This is a port of BBC BASIC 3.10 for the Atari XL/XE computers. 3.10 was the last version that ran on an NMOS 6502. Later versions required a CMOS 65C02, and won't run on an unmodified Atari.
You can download the latest release here.
The BBC BASIC dialect is described in the BBC Microcomputer User Guide. A quick overview can be found here. One of the most attractive features is the inbuilt assembler. It's described in the aformentioned user guide. More information and tips & tricks can be found in Creative Assembler.
BBC BASIC 3.10 for the Atari XL/XE runs on any XL/XE compatible machine with at least 64kB of RAM, and a floppy drive. If you're familiar with Turbo Basic 1.5, this works more or less the same. A small portion of the BBC BASIC interpreter resides in main RAM, but most of the interpreter runs from the RAM under the OS ROM. Also in main RAM is a translation layer that implements most of the MOS (the BBC operating system) calls needed by the BASIC interpreter. See the memory map below for details.
To enter BASIC programs, the usual Atari E: (Editor) device driver is used for keyboard input and screen output.
So unlike the BBC, you don't need a separate editor to comfortably edit you programs.
You can use the cursor keys to move around, make changes, insert and delete characters, and press RETURN to commit the changes.
It correctly distinguishes between logical and physical lines, just like Atari BASIC, unless you use WIDTH to change the terminal width to something other than 255.
Don't do that, as the editor won't know where a logical line starts or ends anymore.
If you want different left or right margins, poke the appropriate Atari OS memory locations (e.g. LMARGN with ?&52=0 and RMARGN with ?&53=30).
The Atari is not an Acorn computer, so in order to turn BBC BASIC into a useful BASIC for the Atari, some compromises had to be made. Instead of doing a poor BBC emulation, some of the commands work differenty than they would on the BBC in order to make them more useful on the Atari.
-
The BREAK key interrupts like ESCape on the BBC. Pressing the RESET key does a coldstart of BASIC, but you can recover your listing by typing
OLD. -
SOUNDworks like Atari BASIC, i.e.SOUND voice, pitch, distortion, volume. If your Atari is equiped with a second Pokey, you can add 8 to the voice parameter to play sounds on the right channel. -
ENVELOPEdoes nothing. -
VDUjust prints the ASCII characters corresponding to the numbers you provide. There is no VDU emulation. -
You can use
ADVAL(255)to check if there's a pending keypress. All other ADC values are ignored. To read joysticks, peek at the shadow registers in RAM (e.g.DIR=?&0278for stick 0). -
OPENUP, EXT# and PTR# do not work because there's no byte accurate way to do fseek/ftell with Atari DOS 2.5.
It's not possible to accurately emulate the BBC graphics modes. The resolutions and the available colors are too different, so instead BBC BASIC for the Atari uses the standard Atari OS graphics modes and its coordinate system. (0,0) is at the top-left corner, and the maximum resolution is 320x192 (MODE 8).
-
MODEworks like Atari BASIC'sGRAPHICS. Supported modes are 0-15. Add 16 to disable the text window at the bottom. -
POINTworks likeLOCATE. For exampleA = POINT(42,42)returns the color of the pixel at position (42,42) in A. -
PLOT action, x, ysupports action 4 (MOVE), 5 (DRAW), 69 (PLOT POINT), and 133 (FILL). All other actions are ignored. -
To plot a single point, use
PLOT 69,x,y. -
MOVE x,yis a shortcut forPLOT 4,x,y. It moves the graphics cursor to location (x,y) (but does not plot a point). -
DRAW x,yshortcut forPLOT 5,x,y. It draws a line from the current position to (x,y) in the current drawing color. -
PLOT 133, x, ycalls the Atari OS flood fill. Note that this not 100% identical to 133 on the BBC, and it has the same quirks the XIO call has in Atari BASIC. -
COLOR numselects the drawing color. -
GCOLacts likeSETCOLORin Atari BASIC, but takes two arguments. The color number 0-4, and what value to set it to (&00-&ff). For example, setting the color of playfield 2 to red isGCOL 2,&34.
The following OSCLI commands are implemented:
*DOS,*QUIT,*BYE- Exit BBC BASIC and return to DOS.*DIR ["D:FILESPEC.EXT"],*CAT ["D:FILESPEC.EXT"],*. ["D:FILESPEC.EXT"]- Show a directory listing.*LOAD "D:FILENAME.EXT" <start>- Load binary file into memory, starting at <start> address.*SAVE "D:FILENAME.EXT" <start> <end>- Save memory from <start> to <end> to a file.*APPEND "D:FILENAME.EXT" <start> <end>- Save memory from <start> to <end> to a file, appending to existing data.
Just like on the BBC, you can make MOS calls directly from inline assembly. The vectors are located in RAM page &2F instead of the usual page &FF on a BBC. Note that the order and the LSBs are identical.
OSFIND = &2FCE
OSBPUT = &2FD4
OSBGET = &2FD7
OSARGS = &2FDA
OSFILE = &2FDD
OSRDCH = &2FE0
OSASCI = &2FE3
OSNEWL = &2FE7
OSWRCH = &2FEE
OSWORD = &2FF1
OSBYTE = &2FF4
OSCLI = &2FF7
All Atari control characters (CHR$0 to CHR$31) can be printed, except for CHR$13 (&0D) which is the end-of-line character (CR, carriage return).
It was not possible to change this to the Atari equivalent 155 (&9b) because that would clash with tknCOS (the internal token value for the COS function call).
Internally BBC BASIC sometimes scans a tokenized line and stops when it encounters the EOL character (&0D). This would fail if EOL and tknCOS are
the same value. If you really need the 'overscore' character, you can either poke &4D directly into the screen memory, bypass OSWRCH and write to CIO channel #0 directly, use COLOUR &0D:PLOT 69,POS,VPOS, redefine an otherwise unused character in the font (see memory map for details), or use BPUT #0,13.
BBC BASIC for the Atari uses the BBC font. Sometimes you need to type the ~ (tilde) if you want to print a hexadecimal
value, or you might want the '}' character somewhere in a string.
To type them, you need to press ESC first to have the actual character show because originally on the Atari they had a special meaning
(e.g. clear screen) and BBC BASIC's keyboard input is done through the standard E:ditor device driver.
So ESC SHIFT-CLEAR is '}', and ESC BACKSPACE is '~'.
You can use inverse characters inside strings, just like standard Atari BASIC. You cannot use them anywhere else, like in REM statements. Inverse characters have an ASCII value larger than 127. BBC BASIC will mistake them for tokens and expand them when listed. It's mostly harmless though, except for inverse CTRL-M (value &8D), which signals a new line number (tknCONST). This will mess up your listing. This is a known BBC BUG, where this could be triggered by including Teletext &8D inside a REM statement.
Here's the memory layout when BBC BASIC is running:
| Range | RAM | ROM | Other |
|---|---|---|---|
| &FFFA - &FFFF | CPU Vectors | Atari OS | |
| &D800 - &FFF9 | BBC BASIC | Atari OS | |
| &D000 - &D7FF | Hardware Registers | ||
| &C000 - &CFFF | BBC BASIC | Atari OS | |
| HIMEM - &CFFF | Screen Memory | ||
| PAGE - HIMEM | Free (ca. 32kB in MODE 0) | ||
| &3800 - &3BFF | BBC BASIC Workspace | ||
| &3000 - &37FF | BBC BASIC | ||
| &2FB9 - &2FFF | MOS Vectors | ||
| &2400 - &2FB8 | MOS Translation Layer | ||
| &2000 - &23FF | BBC Font | ||
| &0700 - &1FFF | Atari DOS | ||
| &0600 - &06FF | Free (page six) | ||
| &0200 - &05FF | Atari OS Variables | ||
| &0100 - &01FF | 6502 Stack | ||
| &00FD - &00FF | MOS Variables | ||
| &00D9 - &00FC | Free | ||
| &00D0 - &00D8 | Translation Layer Variables | ||
| &0080 - &00CF | BBC BASIC Variables | ||
| &0000 - &007F | Atari OS Variables |
Among the sample programs is CLOCKSP.BBC. Here are the results compared to a BBC Micro Model B at 2.0MHz.
With ANTIC DMA enabled:
And with ANTIC DMA disabled:
Which, considering there are still cycles stolen by the RAM refresh circuitry, is pretty close to 1.8MHz. During normal operation, with the screen on, it runs at around 60% of the speed of a BBC Model B.
Note that on an NTSC machine, the timings are off. The BBC BASIC TIME variable counts 100Hz ticks on a PAL machine, but 120Hz ticks on an NTSC machine.
It's similar to how an RTCLOK jiffy in Atari BASIC is 1/50th of a second or 1/60th of a second, depending on where you run it.
Be sure to take this into account if you use the TIME variable to meassure the speed of your code.
BBC BASIC for the Atari uses the BBC character set for all character codes above 32 (space). The first 32 characters are the original Atari control characters.
The reason for this is that with the Atari character set a statement like PRINT ~HIMEM would look like PRINT ◀HIMEM, which is weird. Here's an overview of the complete character set:
Note that the spades and diamonds characters are missing. Instead we have the pound sign, tilde and curly braces. If you really, really, really, need the original Atari font, you can easily copy it from ROM (&E000) to &2000 with a small inline assembly routine. Or you can only redefine a specific subset of the (control) characters to your needs.
To temporarily enable the OS ROM, you can use INC &D301.
Make sure to disable it again with DEC &D301 before returning to BBC BASIC, otherwise your computer will hang.
See the example program COPYBAR.BBC for an example.
Included on the main disk are a few example programs. Type *DIR to show a directory listing.
To build from source you'll need the Mad-Assembler. To create the ATR disk image you'll need atari-tools.
git clone https://github.com/ivop/atari-bbc-basic
cd atari-bbc-basic
make
The BBC BASIC Atari Port is Copyright © 2025 by Ivo van Poorten
BBC BASIC is Copyright © 1982,1983 by Acorn and Sophie Wilson
Parts of the loader are based on Turbo Basic 1.5, Copyright © 1985 by Frank Ostrowski
Credits for the BBC BASIC II/III disassembly are at the bbc-basic github repository.