EightBall is an interpreter for a novel structured programming language. It runs on a number of 6502-based vintage systems and may also be compiled as a 32 bit Linux executable.
EightBall tries to form a balance of the following qualities, in 20K or so of 6502 code:
* Statically typed
* Provides facilities which encourage structured programming ...
* ... Yet makes it easy to fiddle with hardware (PEEK and POKE and bit twiddling)
* Keep the language as simple and small as possible ...
* ... While providing powerful language primitives and encapsulation in subroutines, allowing the language to be extended by writing EightBall routines
The following 6502-based systems are currently supported:
* Apple II - EightBall runs under ProDOS and uses upper/lowercase text. It should run on 64K Apple IIe, IIc or IIgs. It can probably run on Apple II/II+ with an 80 column code, but this has not been tested.
* Commodore 64 - EightBall should run on any C64.
* Commodore VIC-20 - EightBall runs on a VIC-20 with 32K of additional RAM.
There are executables and disk images available to download for Apple II, Commodore 64 and VIC-20. These may be run on real hardware or one of the many emulators that are available.
The language itself is documented in these wiki pages. The best way to learn is to study example programs.
EightBall supports most of C's arithmetic, logical and bitwise operators. They have the same precedence as in C as well. Since the Commodore machines do not have all the ASCII character, some substitutions have been made (shown in parenthesis below.)
EightBall also implements 'star operators' for pointer dereferencing which will also be familiar to C programmers.
The `&` prefix operator returns a pointer to a variable which may be used to read and write the variable's contents. The operator may be applied to scalar variables, whole arrays and individual elements of arrays.
word w = 123;
word A[10] = 0;
pr.dec &w; ' Address of scalar w
pr.dec &A; ' Address of start of array A
pr.dec &A[2] ' Address of third element of array A
### 'Star Operators'
EightBall provides two 'star operators' which dereference pointers in a manner similar to the C star operator. One of these (`*`) operates on word values, the other (`^`) operates on byte values. Each of the operators may be used both for reading and writing through pointers.
Here is an example of a pointer to a word value:
word val = 0; ' Real value stored here
word addr = &val; ' Now addr points to val
*addr = 123; ' Now val is 123
pr.dec *addr; ' Recover the value via the pointer
pr.nl
Here is an example using a pointer to byte. This is similar to `PEEK` and `POKE` in BASIC.
word addr = $c000; ' addr points to hex $c000
byte val = ^addr; ' Read value from $c000 (PEEK)
^val = 0; ' Set value at $c000 to zero (POKE)
### Parenthesis
Parenthesis may be used to control the order of evaluation, for example:
pr.dec (10+2)*3; ' Prints 36
pr.dec 10+2*3; ' Prints 16
### Operator Precedence
| Precedence Level | Operators | Example | Example CBM |
EightBall allows named subroutines to be defined, for example:
sub myFirstSubroutine()
pr.msg "Hello"; pr.nl
endsub
All subroutines must end with `endsub` statement.
A subroutine may have more than one `return` statement returning a numeric value:
sub mySecondSubroutine()
return 2
endsub
If the flow of execution hits the `endsub` then 0 is returned to the caller.
### Simple Subroutine Invocation
The subroutine above can be called as follows:
call myFirstSubroutine()
When `myFirstSubroutine` hits a `return` statement, the flow of execution will return to the statement immediately following the `call`.
### Local Variables
Each subroutine has its own local variable scope. If a local variable is declared with the same name as a global variable, the global will not be available within the scope of the subroutine. When the subroutine returns, the local variables are destroyed.
sub myThirdSubroutine()
word w[10] = 0; ' Local array
endsub
### Argument Passing
Subroutines may take `byte` or `word` arguments, using the following syntax:
sub withArgs(byte flag, word val1, word val2)
' Do stuff
return 0
endsub
This could be called as follows:
word ww = 0; byte b = 0;
call withArgs(b, ww, ww+10)
When `withArgs` runs, the expression passed as the first argument (`b`) will be evaluated and the value assigned to the first formal argument `flag`, which will be created in the subroutine's local scope. Similarly, the second argument (`ww`) will be evaluated and the result assigned to `val1`. Finally, `ww+10` will be evaluated and assigned to `val2`.
Argument passing is by value, which means that `withArgs` can modify `flag`, `val1` or `val2` freely without the changes being visible to the caller.
### Function Invocation
Subroutines may be invoked within an expression. For example, the following subroutine:
sub adder(word a, word b)
return a+b
endsub
Could be used in an expression like this:
pr.dec adder(10, 5); ' Prints 15
Functions may invoke themselves recursively (but you will run out of stack quite fast!)
### Passing by Reference
Passing by reference allows a subroutine to modify a value passed to it. EightBall does this using pointers, in a manner that will be familiar to C programmers. Here is `adder` implemented using this pattern:
sub adder(word a, word b, word resptr)
*resptr = a+b
endsub
Then to call it:
word result
call adder(10, 20, &result)
This code takes the address of variable `result` using the ampersand operator and passes it to subroutine `adder` as `resptr`. The subroutine then uses the star operator to write the result of the addition of the first two arguments (10 + 20 in this example) to the word pointed to by `resptr`.
Unlike C, there are no special pointer types. Pointers must be stored in a `word` variable, since they do not fit in a `byte`.
### Passing an Array by Reference
**Warning: This is currently not implemented in the compiler, only the interpreter.**
It is frequently useful to pass an array into a subroutine. It is not very useful to use pass by value for arrays, since this may mean copying a large object onto the stack. For these reasons, EightBall implements a special pass by reference mode for array variables, which operates in a manner similar to C.
Here is an example of a function which takes a regular variable and an array:
sub clearArray(byte arr[], word sz)
word i = 0
for i = 0 : sz-1
arr[i] = 0
endfor
endsub
This may be invoked like this:
word n = 10
byte A[n] = 99
call clearArray(A, n)
Note that the size of the array is not specified in the subroutine definition - any size array may be passed. Note also that the corresponding argument in the `call` is simply the array name (no [] or other annotation is permitted.)
This mechanism effectively passes a pointer to the array contents 'behind the scenes'.
### End Statement
The `end` statement marks the normal end of execution. This is often used to stop the flow of execution running off the end of the main program and into the subroutines (which causes an error):
Program runs until it hits an `end` statement, an error occurs or it is interrupted by the user.
### Compile Stored Program
comp
The program in memory is compiled to EightBall VM bytecode. This is written to a file `bytecode`.
The `bytecode` file may be executed using the EightBall Virtual Machine that is part of this package.
### Quit EightBall
Easy:
quit
Returns to ProDOS on Apple II, or to CBM BASIC on C64/VIC20.
### Clear Stored Program
new
### Clear All Variables
clear
### Show All Variables
vars
Variables are shown in tabular form. The letter 'b' indicates byte type, while 'w' indicates word type. For scalar variables, the value is shown. For arrays, the dimension(s) are shown.
### Show Free Space
free
The free space available for variables and for program text is shown on the console.
Only console I/O is supported at present. File I/O is planned for a later release.
### Console Output
#### pr.msg
Prints a literal string to the console:
pr.msg "Hello world"
#### pr.dec
Prints an unsigned decimal value to the console:
pr.dec 123/10
#### pr.dec.s
Prints a signed decimal value to the console:
pr.dec.s 12-101
#### pr.hex
Prints a hexadecimal value to the console (prefixed with '$'):
pr.hex 1234
#### pr.nl
Prints a newline to the console:
pr.nl
#### pr.str
Prints a byte array as a string to the console. The string is null terminated (so printing stops at the first 0 character):
pr.str A; ' A is a byte array
### Console Input
#### kbd.ch
Allows a single character to be read from the keyboard. Be careful - this function assumes the argument passed to it a pointer to a byte value into which the character may be stored.
byte c = 0
while 1
kbd.ch &c
pr.ch c
endwhile
#### kbd.ln
Allows a line of input to be read from the keyboard and to be stored to an array of byte values. This statement takes two arguments - the first is an array of byte values into which to write the string, the second is the maximum number of bytes to write.
Be warned that the line editor is rather primitive. However we are trying to save memory.
Editor commands start with the colon character (:).
### Load from Disk
To load a new source file from disk, use the `:r` 'read' command:
:r "myfile.8b"
### Save to Disk
To save the current editor buffer to disk, use the :w 'write' command:
:w "myfile.8b"
On Commodore systems, this must be a new (non-existing) file, or a drive error will result.
### Insert Line(s)
Start inserting text before the specified line. The editor switches to insert mode, indicated by the '>' character (in inverse green on CBM). The following command will start inserting text at the beginning of an empty buffer:
:i0
>
One or more lines of code may then be entered. When you are done, enter a period '.' on a line on its own to return to EightBall immediate mode prompt.
### Append Line(s)
Append is identical to the insert command described above, except that it starts inserting /after/ the specified line. This is often useful to adding lines following the end of an existing program.
### Delete Line(s)
This command allows one or more lines to be deleted. To delete one line:
:d33
or to delete a range of lines:
:d10,12
### Change Line
This command allows an individual line to be replaced (like inserting a new line the deleting the old line). It is different to the insert and append commands in that the text is entered immediately following the command (not on a new line). For example:
:c21:word var1=12
will replace line 21 with `word var1=12`. Note the colon terminator following the line number.
Note that the syntax of this command is contrived to allow the CBM screen editor to work on listed output in a similar way to CBM BASIC. Code may be listed using the `:l` command and the screen may then be interactively edited using the cursor keys and return, just as in BASIC.
### List Line(s)
This allows the program text to be listed to the console. Either the whole program may be displayed or just a range of lines. To show everything:
:l
To show a range of lines:
:l0-20
(The command is the letter Ell, not the number 1!)
