millfork/docs/lang/operators.md

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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.

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:

• enum means any enumeration type

• byte means any numeric one-byte type

• word means any numeric two-byte type, or a byte expanded to a word; pointer is considered to be numeric

• long means any numeric 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 (packed BCD).

On Ricoh-based targets (e.g. Famicom) they require the zeropage register to have size at least 4

• +', -': 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
  enum == enum
byte == byte
simple word == simple word
simple long == simple long

• !=: inequality
  enum != enum
byte != byte
simple word != simple word
simple long != simple long

• >, <, <=, >=: inequality
  byte > byte
simple word > simple word
simple long > simple long

Currently, >, <, <=, >= operators perform signed comparison if any of the types of their arguments is signed, and unsigned comparison otherwise.

WARNING: On targets using Intel 8080 and Sharp LR35902, a signed comparison a OP b is compiled as a - b OP 0, which may give wrong results if the subtraction overflows.

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 enum = enum
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 that has numeric index type: 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.

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

• when a is an array that has index type equal to the type of i: an access to the element of the array a at the location assigned to the key i

• otherwise: a compile error

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)

Furthermore, any type that can be assigned to a variable can be used to convert either from one type either to another type of the same size, or from a 1-byte integer type to a compatible 2-byte integer type.

byte → word
word → pointer
some enum → byte
byte → some enum
but not
word → byte
some enum → word