Enhanced BASIC language reference An advanced BASIC interpreter for the 6502 microprocessor.”
Numbers
Numbers may range from zero to plus or minus 1.70141173×10^38 and will have an
accuracy of just under 1 part in 1.68 x 10^7.
Numbers can be preceded by a sign, + or -, and are written as a string of
numeric digits with or without a decimal point and can also have a positive
or negative exponent as a power of 10 multiplier e.g.

.. are all valid numbers.
Integer numbers, i.e. with no decimal fraction or exponent, can also be in
either hexadecimal or binary. Hexadecimal numbers should be preceded by $
and binary numbers preceded by %, e.g.

.. again are all valid numbers.
Strings
Strings are any string of printable characters enclosed in a pair of quotation marks.
Non printing characters may be converted to single character strings using the
CHR$() functions.

Are all valid strings.
Variables
Variables of both numeric and string type are available. String variables are
distinguished by the $ suffix. As well as simple variables arrays are also
available and these may be either numeric or string and are distinguished by their
bracketed indicies after the variable name.
Variable names may be any length but only the first two name characters are significant
so BL and BLANK will refer to the same variable. The first character must be one of “A”
to “Z” or “a” to “z”, following characters may also include numbers. E.g.

Variable names are case sensitive so AB, Ab, aB and ab are all separate variables.
Variable names may not contain BASIC keywords. Keywords are only valid in upper
case so ‘PRINTER’ is not allowed (it would be interpreted as PRINT ER) but ‘printer’
is.
Note that spaces in variable names are ignored so ‘print e r’, ‘print er’ and ‘pri nter’
will all be interpreted the same way.
BASIC Keywords
Here is a list of BASIC keywords. They are only valid when entered in upper case as
shown and spaces may not be included in them. So GOTO is valid BASIC keyword but GO TO
is not.

• Anything in upper case is part of the command/function structure and must be present
• Anything in lower case enclosed in < > is to be supplied by the user
• Anything enclosed in [ ] is optional
• Anything enclosed in { } and separated by | characters are multi choice options
• Any items followed by an ellipsis, … , may be repeated any number of times
• Any punctuation and symbols, except those above, are part of the structure and must be included

BASIC Commands

END
Terminates program execution and returns control to the command line (direct mode).
END may be placed anywhere in a program, it does not have to be on the last line, and
there may be any number, including none, of ENDs in total.
Note. CONT may be used after and END to resume execution from the next statement.

FOR <var> = <expression> TO <expression> [STEP expression]
Assigns a variable to a loop counter and optionally sets the start value, the end
value and the step size. If STEP expression is omitted then a default step size of
+1 will be assumed.

NEXT [var[,var]…]
Increments or decrements a loop variable and checks for the terminating condition. If
the terminating condition has been reached then execution continues with the next
command, else execution continues with the command after the FOR assignment. See FOR.

DATA [{r|$}[,{r|$}]…]
Defines a constant or series of constants. Real constants are held as
strings in program memory and can be read as numeric values or string
values. String constants may contain spaces but if they need to contain
commas then they must be enclosed in quotes.

INPUT [“$”;] <var>[,var]…
Get a variable, or list of variables from the input stream. A question mark, “?”, is
always output, after the string if there is one, and if further input is required,
i.e. there are more variables in the list than the user entered values, then a double
question mark, “??”, will be output until enough values have been entered.
There are two possible messages that may appear during the execution of an input
statement:
Extra ignored
The user has attempted to enter more values than are required. Program
execution will continue but the extraneous data entered has been discarded.
Redo from start
The user has attempted to enter a string where a number was expected. The
reverse never causes an error as numbers are also valid strings.

DIM <var[$](i1[,i2[,in]…])>[,var[$](i1[,i2[,in]…])]…
Dimension arrays. Creates arrays of either string or numeric variables. The arrays
can have one or more dimensions. The lower limit of any dimension is always
zero and the upper limit is i. If you do not explicitly dimension an array then
it’s number of dimensions will be set when you first access it and the
upper bound will be set to 10 for each dimension.

READ <var>[,var]…
Reads values from DATA statements and assigns them to variables. Trying to
read a string literal into a numeric variable will cause a syntax error.

LET <var> = <expression>
Assign the value of expression to var. Both var and expression bust be of the
same type. The LET command word is optional and just <var> = <expression> will
give exactly the same result. It is only maintained for historical reasons.

DEC <var>[,var]…
Decrement variables. The variables listed will have their values decremented
by one. Trying to decrement a string variable will give a type mismatch error.
DEC A is much faster than doing A=A-1 and DEC A,A is slightly faster than
doing A=A-2.

SWAP <var[$]>,<var[$]>
Swap two variables. The variables listed will have their values exchanged. Both
must be of the same type, numeric or string, and either, or both, may be array
elements. Trying to swap a numeric and string variable will give a type mismatch
error.

GOTO <n>
Continue execution from line number n

RUN [n]
Begins execution of the program currently in memory at the lowest numbered line.
RUN erases all variables and functions, resets FOR .. NEXT, GOSUB .. RETURN and
DO ..LOOP states and sets the data pointer to the program start.

If n is specified then programme execution will start at the specified line number.

IF <expression>[relation expression] THEN<{{statement|n}|{GOTO|GOSUB}n}>
Evaluates expression. If the result of expression is non zero then the statement(s)
after the THEN or the GOTO or GOSUB are executed. If the result of expression is
zero then execution continues with the next line.

RESTORE [n]
Reset the DATA pointer. If n is specified then the pointer will be reset to the
beginning of line n else it will be reset to the start of the program. If n is
specified but doesn’t exist an error will be generated.

GOSUB <n>
Call a subroutine at line n. Program execution is diverted to line n but the
calling point is remembered. Upon encountering a RETURN statement program
execution will continue with the next statement (line) after the GOSUB.

RETIRQ
Returns program execution to the next statement after an interrupt, automatically
restores the IRQ enabled flag. See ON IRQ.

RETNMI
Returns program execution to the next statement after an interrupt, automatically
restores the NMI enabled flag. See ON NMI.

RETURN
Returns program execution to the next statement (line) after the last GOSUB
encountered. See GOSUB. Also returns program execution to the next statement
after an interrupt but does not restore the enabled flags.

REM
Everything following this statement on this program line will be ignored,
even colons.

STOP
Halts program execution and generates a “Break in line n” message where n is the
line in which the STOP was encountered.

OFF
See IRQ or NMI.

ON <expression> {GOTO|GOSUB} <n>[,n]…
The integer value of expression is calculated and then the nth number after the
GOTO or GOSUB is taken (where n is the result of expression). Note that valid
results for expression range only from zero to 255. Any result outside this
range will cause a Function call error.

ON {IRQ|NMI} <n>
Set up the IRQ or NMI routine pointers. This sets up the effective GOSUB line
that is taken when an interrupt happens. When the effective GOSUB is taken the
interrupt, IRQ or NMI, is turned off. This can be turned back on with the interrupt
on command or by using the matching special return. The normal program flow is
resumed by any of RETIRQ, RETNMI or RETURN.

NULL <n>
Sets the number of null characters printed by BASIC after every carriage
return. n may be specified in the range 0 to 255.

INC <var>[,var]…
Increment variables. The variables listed will have their values incremented
by one. Trying to increment a string variable will give a type mismatch error.
INC A is much faster than doing A=A+1 and INC A,A is slightly faster than
doing A=A+2.

WAIT <addr,b1>[,b2]
Program execution will wait at this point until the value of the location addr
exclusive ORed with b2 then ANDed with b1 is non zero. If b2 is not defined then
it is assumed to be zero. Note b1 and b2 must both be byte values.

LOAD
Does nothing in this version but does it via a vector in RAM so is easily patched.

SAVE
Does nothing in this version but does it via a vector in RAM so is easily patched.

DEF FN <name>(<var>) = <statement>
Defines <statement> as function <name>. <name> can be any valid numeric variable
name of one or more characters. <var> must be a simple variable and is used to
pass a numeric argument into the function.
Note that the value of <var> will be unchanged if it is used in the function
so <var> should be considered to be a local variable name.

POKE <addr,b>
Writes the byte value b into the address addr.

DOKE <addr,w>
Writes the word value w into the addresses addr and addr+1, the lower byte of
w is in addr. Note if addr = 65535 ($FFFF) then the high byte will be written
to address zero.

CALL <addr>
CALLs a user subroutine at address addr. No values are passed or returned and
so this is much faster than using USR()

DO
Marks the beginning of a DO .. LOOP loop (See LOOP). No parameters. This
command can be nested like FOR .. NEXT or GOSUB .. RETURN.

LOOP [{UNTIL|WHILE} expression]
Marks the end of a DO .. LOOP loop. There are three possible variations on the
LOOP command ..
LOOP
This loop repeats forever. With just this command control is passed back to the
next command after the corresponding DO.
LOOP UNTIL expression
This loop will repeat until the value of expression is non zero. Once that
occurs execution will continue with the next command after the LOOP UNTIL.
LOOP WHILE expression
This loop will repeat while the value of expression is non zero. When the
value of expression is zero execution will continue with the next command
after the LOOP WHILE.

PRINT [expression][{;|,}expression]…[{;|,}]
Outputs the value of each expressions. If the list of expressions to be output
does not end with a comma or a semi-colon, then a carriage return and linefeed
is output after the values.
Expressions on the line can be separated with either a semi-colon, causing the
next expression to follow immediately, or a comma which will advance the output
to the next tab stop before continuing to print. If there are no expressions and
no comma or semi-colon after the PRINT statement then a carriage return and
linefeed is output.
When entering a program line, or immediate statement, PRINT can be abbreviated to ?

CONT
Continues program execution after CTRL-C has been typed, a STOP has been encountered
during program execution or a null input was given to an INPUT request.

LIST [n1][-n2]
Lists the entire program held in memory. If n1 is specified then the listing will
start from line n1 and run to the end of the program. If -n2 is specified then the
listing will terminate after line n2 has been listed. If n1 and -n2 are specified
then all the lines from n1 to n2 inclusive will be listed.
Note. If n1 does not exist then the list will start from the next line numbered
after n1. If n2 does not exist then the listing will stop with the last line
numbered before n2.
Also note. LIST can be executed from within a program, first a [CR][LF] is printed
and then the specified lines, if any, each terminated with another [CR][LF].
Program execution then continues as normal.

CLEAR
Erases all variables and functions and resets FOR .. NEXT, GOSUB .. RETURN and
DO ..LOOP states.

NEW
Deletes the current program and all variables from memory.

WIDTH {b1|,b2|b1,b2}
Sets the terminal width and TAB spacing. b1 is the terminal width and b2 is the tab
spacing, default is 80 and 14. Width can be zero, for “infinite” terminal width, or
from 16 to 255. The tab size is from 2 to width-1 or 127, whichever is smaller.

GET <var[$]>
Gets a key, if there is one, from the input device. If there is no key waiting then
var will be set to 0 and var$ will return a null string “”. GET does not halt and
execution will continue.

IRQ {ON|OFF|CLEAR}
Enables or disables the IRQ handling subroutine. Note that turning the handler off
does not suppress the interrupt detection and if an interrupt occurs while handling is
off it will be actioned as soon as handling is turned back on. Using CLEAR clears the
interrupt assignment and it can only be restarted with an ON IRQ command

NMI {ON|OFF|CLEAR}
Enables or disables the NMI handling subroutine. Note that turning the handler off
does not suppress the interrupt detection and if an interrupt occurs while handling is
off it will be actioned as soon as handling is turned back on. Using CLEAR clears the
interrupt assignment and it can only be restarted with an ON NMI command

TAB(<expression>)
Sets the cursor position to <expression>. If the cursor is already beyond that
point then the cursor will be left where it is. This command is only valid in a
PRINT statement.

TO
Sets the range in a FOR .. NEXT loop. See FOR.

FN<name>(<expression>)
See DEF.

SPC(<expression>)
Prints <expression> spaces. This command is only valid in a PRINT statement.

THEN
See IF.

NOT <expression>
Generates the bitwise NOT of then signed integer value of <expression>.

STEP
Sets the step size in a FOR .. NEXT loop. See FOR.

UNTIL
See DO and LOOP.

WHILE
See DO and LOOP.

BITCLR <addr>,<b>
Clears bit b of address addr. Valid bit numbers are 0, the least significant bit, to
7, the most significant bit. Values outside this range will cause a function call
error.

BITSET <addr>,<b>
Sets bit b of address addr. Valid bit numbers are 0, the least significant
bit, to 7, the most significant bit. Values outside this range will cause a
function call error.
BASIC Operators
Operators perform mathematical or logical operations on values and return the result.
The operation is usually preceded by a variable name and equality sign or is part of
an IF .. THEN statement.

The three comparison operators can be mixed to provide further operators ..

BASIC Functions
Functions always return a value, be it numeric or string, so are used on the right hand
side of the = sign in assignments, on either side of operators and in commands requiring
an expression e.g. after PRINT, within expressions, or in other functions.

SGN(<expression>)
Returns the sign of <expression>. If the value is +ve SGN returns +1, if
the value is -ve then SGN returns -1. If expression=0 then SGN returns 0.

INT(<expression>)
Returns the integer of <expression>.

ABS(<expression>)
Returns the absolute value of <expression>.

USR(<expression>)
Takes the value of <expression> and places it in FAC1 and then calls the
USeR routine pointed to by the vector at $0B,$0C. What the routine does
with this value is entirely up to the user, it can even be safely ignored
if it isn’t needed. The routine, after the user code has done an RTS, takes
whatever is in FAC1 and returns that. Note it can be either a numeric or
string value.
If no value needs to be passed or returned then CALL is a better option.

FRE(<expression>)
Returns the amount of free program memory. The value of expression is ignored
and can be numeric or string.

POS(<expression>)
Returns the POSition of the cursor on the terminal line. The value of expression
is ignored.

SQR(<expression>)
Returns the square root of <expression>.

RND(<expression>)
Returns a random number in the range 0 to 1. If the value of <expression> is
non zero then it will be used as the seed for the returned pseudo random number
otherwise the next number in the sequence will be returned.

LOG(<expression>)
Returns the natural logarithm (base e) of <expression>.

EXP(<expression>)
Returns e^<expression>. Natural antilog.

COS(<expression>)
Returns the cosine of the angle <expression> radians.

SIN(<expression>)
Returns the sine of the angle <expression> radians.

TAN(<expression>)
Returns the tangent of the angle <expression> radians.

ATN(<expression>)
Returns, in radians, the arctangent of <expression>.

PEEK(<addr>)
Returns the byte value of <addr>.

DEEK(<addr>)
Returns the word value of <addr> and addr+1 as an integer in the range -32768
to 32767. Addr holds the word low byte.

SADD(<{var$|var$()}>)
Returns the address of var$ or var$(). This returns a pointer to the actual string
in memory not the descriptor. If you want the pointer to the descriptor use
VARPTR instead.

LEN(<expression$>)
Returns the length of <expression$>.

STR$(<expression>)
Returns the result of <expression> as a string.

VAL(<expression$>)
Returns the value of <expression$>.

ASC(<expression$>)
Returns the ASCII value of the first character of <expression$>.

LCASE$(<expression$>)
Returns <expression$> with all the alpha characters in lower case.

UCASE$(<expression$>)
Returns <expression$> with all the alpha characters in upper case.

CHR$(b)
Returns single character string of character <b>.

HEX$(<expression>[,b])
Returns <expression> as a hex string. If b is omitted, or if b = 0, then
the string is returned with all leading zeroes removed and is of variable length.
If b is set, permissible values range from 1 to 6, then a string of length b will
be returned. The result is always unsigned and calling this function with expression
> 2^24-1 or b > 6 will cause a function call error.

BIN$(<expression>[,b])
Returns <expression> as a binary string. If b is omitted, or if b = 0, then
the string is returned with all leading zeroes removed and is of variable length.
If b is set, permissible values range from 1 to 24, then a string of length b will
be returned. The result is always unsigned and calling this function with expression
> 2^24-1 or b > 24 will cause a function call error.

BITTST(<addr>,<b>)
Tests bit b of address addr. Valid bit numbers are 0, the least significant bit, to 7,
the most significant bit. Values outside this range will cause a function call error.
Returns zero if the bit was zero, returns -1 if the bit was 1.

MAX(<expression>[,<expression>]…)
Returns the maximum value from a list of numeric expressions. There must be
at least one expression but the upper limit is dictated by the line length.
Each expression is evaluated in turn and the largest of them returned.

MIN(<expression>[,<expression>]…)
Returns the minimum value from a list of numeric expressions. There must be
at least one expression but the upper limit is dictated by the line length.
Each expression is evaluated in turn and the smallest of them returned.

PI
Returns the value of pi as 3.14159274 (closest floating value).

TWOPI
Returns the value of 2*pi as 6.28318548 (closest floating value).

VARPTR(<var[$]>)
Returns a pointer to the variable memory space. If the variable is numeric, or a
numeric array element, then VARPTR returns the pointer to the packed value of that
variable in memory. If the variable is a string, or a string array element, then
VARPTR returns a pointer to the descriptor for that string. If you want the pointer
to the string itself use SADD instead.

LEFT$(<expression$,b>)
Returns the leftmost b characters of <expression$>.

RIGHT$(<expression$,b>)
Returns the rightmost b characters of <expression$>.

MID$(<expression$,b1>[,b2])
Returns the substring string from character b1 of expression$ of length b2. The
characters of expression$ are numbered from 1 starting with the leftmost. If b2 is
omitted then all the characters from b1 to the end of the string are returned.
BASIC Error Messages
These all occur from time to time and, if the error occurred while executing a program,
will be followed by “in line ” where is the number of the line in which the error
occurred.
NEXT without FOR Error
NEXT has been encountered and no matching FOR could be found.
Syntax Error
Just generally wrong. 8^)=
RETURN without GOSUB Error
RETURN has been encountered and no matching GOSUB could be found.
Out of DATA Error
A READ has tried to read data beyond the last item. Usually because you either
mistyped the DATA lines, miscounted the DATA, RESTOREd to the wrong place or
just plain forgot to restore.
Function call Error
Some parameter of a function was outside it’s limits. E.g. Trying to POKE a
value of less than 0 or greater than 255.
Overflow Error
The result of a calculation has exceeded the numerical range of BASIC. This is
plus or minus 1.7014117+E38
Out of memory Error
Anything that uses memory can cause this but mostly it’s writing and running
programmes that does it.
Undefined statement Error
Either a GOTO, GOSUB, RUN or RESTORE was attempted to a line that doesn’t exist
or the line referred to in an ON <expression> {GOTO|GOSUB} or ON {IRQ|NMI}
doesn’t exist.
Array bounds Error
An attempt was made to access an element of an array that was outside it’s
bounding dimensions.
Double dimension Error
An attempt has been made to dimension an already dimensioned array. This could
be because the array was accessed previously causing it to be dimensioned by
default.
Divide by zero Error
The right hand side of an A/B expression was zero.
Illegal direct Error
An attempt was made to execute a command or function in direct mode which is
disallowed in that mode e.g. INPUT or DEF.
Type mismatch Error
An attempt was made to assign a numeric value to a string variable, a string value to
a numeric variable or a value of one type was returned when a value of the other type
was expexted or an attempt at a relational operation between a string and a number
was made.
String too long Error
String lengths can be from zero to 255 characters, more than that and you will
see this.
String too complex Error
A string expression caused an overflow on the descriptor stack. Try splitting
the expression into smaller pieces.
Can’t continue Error
Execution can’t be continued because either the program execution ended because
an error occurred, NEW or CLEAR have been executed since the program was
interrupted or the program has been edited.
Undefined function Error
FN <var> was called but not found.
LOOP without DO Error
LOOP has been encountered and no matching DO could be found.