Chapter 7.

Expressions

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

pression.

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.

7.1. 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.

7.2. Integer Expressions

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

7.2.1. 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 paren 

thesis

)

; or a prefix operator followed by an argument.

7.2.2. 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.

7.2.3. Prefix Operator

as

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

Negation. Two's complement negation.