FSMD is called as Finite State Machine with Datapath. It is mathematical abstraction sometimes used to design digital logic or computer program

Computer program

A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

s.

An FSMD is a digital system composed of a finite-state machine, which controls the program flow, and a `datapath

Datapath

A datapath is a collection of functional units, such as arithmetic logic units or multipliers, that perform data processing operations. Most central processing units consist of a datapath and a control unit, with a large part of the control unit dedicated to regulating the interaction between the...

`, 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.