag is set if the parse is successful. 668 | odigit: 669 | lodsb 670 | cmp al,'0' 671 | jne .odigit_bad 672 | ;! Error checking on o removed for byte count. 673 | lodsb 674 | xor cx,cx 675 | .odigit_loop: 676 | lodsb 677 | ; Convert by subtracting off ASCII 0. 678 | sub al,'0' 679 | ; Note if al < '0', then CF is set to 1. This indicates we are now dealing 680 | ; with a whitespace character, and the parse was successful. 681 | ;! We don't check that the value is not too large. 682 | jb .odigit_good 683 | shl cx,0x3 684 | ; Note that the bottom three bits of cl are zero and al is at most 7, 685 | ; so adding only the lower bytes here is fine; we never overflow cl. 686 | add cl,al 687 | jmp .odigit_loop 688 | .odigit_bad: 689 | ; Clear the carry flag to show parse was bad. 690 | clc 691 | .odigit_good: 692 | ; Basically mov ax,cx, but cx is getting clobbered. 693 | xchg ax,cx 694 | ret 695 | 696 | ;; THESE TABLES MUST APPEAR IN THIS ORDER. ;; 697 | 698 | ;; Lookup table for jump and call instructions (anything requiring relative 699 | ;; relocation with 16-bit addresses). Keys are the hashes, and values are the 700 | ;; opcode. 701 | jmp_and_call_table: 702 | dw_hash 'call' 703 | db 0xe8 704 | dw_hash 'jmp' 705 | db 0xe9 706 | dw_hash 'jb' 707 | db 0x82 708 | dw_hash 'jnb' 709 | db 0x83 710 | dw_hash 'jz' 711 | db 0x84 712 | dw_hash 'jnz' 713 | db 0x85 714 | dw_hash 'jbe' 715 | db 0x86 716 | dw_hash 'jnbe' 717 | db 0x87 718 | ; NOT FOUND 719 | dw 0x0 720 | 721 | ;; Lookup table for instructions which take up one byte and have no arguments. 722 | ;; Keys here are the hashes, values are simply the opcode. 723 | table10: 724 | dw_hash 'cbw' 725 | db 0x98 726 | dw_hash 'movsw' 727 | db 0xa4 728 | dw_hash 'stosb' 729 | db 0xaa 730 | dw_hash 'stosw' 731 | db 0xab 732 | dw_hash 'lodsb' 733 | db 0xac 734 | dw_hash 'lodsw' 735 | db 0xad 736 | dw_hash 'scasw' 737 | db 0xaf 738 | dw_hash 'ret' 739 | db 0xc3 740 | dw_hash 'stc' 741 | db 0xf9 742 | ; NOT FOUND 743 | dw 0x0 744 | 745 | ;; Lookup table for instructions with one opcode byte and one argument. 746 | ;; Keys here are the hashes. Values are the opcode which will be added to 747 | ;; (note that because we start numbering 16-bit registers at 0x8, most of 748 | ;; the values here are shifted down by 0x8). 749 | table11: 750 | dw_hash 'push' 751 | db 0x48 752 | dw_hash 'pop' 753 | db 0x50 754 | dw_hash 'inc' 755 | db 0x38 756 | dw_hash 'dec' 757 | ; Nasty trick here. We overlap outfile and infile with opcode bytes. 758 | ; This makes our input filename "0" and our output filename "@", and 759 | ; saves us three bytes. 760 | outfile: 761 | db 0x40 762 | ; NOT FOUND 763 | dw 0x0 764 | 765 | ;; Lookup table for instructions which take two arguments. Keys here are the 766 | ;; hashes. Values are the opcode byte for non-immediate (register/register) 767 | ;; forms. 768 | table2x: 769 | dw_hash 'add' 770 | db 0x00 771 | dw_hash 'or' 772 | db 0x08 773 | dw_hash 'adc' 774 | db 0x10 775 | dw_hash 'and' 776 | db 0x20 777 | dw_hash 'cmp' 778 | db 0x38 779 | dw_hash 'mov' 780 | db 0x88 781 | dw_hash 'xor' 782 | infile: 783 | db 0x30 784 | ; NOT FOUND 785 | dw 0x0 786 | 787 | ;; The "partial" register table contains only keys -- no values. The actual 788 | ;; register table is initialized using this and initialize_register_table. 789 | partial_register_table: 790 | dw_hash 'al' 791 | dw_hash 'cl' 792 | dw_hash 'dl' 793 | dw_hash 'bl' 794 | dw_hash 'ah' 795 | dw_hash 'ch' 796 | dw_hash 'dh' 797 | dw_hash 'bh' 798 | dw_hash 'ax' 799 | dw_hash 'cx' 800 | dw_hash 'dx' 801 | dw_hash 'bx' 802 | dw_hash 'sp' 803 | dw_hash 'bp' 804 | dw_hash 'si' 805 | dw_hash 'di' 806 | ; NOT FOUND not required. 807 | 808 | bytes_left_message: 809 | %assign bytes_left 0x200-($-$$) 810 | %warning Have bytes_left bytes left. 811 | times bytes_left db 0x90 812 | -------------------------------------------------------------------------------- /BOOTOS-LICENSE: -------------------------------------------------------------------------------- 1 | Copyright (c) 2019 Oscar Toledo G. http://nanochess.org/ 2 | All rights reserved. 3 | 4 | Redistribution and use in source and binary forms, with or without 5 | modification, are permitted provided that the following conditions are met: 6 | 7 | 1. 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Such new versions will 567 | be similar in spirit to the present version, but may differ in detail to 568 | address new problems or concerns. 569 | 570 | Each version is given a distinguishing version number. If the 571 | Program specifies that a certain numbered version of the GNU General 572 | Public License "or any later version" applies to it, you have the 573 | option of following the terms and conditions either of that numbered 574 | version or of any later version published by the Free Software 575 | Foundation. If the Program does not specify a version number of the 576 | GNU General Public License, you may choose any version ever published 577 | by the Free Software Foundation. 578 | 579 | If the Program specifies that a proxy can decide which future 580 | versions of the GNU General Public License can be used, that proxy's 581 | public statement of acceptance of a version permanently authorizes you 582 | to choose that version for the Program. 583 | 584 | Later license versions may give you additional or different 585 | permissions. However, no additional obligations are imposed on any 586 | author or copyright holder as a result of your choosing to follow a 587 | later version. 588 | 589 | 15. Disclaimer of Warranty. 590 | 591 | THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY 592 | APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT 593 | HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY 594 | OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, 595 | THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 596 | PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM 597 | IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF 598 | ALL NECESSARY SERVICING, REPAIR OR CORRECTION. 599 | 600 | 16. Limitation of Liability. 601 | 602 | IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING 603 | WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS 604 | THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY 605 | GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE 606 | USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF 607 | DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD 608 | PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), 609 | EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF 610 | SUCH DAMAGES. 611 | 612 | 17. Interpretation of Sections 15 and 16. 613 | 614 | If the disclaimer of warranty and limitation of liability provided 615 | above cannot be given local legal effect according to their terms, 616 | reviewing courts shall apply local law that most closely approximates 617 | an absolute waiver of all civil liability in connection with the 618 | Program, unless a warranty or assumption of liability accompanies a 619 | copy of the Program in return for a fee. 620 | 621 | END OF TERMS AND CONDITIONS 622 | 623 | How to Apply These Terms to Your New Programs 624 | 625 | If you develop a new program, and you want it to be of the greatest 626 | possible use to the public, the best way to achieve this is to make it 627 | free software which everyone can redistribute and change under these terms. 628 | 629 | To do so, attach the following notices to the program. It is safest 630 | to attach them to the start of each source file to most effectively 631 | state the exclusion of warranty; and each file should have at least 632 | the "copyright" line and a pointer to where the full notice is found. 633 | 634 | 635 | Copyright (C) 636 | 637 | This program is free software: you can redistribute it and/or modify 638 | it under the terms of the GNU General Public License as published by 639 | the Free Software Foundation, either version 3 of the License, or 640 | (at your option) any later version. 641 | 642 | This program is distributed in the hope that it will be useful, 643 | but WITHOUT ANY WARRANTY; without even the implied warranty of 644 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 645 | GNU General Public License for more details. 646 | 647 | You should have received a copy of the GNU General Public License 648 | along with this program. If not, see . 649 | 650 | Also add information on how to contact you by electronic and paper mail. 651 | 652 | If the program does terminal interaction, make it output a short 653 | notice like this when it starts in an interactive mode: 654 | 655 | Copyright (C) 656 | This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'. 657 | This is free software, and you are welcome to redistribute it 658 | under certain conditions; type `show c' for details. 659 | 660 | The hypothetical commands `show w' and `show c' should show the appropriate 661 | parts of the General Public License. Of course, your program's commands 662 | might be different; for a GUI interface, you would use an "about box". 663 | 664 | You should also get your employer (if you work as a programmer) or school, 665 | if any, to sign a "copyright disclaimer" for the program, if necessary. 666 | For more information on this, and how to apply and follow the GNU GPL, see 667 | . 668 | 669 | The GNU General Public License does not permit incorporating your program 670 | into proprietary programs. If your program is a subroutine library, you 671 | may consider it more useful to permit linking proprietary applications with 672 | the library. If this is what you want to do, use the GNU Lesser General 673 | Public License instead of this License. But first, please read 674 | . 675 | -------------------------------------------------------------------------------- /Makefile: -------------------------------------------------------------------------------- 1 | .PHONY: all 2 | all: 0asm.bin os.img README.md 3 | 4 | 0asm.o 0asm.lst: 0asm.asm 5 | nasm -f elf32 -g3 -F dwarf 0asm.asm -l 0asm.lst -o 0asm.o 6 | 7 | 0asm.lst: 0asm.o 8 | 9 | 0asm.elf: 0asm.o 10 | ld -Ttext=0x7c00 -melf_i386 $^ -o $@ 11 | 12 | 0asm.bin: 0asm.elf 13 | objcopy -O binary $^ $@ 14 | 15 | os.img: osbase.img 0asm.bin test.asm 16 | cp osbase.img os.img 17 | ./copy-to-sector.sh test.asm os.img 1 18 | ./copy-to-sector.sh /dev/zero os.img 2 19 | ./copy-to-sector.sh 0asm.bin os.img 3 20 | 21 | README.md: 0asm.asm 22 | sed -n 's/;;; \?\(.*\)/\1/p' $^ > $@ 23 | 24 | .PHONY: run 25 | run: os.img 26 | qemu-system-i386 \ 27 | -drive file=os.img,format=raw,index=0,if=floppy \ 28 | -curses 29 | 30 | .PHONY: debug 31 | debug: os.img 32 | qemu-system-i386 \ 33 | -S -s \ 34 | -drive file=os.img,format=raw,index=0,if=floppy \ 35 | -curses 36 | 37 | .PHONY: gdbdebug 38 | gdbdebug: os.img 39 | gdb -x gdb-real-mode 0asm.elf 40 | 41 | .PHONY: clean 42 | clean: 43 | rm -f os.img 0asm.lst 0asm.elf 0asm.o 0asm.bin README.md 44 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | # 0asm 2 | 3 | 0asm is a 16-bit x86 assembler written in 16-bit x86 assembly which fits 4 | in under 512 bytes of x86 machine code! It runs on 5 | [bootOS](https://github.com/nanochess/bootOS/). 6 | 7 | ## Assembling 8 | 9 | You can assemble and run 0asm using nasm and QEMU (respectively): 10 | 11 | make run 12 | 13 | This also copies the input file `test.asm` into the disk under the filename 14 | `0`. One can then run the assembler in bootOS: 15 | 16 | $0asm 17 | 18 | and then run the output program under `@`: 19 | 20 | $@ 21 | Hello, world! $ 22 | 23 | Further examples are available under the `examples/` directory. 24 | 25 | ## Anticipated FAQ 26 | 27 | ### What does it support? 28 | 29 | 0asm supports several common instructions, labels (absolute and relative 30 | relocations), and octal literals (but only octal literals). It also 31 | supports all 16-bit x86 general purpose registers, but does not support 32 | segment registers. 33 | 34 | Many jump instructions are supported (call, jmp, jb, jnb, jz, jnz, jbe, 35 | jnbe), along with several string instructions (stosb, stosw, lodsb, lodsw, 36 | movsw, cbw, scasw), several stack instructions (push, pop, ret), a few 37 | fundamental arithmetic instructions in both register/register and 38 | register/immediate form (add, or, adc, and, xor, cmp, mov), and some misc 39 | instructions (int, stc, inc, dec, the pseudo-instruction db). 40 | 41 | Errors are not always handled, but the assembler does usually exit cleanly 42 | instead of producing garbage. 43 | 44 | ### How does it work? 45 | 46 | 0asm is a simple 2-pass assembler. The first pass outputs the machine 47 | code for instructions which do not require any relocation; it also collects 48 | labels into a "symbol table", and the addresses of immediates which need 49 | to be fixed into a "fixup table". The first pass also outputs the machine 50 | code for instructions which do not require any relocation. 51 | 52 | ### Is it self-hosting? 53 | 54 | Unfortunately not yet, for several reasons. 55 | 56 | 1. The underlying bootOS only supports 512 byte files. 57 | 2. The instruction encoding produced is not optimal, so it will not fit 58 | in 512 bytes once assembled. 59 | 3. Shift and memory-addressing instructions are not yet supported. 60 | 61 | These problems are not insurmountable, although it seems difficult. 62 | We could easily move the goalpost by typing the entire program using db, 63 | but of course that would be no fun. 64 | 65 | ## Development 66 | 67 | The code is decently commented. Many of the files in this repo are 68 | generated from `0asm.asm`, including the README. You can also debug by 69 | running `make debug` (to start QEMU in one pane) and `make gdbdebug` 70 | (to start GDB along with some helpful default scripts). 71 | 72 | ## License 73 | 74 | Copyright (c) 2019 Keyhan Vakil 75 | 76 | This program is free software: you can redistribute it and/or modify 77 | it under the terms of the GNU General Public License as published by 78 | the Free Software Foundation, either version 3 of the License, or 79 | (at your option) any later version. 80 | 81 | This program is distributed in the hope that it will be useful, 82 | but WITHOUT ANY WARRANTY; without even the implied warranty of 83 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 84 | GNU General Public License for more details. 85 | 86 | You should have received a copy of the GNU General Public License 87 | along with this program. If not, see . 88 | -------------------------------------------------------------------------------- /copy-to-sector.sh: -------------------------------------------------------------------------------- 1 | #!/bin/bash 2 | sector_no=$(($3 * 18)) 3 | dd if=/dev/zero of="$2" count=1 bs=512 seek="$sector_no" conv=notrunc 2>/dev/null 4 | dd if="$1" of="$2" count=1 bs=512 seek="$sector_no" conv=notrunc 2>/dev/null 5 | -------------------------------------------------------------------------------- /examples/all.asm: -------------------------------------------------------------------------------- 1 | ; Show off what we support. 2 | call message 3 | jmp message 4 | jb message 5 | jnb message 6 | jz message 7 | jnz message 8 | jbe message 9 | jnbe message 10 | stosb 11 | stosw 12 | lodsb 13 | lodsw 14 | cbw 15 | push ax 16 | pop bx 17 | ret 18 | add al,ah 19 | and ah,al 20 | xor bx,sp 21 | cmp ax,cx 22 | mov ax,bx 23 | add al,0o123 24 | and ah,0o123 25 | xor bx,0o123 26 | cmp cx,0o123 27 | mov bh,0o123 28 | mov bx,0o123 29 | int 0o20 30 | message: 31 | db 0o0 32 | 33 | -------------------------------------------------------------------------------- /examples/hello_world.asm: -------------------------------------------------------------------------------- 1 | mov si,message 2 | xor ax,ax 3 | lp: 4 | lodsb 5 | push si 6 | push ax 7 | int 0o42 8 | pop ax 9 | pop si 10 | and al,al 11 | jnz lp 12 | int 0o40 13 | message: 14 | db 0o110 15 | db 0o145 16 | db 0o154 17 | db 0o154 18 | db 0o157 19 | db 0o54 20 | db 0o40 21 | db 0o167 22 | db 0o157 23 | db 0o162 24 | db 0o154 25 | db 0o144 26 | db 0o41 27 | db 0o0 28 | -------------------------------------------------------------------------------- /gdb-16bit.xml: -------------------------------------------------------------------------------- 1 | 2 | 7 | 8 | 9 | 10 | i8086 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 | -------------------------------------------------------------------------------- /gdb-real-mode: -------------------------------------------------------------------------------- 1 | # Special mode for GDB that allows to debug/disassemble REAL MODE x86 code 2 | # 3 | # It has been designed to be used with QEMU or BOCHS gdb-stub 4 | # 5 | # 08/2011 Hugo Mercier - GPL v3 license 6 | # 7 | # Freely inspired from "A user-friendly gdb configuration file" widely available 8 | # on the Internet 9 | 10 | set confirm off 11 | set verbose off 12 | set prompt \033[31mreal-mode-gdb$ \033[0m 13 | 14 | set output-radix 0d10 15 | set input-radix 0d10 16 | 17 | # These make gdb never pause in its output 18 | set height 0 19 | set width 0 20 | 21 | # Intel syntax 22 | set disassembly-flavor intel 23 | # Real mode 24 | set architecture i8086 25 | 26 | set $SHOW_CONTEXT = 1 27 | 28 | set $REAL_MODE = 1 29 | 30 | # By default A20 is present 31 | set $ADDRESS_MASK = 0x1FFFFF 32 | 33 | # nb of instructions to display 34 | set $CODE_SIZE = 10 35 | 36 | define enable-a20 37 | set $ADDRESS_MASK = 0x1FFFFF 38 | end 39 | define disable-a20 40 | set $ADDRESS_MASK = 0x0FFFFF 41 | end 42 | 43 | # convert segment:offset address to physical address 44 | define r2p 45 | if $argc < 2 46 | printf "Arguments: segment offset\n" 47 | else 48 | set $ADDR = (((unsigned long)$arg0 & 0xFFFF) << 4) + (((unsigned long)$arg1 & 0xFFFF) & $ADDRESS_MASK) 49 | printf "0x%05X\n", $ADDR 50 | end 51 | end 52 | document r2p 53 | Convert segment:offset address to physical address 54 | Set the global variable $ADDR to the computed one 55 | end 56 | 57 | # get address of Interruption 58 | define int_addr 59 | if $argc < 1 60 | printf "Argument: interruption_number\n" 61 | else 62 | set $offset = (unsigned short)*($arg0 * 4) 63 | set $segment = (unsigned short)*($arg0 * 4 + 2) 64 | r2p $segment $offset 65 | printf "%04X:%04X\n", $segment, $offset 66 | end 67 | end 68 | document int_addr 69 | Get address of interruption 70 | end 71 | 72 | define compute_regs 73 | set $rax = ((unsigned long)$eax & 0xFFFF) 74 | set $rbx = ((unsigned long)$ebx & 0xFFFF) 75 | set $rcx = ((unsigned long)$ecx & 0xFFFF) 76 | set $rdx = ((unsigned long)$edx & 0xFFFF) 77 | set $rsi = ((unsigned long)$esi & 0xFFFF) 78 | set $rdi = ((unsigned long)$edi & 0xFFFF) 79 | set $rbp = ((unsigned long)$ebp & 0xFFFF) 80 | set $rsp = ((unsigned long)$esp & 0xFFFF) 81 | set $rcs = ((unsigned long)$cs & 0xFFFF) 82 | set $rds = ((unsigned long)$ds & 0xFFFF) 83 | set $res = ((unsigned long)$es & 0xFFFF) 84 | set $rss = ((unsigned long)$ss & 0xFFFF) 85 | set $rip = ((((unsigned long)$cs & 0xFFFF) << 4) + ((unsigned long)$eip & 0xFFFF)) & $ADDRESS_MASK 86 | set $r_ss_sp = ((((unsigned long)$ss & 0xFFFF) << 4) + ((unsigned long)$esp & 0xFFFF)) & $ADDRESS_MASK 87 | set $r_ss_bp = ((((unsigned long)$ss & 0xFFFF) << 4) + ((unsigned long)$ebp & 0xFFFF)) & $ADDRESS_MASK 88 | end 89 | 90 | define print_regs 91 | printf "AX: %04X BX: %04X ", $rax, $rbx 92 | printf "CX: %04X DX: %04X\n", $rcx, $rdx 93 | printf "SI: %04X DI: %04X ", $rsi, $rdi 94 | printf "SP: %04X BP: %04X\n", $rsp, $rbp 95 | printf "CS: %04X DS: %04X ", $rcs, $rds 96 | printf "ES: %04X SS: %04X\n", $res, $rss 97 | printf "\n" 98 | printf "IP: %04X EIP:%08X\n", ((unsigned short)$eip & 0xFFFF), $eip 99 | printf "CS:IP: %04X:%04X (0x%05X)\n", $rcs, ((unsigned short)$eip & 0xFFFF), $rip 100 | printf "SS:SP: %04X:%04X (0x%05X)\n", $rss, $rsp, $r_ss_sp 101 | printf "SS:BP: %04X:%04X (0x%05X)\n", $rss, $rbp, $r_ss_bp 102 | end 103 | document print_regs 104 | Print CPU registers 105 | end 106 | 107 | define print_eflags 108 | printf "OF <%d> DF <%d> IF <%d> TF <%d>",\ 109 | (($eflags >> 0xB) & 1), (($eflags >> 0xA) & 1), \ 110 | (($eflags >> 9) & 1), (($eflags >> 8) & 1) 111 | printf " SF <%d> ZF <%d> AF <%d> PF <%d> CF <%d>\n",\ 112 | (($eflags >> 7) & 1), (($eflags >> 6) & 1),\ 113 | (($eflags >> 4) & 1), (($eflags >> 2) & 1), ($eflags & 1) 114 | printf "ID <%d> VIP <%d> VIF <%d> AC <%d>",\ 115 | (($eflags >> 0x15) & 1), (($eflags >> 0x14) & 1), \ 116 | (($eflags >> 0x13) & 1), (($eflags >> 0x12) & 1) 117 | printf " VM <%d> RF <%d> NT <%d> IOPL <%d>\n",\ 118 | (($eflags >> 0x11) & 1), (($eflags >> 0x10) & 1),\ 119 | (($eflags >> 0xE) & 1), (($eflags >> 0xC) & 3) 120 | end 121 | document print_eflags 122 | Print eflags register. 123 | end 124 | 125 | # dump content of bytes in memory 126 | # arg0 : addr 127 | # arg1 : nb of bytes 128 | define _dump_memb 129 | if $argc < 2 130 | printf "Arguments: address number_of_bytes\n" 131 | else 132 | set $_nb = $arg1 133 | set $_i = 0 134 | set $_addr = $arg0 135 | while ($_i < $_nb) 136 | printf "%02X ", *((unsigned char*)$_addr + $_i) 137 | set $_i++ 138 | end 139 | end 140 | end 141 | 142 | # dump content of memory in words 143 | # arg0 : addr 144 | # arg1 : nb of words 145 | define _dump_memw 146 | if $argc < 2 147 | printf "Arguments: address number_of_words\n" 148 | else 149 | set $_nb = $arg1 150 | set $_i = 0 151 | set $_addr = $arg0 152 | while ($_i < $_nb) 153 | printf "%04X ", *((unsigned short*)$_addr + $_i) 154 | set $_i++ 155 | end 156 | end 157 | end 158 | 159 | # display data at given address 160 | define print_data 161 | if ($argc > 0) 162 | set $seg = $arg0 163 | set $off = $arg1 164 | set $raddr = ($arg0 << 16) + $arg1 165 | set $maddr = ($arg0 << 4) + $arg1 166 | 167 | set $w = 16 168 | set $i = (int)0 169 | while ($i < 4) 170 | printf "%08X: ", ($raddr + $i * $w) 171 | set $j = (int)0 172 | while ($j < $w) 173 | printf "%02X ", *(unsigned char*)($maddr + $i * $w + $j) 174 | set $j++ 175 | end 176 | printf " " 177 | set $j = (int)0 178 | while ($j < $w) 179 | set $c = *(unsigned char*)($maddr + $i * $w + $j) 180 | if ($c > 32) && ($c < 128) 181 | printf "%c", $c 182 | else 183 | printf "." 184 | end 185 | set $j++ 186 | end 187 | printf "\n" 188 | set $i++ 189 | end 190 | 191 | 192 | end 193 | end 194 | 195 | define context 196 | printf "---------------------------[ STACK ]---\n" 197 | _dump_memw $r_ss_sp 8 198 | printf "\n" 199 | set $_a = $r_ss_sp + 16 200 | _dump_memw $_a 8 201 | printf "\n" 202 | printf "---------------------------[ DS:SI ]---\n" 203 | print_data $ds $rsi 204 | printf "---------------------------[ ES:DI ]---\n" 205 | print_data $es $rdi 206 | 207 | printf "----------------------------[ CPU ]----\n" 208 | print_regs 209 | print_eflags 210 | printf "---------------------------[ CODE ]----\n" 211 | 212 | set $_code_size = $CODE_SIZE 213 | 214 | # disassemble 215 | # first call x/i with an address 216 | # subsequent calls to x/i will increment address 217 | if ($_code_size > 0) 218 | x /i $rip 219 | set $_code_size-- 220 | end 221 | while ($_code_size > 0) 222 | x /i 223 | set $_code_size-- 224 | end 225 | end 226 | document context 227 | Print context window, i.e. regs, stack, ds:esi and disassemble cs:eip. 228 | end 229 | 230 | define hook-stop 231 | compute_regs 232 | if ($SHOW_CONTEXT > 0) 233 | context 234 | end 235 | end 236 | document hook-stop 237 | !!! FOR INTERNAL USE ONLY - DO NOT CALL !!! 238 | end 239 | 240 | # add a breakpoint on an interrupt 241 | define break_int 242 | set $offset = (unsigned short)*($arg0 * 4) 243 | set $segment = (unsigned short)*($arg0 * 4 + 2) 244 | 245 | break *$offset 246 | end 247 | 248 | define break_int_if_ah 249 | if ($argc < 2) 250 | printf "Arguments: INT_N AH\n" 251 | else 252 | set $addr = (unsigned short)*($arg0 * 4) 253 | set $segment = (unsigned short)*($arg0 * 4 + 2) 254 | break *$addr if ((unsigned long)$eax & 0xFF00) == ($arg1 << 8) 255 | end 256 | end 257 | document break_int_if_ah 258 | Install a breakpoint on INT N only if AH is equal to the expected value 259 | end 260 | 261 | define break_int_if_ax 262 | if ($argc < 2) 263 | printf "Arguments: INT_N AX\n" 264 | else 265 | set $addr = (unsigned short)*($arg0 * 4) 266 | set $segment = (unsigned short)*($arg0 * 4 + 2) 267 | break *$addr if ((unsigned long)$eax & 0xFFFF) == $arg1 268 | end 269 | end 270 | document break_int_if_ax 271 | Install a breakpoint on INT N only if AX is equal to the expected value 272 | end 273 | 274 | define stepo 275 | ## we know that an opcode starting by 0xE8 has a fixed length 276 | ## for the 0xFF opcodes, we can enumerate what is possible to have 277 | 278 | set $lip = $rip 279 | set $offset = 0 280 | 281 | # first, get rid of segment prefixes, if any 282 | set $_byte1 = *(unsigned char *)$rip 283 | # CALL DS:xx CS:xx, etc. 284 | if ($_byte1 == 0x3E || $_byte1 == 0x26 || $_byte1 == 0x2E || $_byte1 == 0x36 || $_byte1 == 0x3E || $_byte1 == 0x64 || $_byte1 == 0x65) 285 | set $lip = $rip + 1 286 | set $_byte1 = *(unsigned char*)$lip 287 | set $offset = 1 288 | end 289 | set $_byte2 = *(unsigned char *)($lip+1) 290 | set $_byte3 = *(unsigned char *)($lip+2) 291 | 292 | set $noffset = 0 293 | 294 | if ($_byte1 == 0xE8) 295 | # call near 296 | set $noffset = 3 297 | else 298 | if ($_byte1 == 0xFF) 299 | # A "ModR/M" byte follows 300 | set $_mod = ($_byte2 & 0xC0) >> 6 301 | set $_reg = ($_byte2 & 0x38) >> 3 302 | set $_rm = ($_byte2 & 7) 303 | #printf "mod: %d reg: %d rm: %d\n", $_mod, $_reg, $_rm 304 | 305 | # only for CALL instructions 306 | if ($_reg == 2 || $_reg == 3) 307 | 308 | # default offset 309 | set $noffset = 2 310 | 311 | if ($_mod == 0) 312 | if ($_rm == 6) 313 | # a 16bit address follows 314 | set $noffset = 4 315 | end 316 | else 317 | if ($_mod == 1) 318 | # a 8bit displacement follows 319 | set $noffset = 3 320 | else 321 | if ($_mod == 2) 322 | # 16bit displacement 323 | set $noffset = 4 324 | end 325 | end 326 | end 327 | 328 | end 329 | # end of _reg == 2 or _reg == 3 330 | 331 | else 332 | # else byte1 != 0xff 333 | if ($_byte1 == 0x9A) 334 | # call far 335 | set $noffset = 5 336 | else 337 | if ($_byte1 == 0xCD) 338 | # INTERRUPT CASE 339 | set $noffset = 2 340 | end 341 | end 342 | 343 | end 344 | # end of byte1 == 0xff 345 | end 346 | # else byte1 != 0xe8 347 | 348 | # if we have found a call to bypass we set a temporary breakpoint on next instruction and continue 349 | if ($noffset != 0) 350 | set $_nextaddress = $eip + $offset + $noffset 351 | printf "Setting BP to %04X\n", $_nextaddress 352 | tbreak *$_nextaddress 353 | continue 354 | # else we just single step 355 | else 356 | nexti 357 | end 358 | end 359 | document stepo 360 | Step over calls 361 | This function will set a temporary breakpoint on next instruction after the call so the call will be bypassed 362 | You can safely use it instead nexti since it will single step code if it's not a call instruction (unless you want to go into the call function) 363 | end 364 | 365 | define step_until_iret 366 | set $SHOW_CONTEXT=0 367 | set $_found = 0 368 | while (!$_found) 369 | if (*(unsigned char*)$rip == 0xCF) 370 | set $_found = 1 371 | else 372 | stepo 373 | end 374 | end 375 | set $SHOW_CONTEXT=1 376 | context 377 | end 378 | 379 | define step_until_ret 380 | set $SHOW_CONTEXT=0 381 | set $_found = 0 382 | while (!$_found) 383 | set $_p = *(unsigned char*)$rip 384 | if ($_p == 0xC3 || $_p == 0xCB || $_p == 0xC2 || $_p == 0xCA) 385 | set $_found = 1 386 | else 387 | stepo 388 | end 389 | end 390 | set $SHOW_CONTEXT=1 391 | context 392 | end 393 | 394 | define step_until_int 395 | set $SHOW_CONTEXT = 0 396 | 397 | while (*(unsigned char*)$rip != 0xCD) 398 | stepo 399 | end 400 | set $SHOW_CONTEXT = 1 401 | context 402 | end 403 | 404 | # Find a pattern in memory 405 | # The pattern is given by a string as arg0 406 | # If another argument is present it gives the starting address (0 otherwise) 407 | define find_in_mem 408 | if ($argc >= 2) 409 | set $_addr = $arg1 410 | else 411 | set $_addr = 0 412 | end 413 | set $_found = 0 414 | set $_tofind = $arg0 415 | while ($_addr < $ADDRESS_MASK) && (!$_found) 416 | if ($_addr % 0x100 == 0) 417 | printf "%08X\n", $_addr 418 | end 419 | set $_i = 0 420 | set $_found = 1 421 | while ($_tofind[$_i] != 0 && $_found == 1) 422 | set $_b = *((char*)$_addr + $_i) 423 | set $_t = (char)$_tofind[$_i] 424 | if ($_t != $_b) 425 | set $_found = 0 426 | end 427 | set $_i++ 428 | end 429 | if ($_found == 1) 430 | printf "Code found at 0x%05X\n", $_addr 431 | end 432 | set $_addr++ 433 | end 434 | end 435 | document find_in_mem 436 | Find a pattern in memory 437 | The pattern is given by a string as arg0 438 | If another argument is present it gives the starting address (0 otherwise) 439 | end 440 | 441 | 442 | define step_until_code 443 | set $_tofind = $arg0 444 | set $SHOW_CONTEXT = 0 445 | 446 | set $_found = 0 447 | while (!$_found) 448 | set $_i = 0 449 | set $_found = 1 450 | 451 | while ($_tofind[$_i] != 0 && $_found == 1) 452 | set $_b = *((char*)$rip + $_i) 453 | set $_t = (char)$_tofind[$_i] 454 | if ($_t != $_b) 455 | set $_found = 0 456 | end 457 | set $_i++ 458 | end 459 | 460 | if ($_found == 0) 461 | stepo 462 | end 463 | end 464 | 465 | set $SHOW_CONTEXT = 1 466 | context 467 | end 468 | 469 | # Added for 0asm. 470 | # Attach to QEMU. 471 | target remote 0:1234 472 | # Use 16-bit architecture. 473 | set tdesc filename gdb-16bit.xml 474 | # Break on entry. 475 | break *0x7c00 476 | continue 477 | -------------------------------------------------------------------------------- /os.img: -------------------------------------------------------------------------------- https://raw.githubusercontent.com/kvakil/0asm/f75a7b86a03815faf4616576d7dc616d3202b2b9/os.img -------------------------------------------------------------------------------- /osbase.img: -------------------------------------------------------------------------------- https://raw.githubusercontent.com/kvakil/0asm/f75a7b86a03815faf4616576d7dc616d3202b2b9/osbase.img -------------------------------------------------------------------------------- /test.asm: -------------------------------------------------------------------------------- 1 | mov si,message 2 | xor ax,ax 3 | lp: 4 | lodsb 5 | push si 6 | push ax 7 | int 0o42 8 | pop ax 9 | pop si 10 | and al,al 11 | jnz lp 12 | int 0o40 13 | message: 14 | db 0o110 15 | db 0o145 16 | db 0o154 17 | db 0o154 18 | db 0o157 19 | db 0o54 20 | db 0o40 21 | db 0o167 22 | db 0o157 23 | db 0o162 24 | db 0o154 25 | db 0o144 26 | db 0o41 27 | db 0o0 28 | --------------------------------------------------------------------------------