FSMD is called as Finite State Machine with Datapath. It is mathematical abstraction sometimes used to design digital logic or computer programs.
An FSMD is a digital system composed of a finite-state machine, which controls the program flow, and a `datapath`, which performs operations.
FSMD is more complex state diagram is essentially a sequential program in which statements have been scheduled into states.
Here, we first convert our program into a complex state diagram in which states and arcs may include arithmetic expressions, and those expressions may use external inputs and outputs as well as variables.
FSM does not use variables, arithmetic operations/conditions. So FSMD's are more powerful than FSM's.
The FSMD level is often referred to as the register-transfer (RT) level.