ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition

I.4 Expressions
This section describes:
General expression syntax
Operators and functions - polymorphism and prototypes on page AppxI-10
Precedence rules on page AppxI-10.
I.4.1 General expression syntax
An expression is one of the following:
a constant
a variable, optionally preceded by a data type name to declare its type
the word UNKNOWN preceded by a data type name to declare its type
the result of applying a language-defined operator to other expressions
the result of applying a function to other expressions.
Pseudocode Definition
Variable names normally consist of alphanumeric and underscore characters, starting with an alphabetic or
underscore character.
Each register described in the text is to be regarded as declaring a correspondingly named bitstring variable,
and that variable has the stated behavior of the register. For example, if a bit of a register is defined as
RAZ/WI, then the corresponding bit of its variable reads as 0 and ignore writes.
An expression like bits(32) UNKNOWN indicates that the result of the expression is a value of the given type,
but the architecture does not specify what value it is and software must not rely on such values. The value
produced must not constitute a security hole and must not be promoted as providing any useful information
to software. (This was called an UNPREDICTABLE value in previous ARM architecture documentation. It is
related to but not the same as UNPREDICTABLE, which says that the entire architectural state becomes
similarly unspecified.)
A subset of expressions are assignable. That is, they can be placed on the left-hand side of an assignment.
This subset consists of:
Variables
The results of applying some operators to other expressions. The description of each
language-defined operator that can generate an assignable expression specifies the circumstances
under which it does so. (For example, those circumstances might include one or more of the
expressions the operator operates on themselves being assignable expressions.)
The results of applying array-like functions to other expressions. The description of an array-like
function specifies the circumstances under which it can generate an assignable expression.
Every expression has a data type. This is determined by:
For a constant, the syntax of the constant.
For a variable, there are three possible sources for the type
— its optional preceding data type name
ARM DDI 0406B Copyright © 1996-1998, 2000, 2004-2008 ARM Limited. All rights reserved. AppxI-9