Using as - 5. Symbols

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# 5. 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.

## 5.1 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.

## 5.2 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 (see section 6. Expressions). This is equivalent to using the `.set` directive. See section 7.59 `.set symbol`, expression.

## 5.3 Symbol Names

Symbol names begin with a letter or with one of ``._'`. On most machines, you can also use `$` in symbol names; exceptions are noted in section 8. Machine Dependent Features. That character may be followed by any string of digits, letters, dollar signs (unless otherwise noted in section 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.

### 5.3.1 Local Symbol Names

Local symbols help compilers and programmers use names temporarily. There are ten local symbol names, which are re-used throughout the program. You may refer to them using the names ``0'` ``1'` ... ``9'`. To define a local symbol, write a label of the form ``N:'` (where N represents any digit). To refer to the most recent previous definition of that symbol write ``Nb'`, using the same digit as when you defined the label. To refer to the next definition of a local label, write ``Nf'`---where N gives you a choice of 10 forward references. The ``b'` stands for "backwards" and the ``f'` stands for "forwards".

Local symbols are not emitted by the current GNU C compiler.

There is no restriction on how you can use these labels, but remember that at any point in the assembly you can refer to at most 10 prior local labels and to at most 10 forward local labels.

Local symbol names are only a notation 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 have these parts:

`L`
  All local labels begin with ``L'`. Normally both `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.
`digit`
  If the label is written ``0:'` then the digit is ``0'`. If the label is written ``1:'` then the digit is ``1'`. And so on up through ``9:'`.
``C-A``
  This unusual character is included so you do not accidentally invent a symbol of the same name. The character has ASCII value ``\001'`.
`ordinal number`
  This is a serial number to keep the labels distinct. The first ``0:'` gets the number ``1'`; The 15th ``0:'` gets the number ``15'`; etc.. Likewise for the other labels ``1:'` through ``9:'`.

For instance, the first `1:` is named `L1`C-A`1`, the 44th `3:` is named `L3`C-A`44`.

## 5.4 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'`.

## 5.5 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.

### 5.5.1 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.

### 5.5.2 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.

### 5.5.3 Symbol Attributes: `a.out`

#### 5.5.3.1 Descriptor

This is an arbitrary 16-bit value. You may establish a symbol's descriptor value by using a `.desc` statement (see section 7.11 `.desc symbol`, abs-expression). A descriptor value means nothing to `as`.

#### 5.5.3.2 Other

This is an arbitrary 8-bit value. It means nothing to `as`.

### 5.5.4 Symbol Attributes for COFF

The COFF format supports a multitude of auxiliary symbol attributes; like the primary symbol attributes, they are set between `.def` and `.endef` directives.

#### 5.5.4.1 Primary Attributes

The symbol name is set with `.def`; the value and type, respectively, with `.val` and `.type`.

#### 5.5.4.2 Auxiliary Attributes

The `as` directives `.dim`, `.line`, `.scl`, `.size`, and `.tag` can generate auxiliary symbol table information for COFF.

### 5.5.5 Symbol Attributes for SOM

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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