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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.
5.1 Labels 5.2 Giving Symbols Other Values 5.3 Symbol Names 5.4 The Special Dot Symbol 5.5 Symbol Attributes
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.
A symbol can be given an arbitrary value by writing a symbol, followed
by an equals sign `=', followed by an expression
(see section 6. Expressions). This is equivalent to using the .set
directive. See section .set.
Symbol names begin with a letter or with one of `._'. On most
machines, you can also use $ in symbol names; exceptions are
noted in 8. Machine Dependent Features. That character may be followed by any
string of digits, letters, dollar signs (unless otherwise noted in
8. 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.
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 definition of a specific local label for a forward reference. It is also worth noting that the first 10 local labels (`0:'...`9:') are implemented in a slightly more efficient manner than the others.
Here is an example:
1: branch 1f 2: branch 1b 1: branch 2f 2: branch 1b |
Which is the equivalent of:
label_1: branch label_3 label_2: branch label_1 label_3: branch label_4 label_4: branch label_3 |
Local symbol names are only a notational device. They are immediately transformed into more conventional symbol names before the assembler uses them. The symbol names stored in the symbol table, appearing in error messages and optionally emitted to the object file. The names are constructed using these parts:
L
as and
ld forget symbols that start with `L'. These labels are
used for symbols you are never intended to see. If you use the
`-L' option then as retains these symbols in the
object file. If you also instruct ld to retain these symbols,
you may use them in debugging.
number
C-B
ordinal number
So for example, the first 1: is named L1C-B1, the 44th
3: is named L3C-B44.
as also supports an even more local form of local labels called
dollar labels. These labels go out of scope (ie they become undefined) as soon
as a non-local label is defined. Thus they remain valid for only a small
region of the input source code. Normal local labels, by contrast, remain in
scope for the entire file, or until they are redefined by another occurrence of
the same local label.
Dollar labels are defined in exactly the same way as ordinary local labels, except that instead of being terminated by a colon, they are terminated by a dollar sign. eg `55$'.
They can also be distinguished from ordinary local labels by their transformed name which uses ASCII character `\001' (control-A) as the magic character to distinguish them from ordinary labels. Thus the 5th defintion of `6$' is named `L6C-A5'.
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'.
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.
5.5.1 Value 5.5.2 Type 5.5.3 Symbol Attributes: a.out5.5.3 Symbol Attributes: a.outSymbol Attributes: a.out,b.out5.5.4 Symbol Attributes for COFF 5.5.5 Symbol Attributes for SOM
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.
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.
a.out
5.5.3.1 Descriptor 5.5.3.2 Other
This is an arbitrary 16-bit value. You may establish a symbol's
descriptor value by using a .desc statement
(see section .desc). A descriptor value means nothing to
as.
This is an arbitrary 8-bit value. It means nothing to as.
The COFF format supports a multitude of auxiliary symbol attributes;
like the primary symbol attributes, they are set between .def and
.endef directives.
The symbol name is set with .def; the value and type,
respectively, with .val and .type.
The as directives .dim, .line, .scl,
.size, and .tag can generate auxiliary symbol table
information for COFF.
The SOM format for the HPPA supports a multitude of symbol attributes set with
the .EXPORT and .IMPORT directives.
The attributes are described in HP9000 Series 800 Assembly
Language Reference Manual (HP 92432-90001) under the IMPORT and
EXPORT assembler directive documentation.
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