Because inline assembly instructions can be mixed with C or C++ statements, they can refer to C or C++ variables by name and use many other elements of those languages.
An __asm block can use the following language elements:
Symbols, including labels and variable and function names
Constants, including symbolic constants and enum members
Macros and preprocessor directives
Comments (both /* */ and // )
Type names (wherever a MASM type would be legal)
typedef names, generally used with operators such as PTR and TYPE or to specify structure or union members
Within an __asm block, you can specify integer constants with either C notation or assembler radix notation (0x100 and 100h are equivalent, for example). This allows you to define (using #define) a constant in C and then use it in both C or C++ and assembly portions of the program. You can also specify constants in octal by preceding them with a 0. For example, 0777 specifies an octal constant.