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 it.

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 shorted 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 than 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
*: 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
<<, >>: bit shifting; shifting pads the result with zeroes
byte << constant byte
word << constant 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, but the latter doesn't compile yet
word >>>> constant byte
>>>>: shifting a byte and returning a 9-bit value; a >>>> b is equivalent to (a << b) & 0x1ff if there was no overflow, but the latter doesn't compile yet
byte <<<< constant byte

Decimal arithmetic operators

These operators work using the decimal arithmetic and will not work on Ricoh CPU's.

+', -': decimal addition/subtraction
byte +' byte
constant word +' constant word
constant long +' constant long
*': 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.

Note that currently in cases like a < f() < b, f() will be evaluated twice!

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

=: 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 <<= 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
*'=: 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.

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