Page 61 - ARM 64 Bit Assembly Language

GNU assembly syntax 45

Note that the four variables are shown to be in the data segment, and the arysize symbol is
marked as an “absolute” symbol, which simply means that it is a number and not an address.
It will have no storage location in the running program, but it’s value can be used whereever
needed while the file is being assembled. The programmer can now use the symbol arysize
to control looping when accessing the array data. If the size of the array is changed by adding
or removing constant values, the value of arysize will change automatically, and the pro-
grammer will not have to search through the code to change the original value, 5, to some
other value in every place it is used.

2.3.5 Functions and objects

There are a few assembler directives that are used for defining the size and type of labels. This
information is stored in the object file along with the code and data, and is used by the linker
and/or debugger.
.size name,expression
The .size directive is used to set the size associated with a symbol. This information
helps the linker to exclude unneeded code and/or data when creating an executable file,
and helps the debugger to keep track of where it is in the program.
.type name,type_description
The .type directive sets the type of a symbol name to be either a function or an object.
Valid values for type_desription in GNU AArch64 assembly include:
%function The symbol is a function name.
%gnu_indirect_function The symbol is an indirect function (may be called
through a function pointer table).
%object The symbol is a data object.
%tls_object The symbol is a thread-local data object.
%common The symbol is a common (shared) object.
%notype The symbol has no type.
Note: Some assemblers, including some versions of the GNU assembler, a may require
the @ character instead of the % character.

The following example shows how a typical function is declared.

1 .type myfunc, %function
2 .global myfunc
3 myfunc: // an example function named myfunc
4
5 // The function code goes here
6
7 .size myfunc,(. - myfunc)