I'd also like to add that, since immediate-operand instructions exist, constants are absolutely not the same as variables at the machine level, since immediates

I'd also like to add that, since immediate-operand instructions exist, constants are absolutely not the same as variables at the machine level, since immediates will never be stored in a register (typically, e.g. "move immediate" will obviously store it in one, and I'm sure there are architectures that use an internal/hidden register that's populated during instruction decode). Also, in Harvard-architecture systems, the constants, being part of the instruction itself, might not even be in the same memory or even address space as variables ([EEP]ROM/Flash vs RAM).