millfork/docs/lang/syntax.md

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Syntax

For information about types, see Types.
For information about literals, see Literals.
For information about assembly, see Using assembly within Millfork programs.

Comments

Comments start with // and last until the end of line.

Declarations

Variable declarations

A variable declaration can happen at either top level of a file (global variables), or a top level of a function (local variables).

Syntax:

[segment(<segment>)] [volatile] [<storage>] <type> <name> [@<address>] [= <initial_value>]

• <segment>: segment name; if absent, then defaults to default.

• volatile means that the variable is volatile. The optimizer shouldn't remove or reorder accesses to volatile variables. Volatile variables cannot be declared as register or `stack.

• <storage> can be only specified for local variables. It can be either stack, static, register or nothing. register is only a hint for the optimizer. See the description of variable storage.

• <address> is a constant expression that defines where in the memory the variable will be located. If not specified, it will be located according to the usual allocation rules. stack variables cannot have a defined address.

• <initial_value> is a constant expression that contains the initial value of the variable. Only global variables can be initialized that way. The behaviour is undefined when targeting a ROM-based platform.

You can declare multiple variables in one declaration, for example:

byte x, y @$c000, z=6, w

will declare 4 variables: uninitialized variables x and w, fixed-address variable y and initialized variable z.

For every variable x larger than a byte, extra subvariables are defined:

• if x is of type word or pointer:

  • constituent bytes, from low to high: x.lo, x.hi

• if x is of type int24:

  • constituent bytes, from low to high: x.b0, x.b1, x.b2

  • partial words: x.loword (=x.b1:x.b0), x.hiword (=x.b2:x.b1)

• if x is of type long:

  • constituent bytes, from low to high: x.b0, x.b1, x.b2, x.b3

  • partial words: x.loword (=x.b1:x.b0), x.hiword (=x.b3:x.b2)

• if x is of a larger integral type:

  • constituent bytes, from low to high: x.b0, x.b1, x.b2, etc.

  • the lowest word: x.loword (=x.b1:x.b0)

Constant declarations

const <type> <name> = <value>

TODO

Alias definitions

alias <alias> = <name> [!]

Sets an alias for a global name. Unless shadowed by a local name, the alias will point to the given global object:

byte x
alias a = x

void f() {
    a = 5 // writes to the global variable x
}

void f() {
    byte a
    a = 5 // writes to the local variable a
}

Aliases can be used for variables, arrays, constants, functions, and types, but not for text encodings, array formats or keywords.

If the alias definition is followed by a !, then the alias overrides any other definition of that name. This allows for overriding definitions of library functions by another library:

void f() {}
void g() {}
alias f = g!
// now the original f is removed and all calls to f will call g instead

Array declarations

An array is a continuous sequence of bytes in memory.

An array declaration can happen at either top level of a file (global arrays), or a top level of a function (local arrays). Regardless of where they were declared, arrays are considered static.

Syntax:

[segment(<segment>)] [const] array [(<element type>)] <name> [[<size>]] [align ( <alignment> )] [@<address>] [= <initial_values>]

• <segment>: segment name; if absent, then defaults to default_code_segment as defined for the platform if the array has initial values, or to default if it doesn't.

• if const is present, the array is read-only. Read-only arrays have to have a fixed address and/or defined contents.
  COMPATIBILITY WARNING! In Millfork 0.3.2 and earlier, arrays couldn't be declared const and on cartridge-based targets, preinitialized arrays were assumed to be immutable and were allocated to ROM. Since 0.3.4, only const arrays can be allocated to ROM, non-const arrays are allocated to RAM and their contents are uninitialized before a call to init_rw_memory. See the ROM vs RAM guide.

• <element type>: type of the elements of the array. If omitted, the default is byte.

• <size>: either a constant number, which then defines the size of the array, or a name of a plain enumeration type, in which case changes the type of the index to that enumeration and declares the array size to be equal to the number of variants in that enumeration. If the size is not specified here, then it's deduced from the <initial_values>. If the declared size and the size deduced from the <initial_values> don't match, then an error is raised.

• <alignment> is either a numeric literal that is a power of 2, or keyword fast. The array will be allocated at the address divisible by alignment. fast means different things depending on the target platform:

  • on 6502, it means that the array will not cross a page boundary
  • on Z80, it means that the array will not cross a page boundary

• <address> is a constant expression that defines where in the memory the array is or will be located.

• <initial_values> is an array literal, see Literals. Local arrays can have initial values only if they're const.

TODO

Function declarations

A function can be declared at the top level. For more details, see Functions

Segment blocks

Instead of repeating the segment name in multiple declarations, you can wrap them in a block:

segment (s) {
    byte x
    word y    
} 

is equivalent to:

segment (s) byte x
segment (s) word y

import statements

import <module>

Adds a module to the program.

The module is looked up first in the current working directory, and then in the include directories.

Usually, the imported module will undergo the first phase of compilation first. This means that the constants in the imported module will be resolved first, allowing you to use them in the importing module.

The only exception to this rule is when the importing graph has a cycle, in which case the order of modules within the cycle is unspecified.

All starting modules are considered to be imported by all source files explicitly mentioned on the command line.

Statements

Expression statement

TODO

if statement

Syntax:

if <expression> {
    <body>
}

if <expression> {
    <body>
} else {
    <body>
}

if <expression> {
    <body>
} else if <expression> {
    <body>
} else {
    <body>
}

return statement

Syntax:

return

return <expression>

return[] statement (return dispatch)

Syntax examples:

return [a + b] {
   0   @ underflow
   255 @ overflow
   default @ nothing
}

return [getF()] {
   1 @ function1
   2 @ function2
   default(5) @ functionDefault
}

return [i] (param1, param2) {
   1,5,8 @ function1(4, 6)
   2     @ function2(9)
   default(0,20) @ functionDefault
}

Return dispatch calculates the value of an index, picks the correct branch, assigns some global variables and jumps to another function.

The index has to evaluate to a byte or to an enum. The functions cannot be macro and shouldn't have parameters. Jumping to a function with parameters gives those parameters undefined values.

The functions are not called, so they don't return to the function the return dispatch statement is in, but to its caller. The return values are passed along. If the dispatching function has a non-void return type different that the type of the function dispatched to, the return value is undefined.

If the default branch exists, then it is used for every missing index. If the index type is an non-empty enum, then the default branch supports all the other values. Otherwise, the default branch handles only the missing values between other supported values. In this case, you can override it with optional parameters to default. They specify the maximum, or both the minimum and maximum supported index value. In the above examples: the first example supports values 0–255, second 1–5, and third 0–20.

If the index has an unsupported value, the behaviour is formally undefined, but in practice the program will simply crash.

Before jumping to the function, the chosen global variables will be assigned parameter values. Variables have to be global byte-sized. Some simple array indexing expressions are also allowed. Parameter values have to be constants. For example, in the third example one of the following will happen:

• if i is 1, 5 or 8, then param1 is assigned 4, param2 is assigned 6 and then function1 is called;

• if i is 2, then param1 is assigned 9, param2 is assigned an undefined value and then function2 is called;

• if i is any other value from 0 to 20, then param1 and param2 are assigned undefined values and then functionDefault is called;

• if i has any other value, then undefined behaviour.

while and do-while statements

Syntax:

while <expression> {
    <body>
}

do {
    <body>
} while <expression>

for statements

Warning: for loops are a bit buggy.

Syntax:

for <variable> , <start> , <direction> , <end> {
}
for <variable> : <enum type> {
}
for <variable> : [ <comma separated expressions> ]  {
}

• <variable> – an already defined numeric variable

• <direction> – the range to traverse:

  • to – from <start> inclusive to <end> inclusive, in ascending order (e.g. 0,to,9 to traverse 0, 1,... 9)

  • downto – from <start> inclusive to <end> inclusive, in descending order (e.g. 9,downto,0 to traverse 9, 8,... 0)

  • until – from <start> inclusive to <end> exclusive, in ascending order (e.g. 0,until,10 to traverse 0, 1,... 9)

  • parallelto – the same as to, but the iterations may be executed in any order

  • paralleluntil – the same as until, but the iterations may be executed in any order

  There is no paralleldownto, because it would do the same as parallelto with swapped arguments.

• <enum type> – traverse enum constants of given type, in arbitrary order

• <comma separated expressions> – traverse every value in the list, in the given order. Values do not have to be constant. If a value is not a constant and its value changes while executing the loop, the behaviour is undefined. Jumps using goto across the scope of this kind of loop are disallowed.

break and continue statements

Syntax:

break
break for
break while
break do
break <variable>
continue
continue for
continue while
continue do
continue <variable>

goto and label

Syntax:

goto <expression>
label <name>

The label statement defines a constant pointer that refers to the current position in the code. Such labels are only visible in the scope of the local function.

The goto expression jumps to the pointer value of the expression.

Jumping using goto across the scope of for loop that uses a fixed list or across functions is not allowed.

Computed gotos are supported:

pointer p
p = x
goto p
label x

asm statements

See Using 6502 assembly within Millfork programs
or Using 8080/LR35902/Z80 assembly within Millfork programs.

Work in progress: For 8086, see the 8086 support disclaimer.