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6. Expressions

An expression specifies an address or numeric value. Whitespace may precede and/or follow an expression.

The result of an expression must be an absolute number, or else an offset into a particular section. If an expression is not absolute, and there is not enough information when as sees the expression to know its section, a second pass over the source program might be necessary to interpret the expression--but the second pass is currently not implemented. as aborts with an error message in this situation.

6.1 Empty Expressions  
6.2 Integer Expressions  

6.1 Empty Expressions

An empty expression has no value: it is just whitespace or null. Wherever an absolute expression is required, you may omit the expression, and as assumes a value of (absolute) 0. This is compatible with other assemblers.

6.2 Integer Expressions

An integer expression is one or more arguments delimited by operators.

6.2.1 Arguments  
6.2.2 Operators  
6.2.3 Prefix Operator  Prefix Operators
6.2.4 Infix Operators  

6.2.1 Arguments

Arguments are symbols, numbers or subexpressions. In other contexts arguments are sometimes called "arithmetic operands". In this manual, to avoid confusing them with the "instruction operands" of the machine language, we use the term "argument" to refer to parts of expressions only, reserving the word "operand" to refer only to machine instruction operands.

Symbols are evaluated to yield {section NNN} where section is one of text, data, bss, absolute, or undefined. NNN is a signed, 2's complement 32 bit integer.

Numbers are usually integers.

A number can be a flonum or bignum. In this case, you are warned that only the low order 32 bits are used, and as pretends these 32 bits are an integer. You may write integer-manipulating instructions that act on exotic constants, compatible with other assemblers.

Subexpressions are a left parenthesis `(' followed by an integer expression, followed by a right parenthesis `)'; or a prefix operator followed by an argument.

6.2.2 Operators

Operators are arithmetic functions, like + or %. Prefix operators are followed by an argument. Infix operators appear between their arguments. Operators may be preceded and/or followed by whitespace.

6.2.3 Prefix Operator

as has the following prefix operators. They each take one argument, which must be absolute.

Negation. Two's complement negation.
Complementation. Bitwise not.

6.2.4 Infix Operators

Infix operators take two arguments, one on either side. Operators have precedence, but operations with equal precedence are performed left to right. Apart from + or `-', both arguments must be absolute, and the result is absolute.

  1. Highest Precedence


    Division. Truncation is the same as the C operator `/'


    Shift Left. Same as the C operator `<<'.

    Shift Right. Same as the C operator `>>'.

  2. Intermediate precedence


    Bitwise Inclusive Or.

    Bitwise And.

    Bitwise Exclusive Or.

    Bitwise Or Not.

  3. Low Precedence

    Addition. If either argument is absolute, the result has the section of the other argument. You may not add together arguments from different sections.

    Subtraction. If the right argument is absolute, the result has the section of the left argument. If both arguments are in the same section, the result is absolute. You may not subtract arguments from different sections.

    Is Equal To
    Is Not Equal To
    Is Less Than
    Is Greater Than
    Is Greater Than Or Equal To
    Is Less Than Or Equal To

    The comparison operators can be used as infix operators. A true results has a value of -1 whereas a false result has a value of 0. Note, these operators perform signed comparisons.

  4. Lowest Precedence

    Logical And.

    Logical Or.

    These two logical operations can be used to combine the results of sub expressions. Note, unlike the comparison operators a true result returns a value of 1 but a false results does still return 0. Also note that the logical or operator has a slightly lower precedence than logical and.

In short, it's only meaningful to add or subtract the offsets in an address; you can only have a defined section in one of the two arguments.

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