Now you can turn your Arduino into an 80's home computer!
A complete BASIC interpreter for the Arduino, using a PS/2 keyboard, and SPI OLED screen. The BASIC supports almost all the usual features, with float and string variables, multi-dimensional arrays, FOR-NEXT, GOSUB-RETURN, etc. Saving and Loading from EEPROM is supported, as well as auto-running a program on power-up. You can also read and write from the analog and digital pins.
There's about 1k of RAM available for your BASIC programs and variables, so its roughly equivalent to my first computer (a Sinclair ZX81). The other 1k of RAM (on an UNO) is used for the keyboard and screen buffers, with a small bit of room left for the CPU stack. That works out quite well, since there's a 1k EEPROM on the arduino so if your program fits in the basic environment, it will fit when saved to EEPROM!
1: An Arduino with at least 2k of RAM. i.e. UNO/Duemilanove/etc. It should also work on the MEGA but is untested. I think the sketch is too big for the Leonardo, since it looks like the bootloader uses more RAM on this model.
2: A PS/2 Keyboard. See http://playground.arduino.cc/Main/PS2Keyboard for wiring details.
3: An SSD1306 based OLED Screen connected using SPI. See e.g. http://www.adafruit.com/product/938. I think it should also work fine with the 128x32 version - you'll just need to change a couple of defines.
4: (Optional) A Piezoelectric buzzer for keyboard clicks and other sounds.
5: (Optional) A external EEPROM (e.g. 24LC256) lets you save more than one file. You can pick these up for about £2/$2 on ebay.
1: Download the zip file, unpack and copy the folder to your arduino sketches directory.
2: Install the PS/2 keyboard library if required. I've included the version I used in the zip file.
3: Install the SSD1306ASCII library. The normal Adafruit library is too RAM hungry for this project, so I'm using a massively cut down driver instead. I've modified this library a bit to get fast SPI transfers which improved the screen updating speed by a factor of about 4. The modified version is included in the zip file.
For both libraries, unzip the files and copy the folder to your arduino libraries directory.
4: Check your wiring corresponds to the pins in the comments/defines at the top of the Arduino_BASIC file.
Variables names can be up to 8 alphanumeric characters but start with a letter e.g. a, bob32 String variable names must end in $ e.g. a$, bob32$ Case is ignored (for all identifiers). BOB32 is the same as Bob32. print is the same as PRINT
Array variables are independent from normal variables. So you can use both:
LET a = 5
DIM a(10)
There is no ambiguity, since a on its own refers to the simple variable 'a', and a(n) referes to an element of the 'a' array.
Arithmetic operators: + - * / MOD
Comparisons: <> (not equals) > >= < <=
Logical: AND, OR, NOT
Expressions can be numerical e.g. 5*(3+2), or string "Hello "+"world". Only the addition operator is supported on strings (plus the functions below).
Commands
PRINT <expr>;<expr>... e.g. PRINT "A=";a
LET variable = <expr> e.g. LET A$="Hello".
variable = <expr> e.g. A=5
LIST [start],[end] e.g. LIST or LIST 10,100
RUN [lineNumber]
GOTO lineNumber
REM <comment> e.g. REM ** My Program ***
STOP
CONT (continue from a STOP)
INPUT variable e.g. INPUT a$ or INPUT a(5,3)
IF <expr> THEN cmd e.g. IF a>10 THEN a = 0: GOTO 20
FOR variable = start TO end STEP step
NEXT variable
NEW
GOSUB lineNumber
RETURN
DIM variable(n1,n2...)
CLS
PAUSE milliseconds
POSITION x,y sets the cursor
PIN pinNum, value (0 = low, non-zero = high)
PINMODE pinNum, mode ( 0 = input, 1 = output)
LOAD (from internal EEPROM)
SAVE (to internal EEPROM) e.g. use SAVE + to set auto-run on boot flag
LOAD "filename", SAVE "filename, DIR, DELETE "filename" if using with external EEPROM.
"Pseudo-identifiers"
INKEY$ - returns (and eats) the last key pressed buffer (non-blocking). e.g. PRINT INKEY$
RND - random number betweeen 0 and 1. e.g. LET a = RND
Functions
LEN(string) e.g. PRINT LEN("Hello") -> 5
VAL(string) e.g. PRINT VAL("1+2")
INT(number) e.g. INT(1.5)-> 1
STR$(number) e.g. STR$(2) -> "2"
LEFT$(string,n)
RIGHT$(string,n)
MID$(string,start,n)
PINREAD(pin) - see Arduino digitalRead()
ANALOGRD(pin) - see Arduino analogRead()