├── .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:
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1 | 0asm.bin
2 | 0asm.elf
3 | 0asm.lst
4 | 0asm.o
5 |
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/0asm.asm:
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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/
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451 | for enforcing compliance by third parties with this License.
452 |
453 | An "entity transaction" is a transaction transferring control of an
454 | organization, or substantially all assets of one, or subdividing an
455 | organization, or merging organizations. If propagation of a covered
456 | work results from an entity transaction, each party to that
457 | transaction who receives a copy of the work also receives whatever
458 | licenses to the work the party's predecessor in interest had or could
459 | give under the previous paragraph, plus a right to possession of the
460 | Corresponding Source of the work from the predecessor in interest, if
461 | the predecessor has it or can get it with reasonable efforts.
462 |
463 | You may not impose any further restrictions on the exercise of the
464 | rights granted or affirmed under this License. For example, you may
465 | not impose a license fee, royalty, or other charge for exercise of
466 | rights granted under this License, and you may not initiate litigation
467 | (including a cross-claim or counterclaim in a lawsuit) alleging that
468 | any patent claim is infringed by making, using, selling, offering for
469 | sale, or importing the Program or any portion of it.
470 |
471 | 11. Patents.
472 |
473 | A "contributor" is a copyright holder who authorizes use under this
474 | License of the Program or a work on which the Program is based. The
475 | work thus licensed is called the contributor's "contributor version".
476 |
477 | A contributor's "essential patent claims" are all patent claims
478 | owned or controlled by the contributor, whether already acquired or
479 | hereafter acquired, that would be infringed by some manner, permitted
480 | by this License, of making, using, or selling its contributor version,
481 | but do not include claims that would be infringed only as a
482 | consequence of further modification of the contributor version. For
483 | purposes of this definition, "control" includes the right to grant
484 | patent sublicenses in a manner consistent with the requirements of
485 | this License.
486 |
487 | Each contributor grants you a non-exclusive, worldwide, royalty-free
488 | patent license under the contributor's essential patent claims, to
489 | make, use, sell, offer for sale, import and otherwise run, modify and
490 | propagate the contents of its contributor version.
491 |
492 | In the following three paragraphs, a "patent license" is any express
493 | agreement or commitment, however denominated, not to enforce a patent
494 | (such as an express permission to practice a patent or covenant not to
495 | sue for patent infringement). To "grant" such a patent license to a
496 | party means to make such an agreement or commitment not to enforce a
497 | patent against the party.
498 |
499 | If you convey a covered work, knowingly relying on a patent license,
500 | and the Corresponding Source of the work is not available for anyone
501 | to copy, free of charge and under the terms of this License, through a
502 | publicly available network server or other readily accessible means,
503 | then you must either (1) cause the Corresponding Source to be so
504 | available, or (2) arrange to deprive yourself of the benefit of the
505 | patent license for this particular work, or (3) arrange, in a manner
506 | consistent with the requirements of this License, to extend the patent
507 | license to downstream recipients. "Knowingly relying" means you have
508 | actual knowledge that, but for the patent license, your conveying the
509 | covered work in a country, or your recipient's use of the covered work
510 | in a country, would infringe one or more identifiable patents in that
511 | country that you have reason to believe are valid.
512 |
513 | If, pursuant to or in connection with a single transaction or
514 | arrangement, you convey, or propagate by procuring conveyance of, a
515 | covered work, and grant a patent license to some of the parties
516 | receiving the covered work authorizing them to use, propagate, modify
517 | or convey a specific copy of the covered work, then the patent license
518 | you grant is automatically extended to all recipients of the covered
519 | work and works based on it.
520 |
521 | A patent license is "discriminatory" if it does not include within
522 | the scope of its coverage, prohibits the exercise of, or is
523 | conditioned on the non-exercise of one or more of the rights that are
524 | specifically granted under this License. You may not convey a covered
525 | work if you are a party to an arrangement with a third party that is
526 | in the business of distributing software, under which you make payment
527 | to the third party based on the extent of your activity of conveying
528 | the work, and under which the third party grants, to any of the
529 | parties who would receive the covered work from you, a discriminatory
530 | patent license (a) in connection with copies of the covered work
531 | conveyed by you (or copies made from those copies), or (b) primarily
532 | for and in connection with specific products or compilations that
533 | contain the covered work, unless you entered into that arrangement,
534 | or that patent license was granted, prior to 28 March 2007.
535 |
536 | Nothing in this License shall be construed as excluding or limiting
537 | any implied license or other defenses to infringement that may
538 | otherwise be available to you under applicable patent law.
539 |
540 | 12. No Surrender of Others' Freedom.
541 |
542 | If conditions are imposed on you (whether by court order, agreement or
543 | otherwise) that contradict the conditions of this License, they do not
544 | excuse you from the conditions of this License. If you cannot convey a
545 | covered work so as to satisfy simultaneously your obligations under this
546 | License and any other pertinent obligations, then as a consequence you may
547 | not convey it at all. For example, if you agree to terms that obligate you
548 | to collect a royalty for further conveying from those to whom you convey
549 | the Program, the only way you could satisfy both those terms and this
550 | License would be to refrain entirely from conveying the Program.
551 |
552 | 13. Use with the GNU Affero General Public License.
553 |
554 | Notwithstanding any other provision of this License, you have
555 | permission to link or combine any covered work with a work licensed
556 | under version 3 of the GNU Affero General Public License into a single
557 | combined work, and to convey the resulting work. The terms of this
558 | License will continue to apply to the part which is the covered work,
559 | but the special requirements of the GNU Affero General Public License,
560 | section 13, concerning interaction through a network will apply to the
561 | combination as such.
562 |
563 | 14. Revised Versions of this License.
564 |
565 | The Free Software Foundation may publish revised and/or new versions of
566 | the GNU General Public License from time to time. 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 |
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--------------------------------------------------------------------------------
/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 |
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/os.img:
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https://raw.githubusercontent.com/kvakil/0asm/f75a7b86a03815faf4616576d7dc616d3202b2b9/os.img
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/osbase.img:
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https://raw.githubusercontent.com/kvakil/0asm/f75a7b86a03815faf4616576d7dc616d3202b2b9/osbase.img
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/test.asm:
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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 |
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