BBC BASIC

BBC BASIC is the native BASIC interpreter on the BBC Micro — and it's a cut above most other 8-bit BASICs. Designed by Sophie Wilson at Acorn, it includes inline 6502 assembly, named procedures and functions, REPEAT...UNTIL and WHILE...ENDWHILE loops, local variables, and a proper ELSE clause. It feels surprisingly modern compared to Commodore BASIC V2 or Spectrum BASIC.

In the IDE, select the BBC Micro platform and the BBC BASIC preset to get a JSBeeb-powered emulator running in your browser.

The basics

BBC BASIC programs use line numbers, like most 8-bit BASICs. But the structured control flow means you use them far less than on the C64.

10 PRINT "HELLO FROM BBC BASIC!"
20 PRINT "THE YEAR IS ";1982
30 END

Run it in the emulator

After building in the IDE, the BBC Micro emulator boots and runs your program automatically. You can type at the BASIC prompt in the emulator for quick tests.

Variables

BBC BASIC has a more flexible type system than Commodore BASIC V2:

Type	Suffix	Example	Notes
Float	(none)	`X = 3.14`	Default numeric type
Integer	`%`	`X% = 100`	Faster; 32-bit range
String	`$`	`N$ = "CHRIS"`	Variable length
Integer array	`%()`	`A%(10)`	Declared with DIM

Unlike Commodore BASIC, BBC BASIC integers are 32-bit — very handy for scores and coordinates without overflow worries.

Control flow

IF / THEN / ELSE

10 IF score% > 100 THEN PRINT "HIGH" ELSE PRINT "LOW"

20 IF lives% = 0 THEN
30   PRINT "GAME OVER"
40   END
50 ENDIF

FOR / NEXT

10 FOR I% = 1 TO 10
20   PRINT I%
30 NEXT I%

40 FOR I% = 0 TO 255 STEP 16
50   COLOUR I% MOD 8
60 NEXT I%

REPEAT / UNTIL

10 REPEAT
20   REM ... game loop ...
30 UNTIL quit% = TRUE

WHILE / ENDWHILE

10 WHILE lives% > 0
20   PROC_game_loop
30 ENDWHILE

CASE ... OF (BBC BASIC V)

10 CASE key$ OF
20   WHEN "W" : y% = y% - 1
30   WHEN "S" : y% = y% + 1
40   WHEN "A" : x% = x% - 1
50   WHEN "D" : x% = x% + 1
60   OTHERWISE : REM No key
70 ENDCASE

Procedures and functions

This is where BBC BASIC really shines. You can define proper named subroutines with local variables:

100 DEF PROC_draw_score
110   LOCAL col%
120   col% = score% MOD 7 + 1   : REM Colour cycles with score
130   COLOUR col%
140   PRINT TAB(0,0);"SCORE: ";score%
150 ENDPROC

200 DEF FN_clamp(val%, lo%, hi%)
210 IF val% < lo% THEN = lo%
220 IF val% > hi% THEN = hi%
230 = val%

PROC_ prefix by convention for procedures; FN_ prefix for functions that return values. LOCAL variables are scoped to the procedure — a huge improvement over global-only BASIC V2.

Screen and graphics

The BBC Micro has several graphics modes, selectable with MODE:

Mode	Resolution	Colours	Characters
0	640×256	2	80×32
1	320×256	4	40×32
2	160×256	16	20×32
4	320×256	2	40×32
5	160×256	4	20×32
7	Teletext	8+flash	40×25

10 MODE 2          : REM 160x256, 16 colours
20 CLS             : REM Clear screen
30 COLOUR 3        : REM Set text colour (3 = yellow in mode 2)
40 PRINT "HELLO"

Graphics primitives

BBC BASIC has built-in graphics commands — no POKE required:

10 MODE 1          : REM 320x256, 4 colours
20 GCOL 0, 3       : REM Set graphics colour to 3
30 DRAW 320, 256   : REM Draw line to (320, 256)
40 MOVE 0, 0       : REM Move graphics cursor
50 PLOT 69, 160, 128  : REM Plot filled triangle vertex

60 CLG             : REM Clear graphics area

Graphics coordinates in BBC BASIC use a 1280×1024 virtual grid regardless of mode — the hardware scales for you.

10 MODE 2
20 FOR angle = 0 TO 360 STEP 5
30   x% = 640 + 400 * COS(RAD(angle))
40   y% = 512 + 400 * SIN(RAD(angle))
50   GCOL 0, angle MOD 8
60   PLOT 69, x%, y%
70 NEXT angle

Input

Reading the keyboard

10 k$ = INKEY$(0)    : REM Non-blocking — returns "" if no key
20 IF k$ = "W" THEN y% = y% - 1

30 k$ = GET$         : REM Blocking — waits for keypress

INKEY$ with a timeout of 0 is equivalent to Commodore BASIC's GET — non-blocking, perfect for game loops.

You can also check specific keys by their scan code:

IF INKEY(-105) THEN PRINT "SPACE PRESSED"  : REM -105 = Space
IF INKEY(-58)  THEN PRINT "Z PRESSED"

Reading the joystick / ADC

ADVAL(1)   : REM Joystick 1 X axis (0-65520)
ADVAL(2)   : REM Joystick 1 Y axis
ADVAL(3)   : REM Joystick 2 X axis
ADVAL(4)   : REM Joystick 2 Y axis
ADVAL(0)   : REM Fire buttons (bit field)

Inline 6502 assembly

One of BBC BASIC's killer features — you can write assembly inline with [:

10 DIM code% 20          : REM Reserve 20 bytes for ML code
20 P% = code%            : REM Set assembler PC to start of buffer
30 [
40 OPT 2                 : REM Assemble + list
50 LDA #2
60 STA &D020             : REM Not valid on BBC — this is C64 syntax, just for demo
70 RTS
80 ]
90 CALL code%            : REM Execute the assembled code

CALL address executes machine code. USR(address) calls it and returns the value in A (and X in the high byte). The assembler is built right into BBC BASIC — no separate tool needed.

Sound

10 SOUND 1, -15, 53, 20    : REM Channel, amplitude, pitch, duration
20 SOUND 1, -15, 101, 10
30 ENVELOPE 1, 3, 1, -1, -1, 5, 5, 5, 126, 0, 0, -126, 126, 0

BBC BASIC's SOUND and ENVELOPE commands give you direct access to the BBC Micro's 4-channel sound chip (SN76489). ENVELOPE defines ADSR envelopes for more interesting tones.

A minimal game loop

1  REM ** SIMPLE MOVER **
5  MODE 4 : VDU 23,1,0;0;0;0;   : REM Mode 4, hide cursor
10 x% = 320 : y% = 128

100 REM ---- MAIN LOOP ----
110 GCOL 0, 0 : PLOT 69, x%, y% : REM Erase (plot in background colour)
120 k$ = INKEY$(0)
130 IF k$ = CHR$(136) THEN x% = x% - 8  : REM Cursor left
140 IF k$ = CHR$(137) THEN x% = x% + 8  : REM Cursor right
150 IF k$ = CHR$(138) THEN y% = y% - 8  : REM Cursor down
160 IF k$ = CHR$(139) THEN y% = y% + 8  : REM Cursor up
170 x% = FN_clamp(x%, 0, 1280)
180 y% = FN_clamp(y%, 0, 1024)
190 GCOL 0, 3 : PLOT 69, x%, y% : REM Draw in colour 3
200 GOTO 100

1000 DEF FN_clamp(v%, lo%, hi%)
1010 IF v% < lo% THEN = lo%
1020 IF v% > hi% THEN = hi%
1030 = v%

Common mistakes

Forgetting ENDPROC — every DEF PROC_... must end with ENDPROC. Missing it gives a No ENDPROC error.

FN_ functions return with = value — there's no RETURN value syntax. The last executed = expression is the return value.

Graphics coordinates vs text coordinates — PRINT TAB(x,y) uses text column/row coordinates; MOVE x,y and DRAW x,y use the 1280×1024 graphics coordinate system. They're independent.

LOCAL variable scope — local variables in a PROC are only visible inside that procedure. You can't read them from the calling code. This is a feature, not a bug — use it to avoid name collisions.