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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: sde-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  


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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.


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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 .set.


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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 8. MIPS Dependent Features. That character may be followed by any string of digits, letters, dollar signs (unless otherwise noted in 8. MIPS Dependent Features), and underscores.

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 Symbol Names

Local symbols help compilers and programmers use names temporarily. You can define and use as many local symbol names as you require, and they can be re-used throughout the program. You may refer to them using a positive decimal number. To define a local symbol, write a label of the form `N:' (where N represents any positive number, or zero). To refer to the most recent previous definition of that symbol write `Nb', using the same value of N 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".

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

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', or in the case of ELF format `.L'. Normally both sde-as and sde-ld don't generate symbol table entries for local labels. These labels are used for symbols you are never intended to see. If you use the `-L' option then sde-as retains these symbols in the object file. If you also instruct sde-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.

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 L1C-A1, the 44th 3: is named L3C-A44.


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5.4 The Special Dot Symbol

The special symbol `.' refers to the current address that sde-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'.


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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, sde-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  


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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 sde-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 sde-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.


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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.


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