The @code identifier is a Text-EquateName created by the assembler when a .MODEL directive is encountered, at which time the assembler performs an automatic ASSUME CS:@code operation. The @code symbol is not defined if a .MODEL directive has not been issued.
Under MASM 5.10 emulation, the @code symbol is set to the name of the implicitly-defined default code segment (the segment opened when a .CODE directive is used) and its value is never changed. In other modes, the @code symbol is updated to reflect whatever segment is opened by using .CODE, whether defined implicitly or as an explicit parameter to the .CODE directive.
The value assigned to the @code symbol when the default code segment is opened is determined by the memory model as follows:
Memory Model
The module entry is replaced with base file name of the top-level module being assembled.