Complex sentence: Modern CPUs are typically 2-operand or 3-operand machines--the additional operands specify which one of many general purpose registers (also called "general purpose accumulators") are used as the source and destination for calculations.
Split into simpler sentences: Modern CPUs are typically 2-operand or 3-operand machines. The additional operands specify which one of many general purpose registers (also called "general purpose accumulators") are used as the source and destination for calculations.