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242 lines
7.7 KiB
Markdown
242 lines
7.7 KiB
Markdown
[< back to index](../index.md)
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# Operators
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Unlike in high-level languages, operators in Millfork have limited applicability.
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Not every well-formed expression is actually compilable.
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Most expressions involving single bytes compile,
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but for larger types usually you need to use in-place modification operators.
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Further improvements to the compiler may increase the number of acceptable combinations.
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Certain expressions require the commandline flag `-fzp-register` (`.ini` equivalent: `zeropage_register`) to be enabled.
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They will be marked with (zpreg) next to them.
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The flag is enabled by default, but you can disable it if you need to.
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## Precedence
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Millfork has different operator precedence compared to most other languages. From highest to lowest it goes:
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* `*`, `*'`
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* `+`, `+'`, `-`, `-'`, `|`, `&`, `^`, `>>`, `>>'`, `<<`, `<<'`, `>>>>`
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* `:`
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* `==`, `!=`, `<`, `>`, `<=`, `>=`
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* `&&`
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* `||`
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* assignment and in-place modification operators
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You cannot use two different operators at the same precedence levels without using parentheses to disambiguate.
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It is to prevent confusion about whether `a + b & c << d` means `(a + b) & (c << d)` `((a + b) & c) << d` or something else.
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The only exceptions are `+` and `-`, and `+'` and `-'`.
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They are interpreted as expected: `5 - 3 + 2 == 4` and `5 -' 3 +' 2 == 4`.
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Note that you cannot mix `+'` and `-'` with `+` and `-`.
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## Argument types
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In the descriptions below, arguments to the operators are explained as follows:
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* `enum` means any enumeration type
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* `byte` means any numeric one-byte type
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* `word` means any numeric two-byte type, or a byte expanded to a word; `pointer` is considered to be numeric
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* `long` means any numeric type longer than two bytes, or a shorter type expanded to such length to match the other argument
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* `constant` means a compile-time constant
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* `simple` means either: a constant, a non-stack variable,
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a pointer indexed with a constant, a pointer indexed with a non-stack variable,
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an array indexed with a constant, an array indexed with a non-stack variable,
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an array indexed with a sum of a constant and a non-stack variable,
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or a split-word expression made of two simple expressions.
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Examples: `1`, `a`, `p[2]`, `p[i]`, `arr[2]`, `arr[i]`, `arr[i+2]`, `h:l`, `h[i]:l[i]`
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Such expressions have the property that the only register they may clobber is Y.
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* `mutable` means an expression that can be assigned to
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## Split-word operator
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Expressions of the shape `h:l` where `h` and `l` are of type byte, are considered expressions of type word.
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If and only if both `h` and `l` are assignable expressions, then `h:l` is also an assignable expression.
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## Binary arithmetic operators
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* `+`, `-`:
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`byte + byte`
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`constant word + constant word`
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`constant long + constant long`
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`constant word + byte`
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`word + word` (zpreg)
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* `*`: multiplication; the size of the result is the same as the size of the arguments
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`byte * constant byte`
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`constant byte * byte`
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`constant word * constant word`
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`constant long * constant long`
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`byte * byte` (zpreg)
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There are no division, remainder or modulo operators.
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## Bitwise operators
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* `|`, `^`, `&`: OR, EXOR and AND
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`byte | byte`
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`constant word | constant word`
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`constant long | constant long`
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`word | word` (zpreg)
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* `<<`, `>>`: bit shifting; shifting pads the result with zeroes
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`byte << byte`
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`word << byte` (zpreg)
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`constant word << constant byte`
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`constant long << constant byte`
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* `>>>>`: shifting a 9-bit value and returning a byte; `a >>>> b` is equivalent to `(a & $1FF) >> b`
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`word >>>> constant byte`
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## Decimal arithmetic operators
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These operators work using the decimal arithmetic (packed BCD).
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On Ricoh-based targets (e.g. Famicom) they require the zeropage register to have size at least 4
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* `+'`, `-'`: decimal addition/subtraction
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`byte +' byte`
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`constant word +' constant word`
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`constant long +' constant long`
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`word +' word` (zpreg)
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* `*'`: decimal multiplication
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`constant *' constant`
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* `<<'`, `>>'`: decimal multiplication/division by power of two
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`byte <<' constant byte`
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## Comparison operators
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These operators (except for `!=`) can accept more than 2 arguments.
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In such case, the result is true if each comparison in the group is true.
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Note you cannot mix those operators, so `a <= b < c` is not valid.
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**WARNING:** Currently in cases like `a < f() < b`, `f()` may be evaluated an undefined number of times
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(the current implementation calls it twice, but do not rely on this behaviour).
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* `==`: equality
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`enum == enum`
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`byte == byte`
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`simple word == simple word`
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`simple long == simple long`
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* `!=`: inequality
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`enum != enum`
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`byte != byte`
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`simple word != simple word`
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`simple long != simple long`
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* `>`, `<`, `<=`, `>=`: inequality
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`byte > byte`
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`simple word > simple word`
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`simple long > simple long`
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Currently, `>`, `<`, `<=`, `>=` operators perform signed comparison
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if any of the types of their arguments is signed,
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and unsigned comparison otherwise.
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**WARNING:** On targets using Intel 8080 and Sharp LR35902, a signed comparison `a OP b` is compiled as `a - b OP 0`,
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which may give wrong results if the subtraction overflows.
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## Assignment and in-place modification operators
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**WARNING:** Unlike other languages, Millfork does not provide any guarantees about how many times the left hand side will be evaluated.
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An expression of form `a[f()] += b` may call `f` an undefined number of times.
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* `=`: normal assignment
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`mutable enum = enum`
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`mutable byte = byte`
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`mutable word = word`
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`mutable long = long`
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* `+=`, `+'=`, `|=`, `^=`, `&=`: modification in place
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`mutable byte += byte`
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`mutable word += word`
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`mutable long += long`
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* `<<=`, `>>=`: shift in place
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`mutable byte <<= byte`
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`mutable word <<= byte`
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`mutable long <<= byte`
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* `<<'=`, `>>'=`: decimal shift in place
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`mutable byte <<'= constant byte`
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`mutable word <<'= constant byte`
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`mutable long <<'= constant byte`
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* `-=`, `-'=`: subtraction in place
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`mutable byte -= byte`
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`mutable word -= simple word`
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`mutable long -= simple long`
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* `*=`: multiplication in place
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`mutable byte *= constant byte`
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`mutable byte *= byte` (zpreg)
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* `*'=`: decimal multiplication in place
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`mutable byte *'= constant byte`
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## Indexing
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While Millfork does not consider indexing an operator, this is a place as good as any to discuss it.
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An expression of form `a[i]`, where `i` is an expression of type `byte`, is:
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* when `a` is an array that has numeric index type: an access to the `i`-th element of the array `a`
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* when `a` is a pointer variable: an access to the byte in memory at address `a + i`
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Those expressions are of type `byte`. If `a` is any other kind of expression, `a[i]` is invalid.
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If the zeropage register is enabled, `i` can also be of type `word`.
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An expression of form `a[i]`, where `i` is an expression of a enumeration type, is:
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* when `a` is an array that has index type equal to the type of `i`:
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an access to the element of the array `a` at the location assigned to the key `i`
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* otherwise: a compile error
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## Built-in functions
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* `not`: negation of a boolean expression
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`not(bool)`
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* `nonet`: expansion of an 8-bit operation to a 9-bit operation
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`nonet(byte + byte)`
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`nonet(byte +' byte)`
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`nonet(byte << constant byte)`
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`nonet(byte <<' constant byte)`
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Other kinds of expressions than the above (even `nonet(byte + byte + byte)`) will not work as expected.
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* `hi`, `lo`: most/least significant byte of a word
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`hi(word)`
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Furthermore, any type that can be assigned to a variable can be used to convert
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either from one type either to another type of the same size,
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or from a 1-byte integer type to a compatible 2-byte integer type.
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`byte` → `word`
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`word` → `pointer`
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some enum → `byte`
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`byte` → some enum
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but not
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`word` → `byte`
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some enum → `word`
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