Operator documentation

doc/README.md

@@ -16,6 +16,8 @@
 
 ## Language reference
 
+* [Operators](lang/operators.md)
+
 * [Inline assembly syntax](lang/assembly.md)
 
 * [Important guidelines regarding reentrancy](lang/reentrancy.md)

doc/lang/operators.md

@@ -0,0 +1,142 @@
+# Operators
+
+Unlike in high-level languages, operators in Millfork have limited applicability. 
+Not every well-formed expression is actually compilable. 
+Further improvements to the compiler may increase the number of acceptable combinations. 
+
+## 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 interpeted 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
+
+## Binary arithmetic operators
+
+* `+`, `-`:  
+`byte + byte`  
+`word + word`  
+`long + long`
+
+* `*`: 8-bit multiplication  
+`byte * constant byte`  
+`constant byte * byte`
+
+There are no division, remainder or modulo operators.
+
+## Bitwise operators
+
+* `|`, `^`, `&`: OR, EXOR and AND  
+`byte | byte`
+
+* `<<`, `>>`: bit shifting; shifting right pads the result with zeroes  
+`byte * 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`  
+
+## Decimal arithmetic operators
+
+These operators work using the decimal arithmetic and will not work on Ricoh CPU's.
+
+* `+'`, `-'`: decimal addition/subtraction  
+`byte +' byte`  
+`word +' word`  
+`long +' 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.
+
+* `==`: 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 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`
+
+There are no `*'=` operator yet.