Chapter 6.

Symbols

Symbols are a central concept: the programmer uses symbols to name things, the linker uses symbols

to link, and the debugger uses symbols to debug.

Warning: as does not place symbols in the object file in the same order they were declared. This may break

some debuggers.

6.1. Labels

A label is written as a symbol immediately followed by a colon

:

. The symbol then represents the

current value of the active location counter, and is, for example, a suitable instruction operand. You are

warned if you use the same symbol to represent two different locations: the first definition overrides

any other definitions.

On the HPPA, the usual form for a label need not be immediately followed by a colon, but instead

must start in column zero. Only one label may be defined on a single line. To work around this, the

HPPA version of

as

also provides a special directive

.label

for defining labels more flexibly.

6.2. Giving Symbols Other Values

A symbol can be given an arbitrary value by writing a symbol, followed by an equals sign

=

, followed

by an expression (Chapter 7 Expressions). This is equivalent to using the

.set

directive. Section 8.79

.set symbol, expression

.

6.3. Symbol Names

Symbol names begin with a letter or with one of

._

. On most machines, you can also use

$

in sym 

bol names; exceptions are noted in Chapter 9 Machine Dependent Features. That character may be

followed by any string of digits, letters, dollar signs (unless otherwise noted in Chapter 9 Machine

Dependent Features), and underscores. For the AMD 29K family,

?

is also allowed in the body of a

symbol name, though not at its beginning.

Case of letters is significant:

foo

is a different symbol name than

Foo

.

Each symbol has exactly one name. Each name in an assembly language program refers to exactly

one symbol. You may use that symbol name any number of times in a program.

6.3.1. Local Symbol Names

Local symbols help compilers and programmers use names temporarily. They create symbols which

are guaranteed to be unique over the entire scope of the input source code and which can be referred

to by a simple notation. To define a local symbol, write a label of the form

N:

(where N represents

any positive integer). To refer to the most recent previous definition of that symbol write

Nb

, using the

same number as when you defined the label. To refer to the next definition of a local label, write

Nf

The

b

stands for"backwards" and the

f

stands for "forwards".

There is no restriction on how you can use these labels, and you can reuse them too. So that it is

possible to repeatedly define the same local label (using the same number

N

), although you can only

refer to the most recently defined local label of that number (for a backwards reference) or the next