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basic.lst
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1 ;
2 ; bootBASIC interpreter in 512 bytes (boot sector)
3 ;
4 ; by Oscar Toledo G.
5 ; http://nanochess.org/
6 ;
7 ; (c) Copyright 2019-2020 Oscar Toledo G.
8 ;
9 ; Creation date: Jul/19/2019. 10pm to 12am.
10 ; Revision date: Jul/20/2019. 10am to 2pm.
11 ; Added assignment statement. list now
12 ; works. run/goto now works. Added
13 ; system and new.
14 ; Revision date: Jul/22/2019. Boot image now includes 'system'
15 ; statement.
16 ;
17
18 ;
19 ; USER'S MANUAL:
20 ;
21 ; Line entry is done with keyboard, finish the line with Enter.
22 ; Only 19 characters per line as maximum.
23 ;
24 ; Backspace can be used, don't be fooled by the fact
25 ; that screen isn't deleted (it's all right in the buffer).
26 ;
27 ; All statements must be in lowercase.
28 ;
29 ; Line numbers can be 1 to 999.
30 ;
31 ; 26 variables are available (a-z)
32 ;
33 ; Numbers (0-65535) can be entered and display as unsigned.
34 ;
35 ; To enter new program lines:
36 ; 10 print "Hello all!"
37 ;
38 ; To erase program lines:
39 ; 10
40 ;
41 ; To test statements directly (interactive syntax):
42 ; print "Hello all!"
43 ;
44 ; To erase the current program:
45 ; new
46 ;
47 ; To run the current program:
48 ; run
49 ;
50 ; To list the current program:
51 ; list
52 ;
53 ; To exit to command-line:
54 ; system
55 ;
56 ; Statements:
57 ; var=expr Assign expr value to var (a-z)
58 ;
59 ; print expr Print expression value, new line
60 ; print expr; Print expression value, continue
61 ; print "hello" Print string, new line
62 ; print "hello"; Print string, continue
63 ;
64 ; input var Input value into variable (a-z)
65 ;
66 ; goto expr Goto to indicated line in program
67 ;
68 ; if expr1 goto expr2
69 ; If expr1 is non-zero then go to line,
70 ; else go to following line.
71 ;
72 ; Examples of if:
73 ;
74 ; if c-5 goto 20 If c isn't 5, go to line 20
75 ;
76 ; Expressions:
77 ;
78 ; The operators +, -, / and * are available with
79 ; common precedence rules and signed operation.
80 ; Integer-only arithmetic.
81 ;
82 ; You can also use parentheses:
83 ;
84 ; 5+6*(10/2)
85 ;
86 ; Variables and numbers can be used in expressions.
87 ;
88 ; The rnd function (without arguments) returns a
89 ; value between 0 and 255.
90 ;
91 ; Sample program (counting 1 to 10):
92 ;
93 ; 10 a=1
94 ; 20 print a
95 ; 30 a=a+1
96 ; 40 if a-11 goto 20
97 ;
98 ; Sample program (Pascal's triangle, each number is the sum
99 ; of the two over it):
100 ;
101 ; 10 input n
102 ; 20 i=1
103 ; 30 c=1
104 ; 40 j=0
105 ; 50 t=n-i
106 ; 60 if j-t goto 80
107 ; 70 goto 110
108 ; 80 print " ";
109 ; 90 j=j+1
110 ; 100 goto 50
111 ; 110 k=1
112 ; 120 if k-i-1 goto 140
113 ; 130 goto 190
114 ; 140 print c;
115 ; 150 c=c*(i-k)/k
116 ; 160 print " ";
117 ; 170 k=k+1
118 ; 180 goto 120
119 ; 190 print
120 ; 200 i=i+1
121 ; 210 if i-n-1 goto 30
122 ;
123 ; Sample program of guessing the dice:
124 ;
125 ; 10 print "choose ";
126 ; 20 print "a number ";
127 ; 30 print "(1-6)"
128 ; 40 input a
129 ; 50 b=rnd
130 ; 60 b=b-b/6*6
131 ; 70 b=b+1
132 ; 80 if a-b goto 110
133 ; 90 print "good"
134 ; 100 goto 120
135 ; 110 print "miss"
136 ; 120 print b
137 ;
138
139 cpu 8086
140
141 %ifndef com_file ; If not defined create a boot sector
142 com_file: equ 0
143 %endif
144
145 %if com_file
146 org 0x0100
147 %else
148 org 0x7c00
149 %endif
150
151 vars: equ 0x7e00 ; Variables (multiple of 256)
152 line: equ 0x7e80 ; Line input
153
154 program: equ 0x8000 ; Program address
155 ; (notice optimizations dependent on this address)
156
157 stack: equ 0xff00 ; Stack address
158 max_line: equ 1000 ; First unavailable line number
159 max_length: equ 20 ; Maximum length of line
160 max_size: equ max_line*max_length ; Max. program size
161
162 start:
163 %if com_file
164 %else
165 00000000 0E push cs ; For boot sector
166 00000001 0E push cs ; it needs to setup
167 00000002 0E push cs ; DS, ES and SS.
168 00000003 1F pop ds
169 00000004 07 pop es
170 00000005 17 pop ss
171 %endif
172 00000006 FC cld ; Clear Direction flag
173 00000007 BF0080 mov di,program ; Point to program
174 0000000A C6050D f14: mov byte [di],0x0d ; Fill with Carriage Return (CR) character
175 0000000D 47 inc di ; Until reaching maximum 64K (DI becomes zero)
176 0000000E 75FA jne f14
177
178 ;
179 ; Main loop
180 ;
181 main_loop:
182 00000010 BC00FF mov sp,stack ; Reinitialize stack pointer
183 00000013 B8[1000] mov ax,main_loop
184 00000016 50 push ax
185 00000017 B03E mov al,'>' ; Show prompt
186 00000019 E84F01 call input_line ; Accept line
187 0000001C E81101 call input_number ; Get number
188 0000001F 09C0 or ax,ax ; No number or zero?
189 00000021 740E je statement ; Yes, jump
190 00000023 E89601 call find_line ; Find the line
191 00000026 97 xchg ax,di
192 ; mov cx,max_length ; CX loaded with this value in 'find_line'
193 00000027 F3A4 rep movsb ; Copy entered line into program
194 00000029 C3 ret
195
196 ;
197 ; Handle 'if' statement
198 ;
199 if_statement:
200 0000002A E86C00 call expr ; Process expression
201 0000002D 09C0 or ax,ax ; Is it zero?
202 0000002F 7458 je f6 ; Yes, return (ignore if)
203 statement:
204 00000031 E8E200 call spaces ; Avoid spaces
205 00000034 803C0D cmp byte [si],0x0d ; Empty line?
206 00000037 7450 je f6 ; Yes, return
207 00000039 BF[C501] mov di,statements ; Point to statements list
208 0000003C 8A05 f5: mov al,[di] ; Read length of the string
209 0000003E 47 inc di ; Avoid length byte
210 0000003F 98 cbw ; Make AH zero
211 00000040 48 dec ax ; Is it zero?
212 00000041 7413 je f4 ; Yes, jump
213 00000043 91 xchg ax,cx
214 00000044 56 push si ; Save current position
215 00000045 F3A6 f16: rep cmpsb ; Compare statement
216 00000047 7506 jne f3 ; Equal? No, jump
217 00000049 58 pop ax
218 0000004A E8C900 call spaces ; Avoid spaces
219 0000004D FF25 jmp word [di] ; Jump to process statement
220
221 0000004F 01CF f3: add di,cx ; Advance the list pointer
222 00000051 47 inc di ; Avoid the address
223 00000052 47 inc di
224 00000053 5E pop si
225 00000054 EBE6 jmp f5 ; Compare another statement
226
227 00000056 E8B400 f4: call get_variable ; Try variable
228 00000059 50 push ax ; Save address
229 0000005A AC lodsb ; Read a line letter
230 0000005B 3C3D cmp al,'=' ; Is it assignment '=' ?
231 0000005D 7434 je assignment ; Yes, jump to assignment.
232
233 ;
234 ; An error happened
235 ;
236 error:
237 0000005F BE[6700] mov si,error_message
238 00000062 E82601 call print_2 ; Show error message
239 00000065 EBA9 jmp main_loop ; Exit to main loop
240
241 error_message:
242 00000067 4023210D db "@#!",0x0d ; Guess the words :P
243
244 ;
245 ; Handle 'list' statement
246 ;
247 list_statement:
248 0000006B 31C0 xor ax,ax ; Start from line zero
249 0000006D 50 f29: push ax
250 0000006E E84B01 call find_line ; Find program line
251 00000071 96 xchg ax,si
252 00000072 803C0D cmp byte [si],0x0d ; Empty line?
253 00000075 740B je f30 ; Yes, jump
254 00000077 58 pop ax
255 00000078 50 push ax
256 00000079 E8A000 call output_number ; Show line number
257 0000007C AC f32: lodsb ; Show line contents
258 0000007D E82901 call output
259 00000080 75FA jne f32 ; Jump if it wasn't 0x0d (CR)
260 00000082 58 f30: pop ax
261 00000083 40 inc ax ; Go to next line
262 00000084 3DE803 cmp ax,max_line ; Finished?
263 00000087 75E4 jne f29 ; No, continue
264 f6:
265 00000089 C3 ret
266
267 ;
268 ; Handle 'input' statement
269 ;
270 input_statement:
271 0000008A E88000 call get_variable ; Get variable address
272 0000008D 50 push ax ; Save it
273 0000008E B03F mov al,'?' ; Prompt
274 00000090 E8D800 call input_line ; Wait for line
275 ;
276 ; Second part of the assignment statement
277 ;
278 assignment:
279 00000093 E80300 call expr ; Process expression
280 00000096 5F pop di
281 00000097 AB stosw ; Save onto variable
282 00000098 C3 ret
283
284 ;
285 ; Handle an expression.
286 ; First tier: addition & subtraction.
287 ;
288 expr:
289 00000099 E81C00 call expr1 ; Call second tier
290 0000009C 803C2D f20: cmp byte [si],'-' ; Subtraction operator?
291 0000009F 740E je f19 ; Yes, jump
292 000000A1 803C2B cmp byte [si],'+' ; Addition operator?
293 000000A4 75E3 jne f6 ; No, return
294 000000A6 50 push ax
295 000000A7 E80D00 call expr1_2 ; Call second tier
296 000000AA 59 f15: pop cx
297 000000AB 01C8 add ax,cx ; Addition
298 000000AD EBED jmp f20 ; Find more operators
299
300 f19:
301 000000AF 50 push ax
302 000000B0 E80400 call expr1_2 ; Call second tier
303 000000B3 F7D8 neg ax ; Negate it (a - b converted to a + -b)
304 000000B5 EBF3 jmp f15
305
306 ;
307 ; Handle an expression.
308 ; Second tier: division & multiplication.
309 ;
310 expr1_2:
311 000000B7 46 inc si ; Avoid operator
312 expr1:
313 000000B8 E81F00 call expr2 ; Call third tier
314 000000BB 803C2F f21: cmp byte [si],'/' ; Division operator?
315 000000BE 740E je f23 ; Yes, jump
316 000000C0 803C2A cmp byte [si],'*' ; Multiplication operator?
317 000000C3 75C4 jne f6 ; No, return
318
319 000000C5 50 push ax
320 000000C6 E81000 call expr2_2 ; Call third tier
321 000000C9 59 pop cx
322 000000CA F7E9 imul cx ; Multiplication
323 000000CC EBED jmp f21 ; Find more operators
324
325 f23:
326 000000CE 50 push ax
327 000000CF E80700 call expr2_2 ; Call third tier
328 000000D2 59 pop cx
329 000000D3 91 xchg ax,cx
330 000000D4 99 cwd ; Expand AX to DX:AX
331 000000D5 F7F9 idiv cx ; Signed division
332 000000D7 EBE2 jmp f21 ; Find more operators
333
334 ;
335 ; Handle an expression.
336 ; Third tier: parentheses, numbers and vars.
337 ;
338 expr2_2:
339 000000D9 46 inc si ; Avoid operator
340 expr2:
341 000000DA E83900 call spaces ; Jump spaces
342 000000DD AC lodsb ; Read character
343 000000DE 3C28 cmp al,'(' ; Open parenthesis?
344 000000E0 750B jne f24
345 000000E2 E8B4FF call expr ; Process inner expr.
346 000000E5 803C29 cmp byte [si],')' ; Closing parenthesis?
347 000000E8 742B je spaces_2 ; Yes, avoid spaces
348 000000EA E972FF jmp error ; No, jump to error
349
350 000000ED 3C40 f24: cmp al,0x40 ; Variable?
351 000000EF 7306 jnc f25 ; Yes, jump
352 000000F1 4E dec si ; Back one letter...
353 000000F2 E83B00 call input_number ; ...to read number
354 000000F5 EB1F jmp short spaces
355
356 000000F7 3C72 f25: cmp al,0x72
357 000000F9 750B jne f22
358 000000FB 803C6E cmp byte [si],0x6e
359 000000FE 7506 jne f22
360 00000100 AD lodsw ; Advance SI by 2
361 00000101 E440 in al,0x40 ; Read timer counter 0
362 00000103 B400 mov ah,0
363 00000105 C3 ret
364
365 00000106 E80500 f22: call get_variable_2 ; Get variable address
366 00000109 93 xchg ax,bx
367 0000010A 8B07 mov ax,[bx] ; Read
368 0000010C C3 ret ; Return
369
370 ;
371 ; Get variable address.
372 ; Also avoid spaces.
373 ;
374 get_variable:
375 0000010D AC lodsb ; Read source
376 get_variable_2:
377 0000010E 241F and al,0x1f ; 0x61-0x7a -> 0x01-0x1a
378 00000110 00C0 add al,al ; x 2 (each variable = word)
379 00000112 B47E mov ah,vars>>8 ; Setup high-byte of address
380 00000114 4E dec si
381 ;
382 ; Avoid spaces after current character
383 ;
384 spaces_2:
385 00000115 46 inc si
386 ;
387 ; Avoid spaces
388 ; The interpreter depends on this routine not modifying AX
389 ;
390 spaces:
391 00000116 803C20 cmp byte [si],' ' ; Space found?
392 00000119 74FA je spaces_2 ; Yes, move one character ahead.
393 0000011B C3 ret ; No, return.
394
395 ;
396 ; Output unsigned number
397 ; AX = value
398 ;
399 output_number:
400 f26:
401 0000011C 31D2 xor dx,dx ; DX:AX
402 0000011E B90A00 mov cx,10 ; Divisor = 10
403 00000121 F7F1 div cx ; Divide
404 00000123 09C0 or ax,ax ; Nothing at left?
405 00000125 52 push dx
406 00000126 7403 je f8 ; No, jump
407 00000128 E8F1FF call f26 ; Yes, output left side
408 0000012B 58 f8: pop ax
409 0000012C 0430 add al,'0' ; Output remainder as...
410 0000012E EB79 jmp short output ; ...ASCII digit
411
412 ;
413 ; Read number in input.
414 ; AX = result
415 ;
416 input_number:
417 00000130 31DB xor bx,bx ; BX = 0
418 00000132 AC f11: lodsb ; Read source
419 00000133 2C30 sub al,'0'
420 00000135 3C0A cmp al,10 ; Digit valid?
421 00000137 98 cbw
422 00000138 93 xchg ax,bx
423 00000139 7309 jnc f12 ; No, jump
424 0000013B B90A00 mov cx,10 ; Multiply by 10
425 0000013E F7E1 mul cx
426 00000140 01C3 add bx,ax ; Add new digit
427 00000142 EBEE jmp f11 ; Continue
428
429 00000144 4E f12: dec si ; SI points to first non-digit
430 00000145 C3 ret
431
432 ;
433 ; Handle 'system' statement
434 ;
435 system_statement:
436 00000146 CD20 int 0x20
437
438 ;
439 ; Handle 'goto' statement
440 ;
441 goto_statement:
442 00000148 E84EFF call expr ; Handle expression
443 0000014B B9 db 0xb9 ; MOV CX to jump over XOR AX,AX
444
445 ;
446 ; Handle 'run' statement
447 ; (equivalent to 'goto 0')
448 ;
449 run_statement:
450 0000014C 31C0 xor ax,ax
451 f10:
452 0000014E E86B00 call find_line ; Find line in program
453 00000151 81FCFEFE f27: cmp sp,stack-2 ; In interactive mode?
454 00000155 7404 je f31 ; Yes, jump.
455 00000157 A3FCFE mov [stack-4],ax ; No, replace the saved address of next line
456 0000015A C3 ret
457 f31:
458 0000015B 50 push ax
459 0000015C 5E pop si
460 0000015D 051400 add ax,max_length ; Point to next line
461 00000160 50 push ax ; Save for next time (this goes into address stack-4)
462 00000161 E8CDFE call statement ; Process current statement
463 00000164 58 pop ax ; Restore address of next line (could have changed)
464 00000165 3D20CE cmp ax,program+max_size ; Reached the end?
465 00000168 75F1 jne f31 ; No, continue
466 0000016A C3 ret ; Yes, return
467
468 ;
469 ; Input line from keyboard
470 ; Entry:
471 ; al = prompt character
472 ; Result:
473 ; buffer 'line' contains line, finished with CR
474 ; SI points to 'line'.
475 ;
476 input_line:
477 0000016B E83B00 call output
478 0000016E BE807E mov si,line
479 00000171 56 push si
480 00000172 5F pop di ; Target for writing line
481 00000173 E82F00 f1: call input_key ; Read keyboard
482 00000176 AA stosb ; Save key in buffer
483 00000177 3C08 cmp al,0x08 ; Backspace?
484 00000179 7502 jne f2 ; No, jump
485 0000017B 4F dec di ; Get back one character
486 0000017C 4F dec di
487 0000017D 3C0D f2: cmp al,0x0d ; CR pressed?
488 0000017F 75F2 jne f1 ; No, wait another key
489 00000181 C3 ret ; Yes, return
490
491 ;
492 ; Handle "print" statement
493 ;
494 print_statement:
495 00000182 AC lodsb ; Read source
496 00000183 3C0D cmp al,0x0d ; End of line?
497 00000185 7426 je new_line ; Yes, generate new line and return
498 00000187 3C22 cmp al,'"' ; Double quotes?
499 00000189 750D jne f7 ; No, jump
500 print_2:
501 f9:
502 0000018B AC lodsb ; Read string contents
503 0000018C 3C22 cmp al,'"' ; Double quotes?
504 0000018E 740F je f18 ; Yes, jump
505 00000190 E81600 call output ; Output character
506 00000193 3C0D cmp al,0x0d ;
507 00000195 75F4 jne f9 ; Jump if not finished with 0x0d (CR)
508 00000197 C3 ret ; Return
509
510 00000198 4E f7: dec si
511 00000199 E8FDFE call expr ; Handle expression
512 0000019C E87DFF call output_number ; Output result
513 0000019F AC f18: lodsb ; Read next character
514 000001A0 3C3B cmp al,';' ; Is it semicolon?
515 000001A2 7509 jne new_line ; No, jump to generate new line
516 000001A4 C3 ret ; Yes, return
517
518 ;
519 ; Read a key into al
520 ; Also outputs it to screen
521 ;
522 input_key:
523 000001A5 B400 mov ah,0x00
524 000001A7 CD16 int 0x16
525 ;
526 ; Screen output of character contained in al
527 ; Expands 0x0d (CR) into 0x0a 0x0d (LF CR)
528 ;
529 output:
530 000001A9 3C0D cmp al,0x0d
531 000001AB 7507 jne f17
532 ;
533 ; Go to next line (generates LF+CR)
534 ;
535 new_line:
536 000001AD B00A mov al,0x0a
537 000001AF E80200 call f17
538 000001B2 B00D mov al,0x0d
539 f17:
540 000001B4 B40E mov ah,0x0e
541 000001B6 BB0700 mov bx,0x0007
542 000001B9 CD10 int 0x10
543 000001BB C3 ret
544
545 ;
546 ; Find line in program
547 ; Entry:
548 ; ax = line number
549 ; Result:
550 ; ax = pointer to program.
551 ; cx = max. length allowed for line.
552 find_line:
553 000001BC B91400 mov cx,max_length
554 000001BF F7E1 mul cx
555 000001C1 050080 add ax,program
556 000001C4 C3 ret
557
558 ;
559 ; List of statements of bootBASIC
560 ; First one byte with length of string
561 ; Then string with statement
562 ; Then a word with the address of the code
563 ;
564 statements:
565 000001C5 046E6577 db 4,"new"
566 000001C9 [0000] dw start
567
568 000001CB 056C697374 db 5,"list"
569 000001D0 [6B00] dw list_statement
570
571 000001D2 0472756E db 4,"run"
572 000001D6 [4C01] dw run_statement
573
574 000001D8 067072696E74 db 6,"print"
575 000001DE [8201] dw print_statement
576
577 000001E0 06696E707574 db 6,"input"
578 000001E6 [8A00] dw input_statement
579
580 000001E8 036966 db 3,"if"
581 000001EB [2A00] dw if_statement
582
583 000001ED 05676F746F db 5,"goto"
584 000001F2 [4801] dw goto_statement
585
586 000001F4 0773797374656D db 7,"system"
587 000001FB [4601] dw system_statement
588
589 000001FD 01 db 1
590
591 ;
592 ; Boot sector filler
593 ;
594 %if com_file
595 %else
596 times 510-($-$$) db 0x4f
597 000001FE 55AA db 0x55,0xaa ; Make it a bootable sector
598 %endif
599