Chapter 6. Symbols

37

They can also be distinguished from ordinary local labels by their transformed name which uses

ASCII character

\001

(control A) as the magic character to distinguish them from ordinary labels.

Thus the 5th defintion of

6$

is named

L6C A5

.

6.4. The Special Dot Symbol

The special symbol

.

refers to the current address that

as

is assembling into. Thus, the expression

melvin: .long .

defines

melvin

to contain its own address. Assigning a value to

.

is treated the

same as a

.org

directive. Thus, the expression

.=.+4

is the same as saying

.space 4

.

6.5. Symbol Attributes

Every symbol has, as well as its name, the attributes "Value" and "Type". Depending on output format,

symbols can also have auxiliary attributes.

If you use a symbol without defining it,

as

assumes zero for all these attributes, and probably won't

warn you. This makes the symbol an externally defined symbol, which is generally what you would

want.

6.5.1. Value

The value of a symbol is (usually) 32 bits. For a symbol which labels a location in the text, data, bss

or absolute sections the value is the number of addresses from the start of that section to the label.

Naturally for text, data and bss sections the value of a symbol changes as

ld

changes section base

addresses during linking. Absolute symbols' values do not change during linking: that is why they are

called absolute.

The value of an undefined symbol is treated in a special way. If it is 0 then the symbol is not defined

in this assembler source file, and

ld

tries to determine its value from other files linked into the same

program. You make this kind of symbol simply by mentioning a symbol name without defining it. A

non zero value represents a

.comm

common declaration. The value is how much common storage to

reserve, in bytes (addresses). The symbol refers to the first address of the allocated storage.

6.5.2. Type

The type attribute of a symbol contains relocation (section) information, any flag settings indicating

that a symbol is external, and (optionally), other information for linkers and debuggers. The exact

format depends on the object code output format in use.

6.5.3. Symbol Attributes:

a.out

6.5.3.1. Descriptor

This is an arbitrary 16 bit value. You may establish a symbol's descriptor value by using a

.desc

statement (Section 8.21

.desc symbol, abs expression

). A descriptor value means nothing to

as

.