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122 lines
4.9 KiB
Plaintext
122 lines
4.9 KiB
Plaintext
C02 for C programmers
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TYPES
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C02 only supports one data type: unsigned char.
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POINTERS
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C02 does not support pointer type variables or parameters. However, the
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address-of operator may be used in function calls and the inline
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statement.
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DIRECTIVES
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C02 does not use a preprocessor. All directives are directly processed
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by the compiler. C02 supports the directives #include and #pragma. The
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directive #define is currently unimplemented and generates an error.
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The #if, #else, and #endif directives are not recognized by C02.
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CONSTANTS
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The syntax of the const keyword in C02 differs from that use in standard C.
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In C02, all constant names are prefixed with a # symbol.
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ENUMERATION
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The syntax of an enum statement in C02 is similar to standard C, except
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that no type name is specified and values may not be explicitly assigned
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to the enumerated constants.
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DECLARATIONS
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Variable and function names may be no more than six characters in length.
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Multiple variable declarations separated by commas are allowed.
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A variable in a declaration may be initialized by following it with an
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equal sign and a constant, however this declaration is done at compile
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time, so no re-initialization will occur during code execution.
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Array declarations using bracket syntax specify the upper bound, rather
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than the array size. Therefore, the array will be allocated with one more
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element than the specified number.
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Only one-dimensional arrays of type char are allowed and are limited to
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a total of 256 elements. Multi-dimensional arrays, arrays of arrays, and
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arrays of structs are not supported.
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Struct definition and syntax is similar to standard C. When defining a
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struct, the char keyword is optional, since char the char type is implied.
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Struct members may only be simple variables or arrays. The maximum total
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size of a struct is 256 bytes. Initialization of members during declaration
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of a struct variable is not supported.
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REGISTERS
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The 6502 registers may be explicitly referenced by name, i.e. A, X, and Y.
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These may only be used in specific places due to the nature of the compiled
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code a well as the limitations of the 6502 instruction set.
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EXPRESSIONS
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C02 supports the addition, subtraction, bitwise-and, bitwise-or, and
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exclusive-or operators. The multiplication, division, and binary shift
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operators are not supported. These can be implemented through functions.
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Unary minus may only be used at the beginning of an expression, in which
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case it is treated as a literal 0 and the subtraction operater, and the
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term following the minus is used as the second term of the expression.
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Functions calls may only be used in the first term of an expression.
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However, the first argument of any function call may be any expression
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(including one with a function call as the first term).
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As in standard C, subscripted array elements may be used in any term of
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an exptression and the index may be any expression (including one with
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a function call as the first term).
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The sizeof operator in C02 is the at sign @. It may only be used with
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declared variables and struct members, not types.
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CONDITIONALS
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Conditional operations, including comparisons and logical operators,
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may not be used within or in place of normal expressions, and are only
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allowed in if, while, and for statements, as well as shortcut-ifs.
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The comparison operators are ==, <, <=, >=, >, and <>. Unlike standard
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C, the not-equals operator is <> instead of !=. In addition, = may be
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substituted for ==. The logical not operator ! negates an entire
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comparison rather than an individual term within the comparison, thus
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the C02 code !b<x is equivalent to the C code !(b<x).
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C02 uses the logical operators "and" and "or" instead of "&&" and "||",
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and performs short-circuit evaluation of logical operators, making them
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effectively right associative.
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FUNCTIONS
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Parameter passing uses the 6502's A, Y, and X registers. This limits
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function calls to a maximum of three char variables or address reference
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plus and optional char variable. However this also allows the return
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of up to three char variables (see assignments below). However, the
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non-standard push and pop statements allow explicit parameter passing
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via the stack, and the inline keyword has been re-purposed to allow
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explicit passing of inline parameters.
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ASSIGNMENTS
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Unlike standard C, struct contents may not be copied via simple assignment.
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Struct variables may be copied byte by byte or using memory move functions.
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Up to three comma-separated variables may be specified on the left side of
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the = assignment operator. This is generally only useful when the expression
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contains a function with multiple return values.
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STATEMENTS
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Instead of the switch statement, C02 uses the select statement. The
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select statement works almost identically to the switch statement except
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that case blocks do not fall through.
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