Operators

Unlike in high-level languages, operators in Millfork have limited applicability. Not every well-formed expression is actually compilable. Most expressions involving single bytes compile, but for larger types usually you need to use in-place modification operators.
Further improvements to the compiler may increase the number of acceptable combinations.

Certain expressions require the commandline flag -fzp-register (.ini equivalent: zeropage_register) to be enabled. They will be marked with (zpreg) next to them. The flag is enabled by default, but you can disable it if you need to.

Work in progress: Certain expressions may not work on non-6502 targets yet. This should improve in the future.

Precedence

Millfork has different operator precedence compared to most other languages. From highest to lowest it goes:

*, *'
+, +', -, -', |, &, ^, >>, >>', <<, <<', >>>>
:
==, !=, <, >, <=, >=
&&
||
assignment and in-place modification operators

You cannot use two different operators at the same precedence levels without using parentheses to disambiguate. It is to prevent confusion about whether a + b & c << d means (a + b) & (c << d) ((a + b) & c) << d or something else.
The only exceptions are + and -, and +' and -'. They are interpreted as expected: 5 - 3 + 2 == 4 and 5 -' 3 +' 2 == 4.
Note that you cannot mix +' and -' with + and -.

Argument types

In the descriptions below, arguments to the operators are explained as follows:

byte means any one-byte type
word means any two-byte type, or a byte expanded to a word
long means any type longer than two bytes, or a shorter type expanded to such length to match the other argument
constant means a compile-time constant
simple means either: a constant, a non-stack variable, a pointer indexed with a constant, a pointer indexed with a non-stack variable, an array indexed with a constant, an array indexed with a non-stack variable, an array indexed with a sum of a constant and a non-stack variable, or a split-word expression made of two simple expressions. Examples: 1, a, p[2], p[i], arr[2], arr[i], arr[i+2], h:l, h[i]:l[i] Such expressions have the property that the only register they may clobber is Y.
mutable means an expression that can be assigned to

Split-word operator

Expressions of the shape h:l where h and l are of type byte, are considered expressions of type word.
If and only if both h and l are assignable expressions, then h:l is also an assignable expression.

Binary arithmetic operators

+, -:
byte + byte
constant word + constant word
constant long + constant long
constant word + byte
word + word (zpreg)
*: multiplication; the size of the result is the same as the size of the arguments
byte * constant byte
constant byte * byte
constant word * constant word
constant long * constant long
byte * byte (zpreg)

There are no division, remainder or modulo operators.

Bitwise operators

|, ^, &: OR, EXOR and AND
byte | byte
constant word | constant word
constant long | constant long
word | word (zpreg)
<<, >>: bit shifting; shifting pads the result with zeroes
byte << byte
word << byte (zpreg)
constant word << constant byte
constant long << constant byte
>>>>: shifting a 9-bit value and returning a byte; a >>>> b is equivalent to (a & $1FF) >> b
word >>>> constant byte

Decimal arithmetic operators

These operators work using the decimal arithmetic and will not work on Ricoh CPU's. The compiler issues a warning if these operators appear in the code.

+', -': decimal addition/subtraction
byte +' byte
constant word +' constant word
constant long +' constant long
word +' word (zpreg)
*': decimal multiplication
constant *' constant
<<', >>': decimal multiplication/division by power of two
byte <<' constant byte

Comparison operators

These operators (except for !=) can accept more than 2 arguments. In such case, the result is true if each comparison in the group is true. Note you cannot mix those operators, so a <= b < c is not valid.

WARNING: Currently in cases like a < f() < b, f() may be evaluated an undefined number of times (the current implementation calls it twice, but do not rely on this behaviour).

==: equality
byte == byte
simple word == simple word
simple long == simple long
!=: inequality
byte != byte
simple word != simple word
simple long != simple long
>, <, <=, >=: inequality
byte > byte
simple word > simple word
simple long > simple long

Currently, >, <, <=, >= operators perform unsigned comparison if none of the types of their arguments is signed, and fail to compile otherwise. This will be changed in the future.

Assignment and in-place modification operators

WARNING: Unlike other languages, Millfork does not provide any guarantees about how many times the left hand side will be evaluated. An expression of form a[f()] += b may call f an undefined number of times.

=: normal assignment
mutable byte = byte
mutable word = word mutable long = long
+=, +'=, |=, ^=, &=: modification in place
mutable byte += byte
mutable word += word
mutable long += long
<<=, >>=: shift in place
mutable byte <<= byte
mutable word <<= byte
mutable long <<= byte
<<'=, >>'=: decimal shift in place
mutable byte <<= constant byte
mutable word <<= constant byte
mutable long <<= constant byte
-=, -'=: subtraction in place
mutable byte -= byte
mutable word -= simple word
mutable long -= simple long
*=: multiplication in place
mutable byte *= constant byte
mutable byte *= byte (zpreg)
*'=: decimal multiplication in place
mutable byte *'= constant byte

Indexing

While Millfork does not consider indexing an operator, this is a place as good as any to discuss it.

An expression of form a[i], where i is an expression of type byte, is:

when a is an array: an access to the i-th element of the array a
when a is a pointer variable: an access to the byte in memory at address a + i

Those expressions are of type byte. If a is any other kind of expression, a[i] is invalid.

If the zeropage register is enabled, i can also be of type word.

Built-in functions

not: negation of a boolean expression
not(bool)
nonet: expansion of an 8-bit operation to a 9-bit operation
nonet(byte + byte)
nonet(byte +' byte)
nonet(byte << constant byte)
nonet(byte <<' constant byte)
Other kinds of expressions than the above (even nonet(byte + byte + byte)) will not work as expected.
hi, lo: most/least significant byte of a word
hi(word)

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