Symbol names begin with a letter or with one of ._. On most machines, you can also use $ in symbol 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.
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:
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.
This is the number that was used in the local label definition. So if the label is written 55: then the number is 55.
This unusual character is included so you do not accidentally invent a symbol of the same name. The character has ASCII value of \002 (control-B).
This is a serial number to keep the labels distinct. The first definition of 0: gets the number 1. The 15th definition of 0: gets the number 15, and so on. Likewise the first definition of 1: gets the number 1 and its 15th defintion gets 15 as well.
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.