In most register allocators, each variable is assigned to either a CPU register or to main memory. The advantage of using a register is speed. Computers have a limited number of registers, so not all variables can be assigned to registers. A “spilled variable” is a variable in main memory rather than in a CPU register. The operation of moving a variable from a register to memory is called spilling, while the reverse operation of moving a variable from memory to a register is called filling. For example, a 32-bit variable spilled to memory gets 32 bits of stack space allocated and all references to the variable are then to that memory. Such a variable has a much slower processing speed than a variable in a register. When deciding which variables to spill, multiple factors are considered: execution time, code space, data space.