Final answer:
Register variables are stored in the CPU's registers, with the specific registers used depending on the computer's architecture. In the x86 architecture, common registers include AX, BX, CX, and DX. The compiler decides which variables are stored as register variables.
Step-by-step explanation:
In computer programming, especially in the context of low-level languages like C or C++, register variables are a type of variable that is stored in the CPU's registers rather than in RAM. Registers are the fastest storage available in a computer, thus variables stored in them can be accessed quickly. The specific registers used depend on the computer's architecture and the compiler's register allocation algorithms. In the x86 architecture, for instance, registers such as AX, BX, CX, and DX (and their extended versions like EAX, EBX, ECX, EDX for 32-bit operations) are commonly used for this purpose. However, not all variables can be 'register variables'; the compiler makes the decision based on various factors, including the frequency of variable use and the current register availability.