\[ \textbf{Given Statements:} \] \[ \begin{array}{ll} \textbf{I.} & \text{Hardware control units are generally faster but more difficult to modify than microprogrammed control units.} \\[6pt] \textbf{II.} & \text{In a horizontal microprogrammed control unit, each control signal has a separate bit in the control word.} \\[6pt] \textbf{III.} & \text{Vertical microprogramming leads to longer control words but provides greater parallelism.} \\[6pt] \textbf{IV.} & \text{Microprogrammed control units are typically easier to implement and modify than hardware control units.} \end{array} \] --- \[ \textbf{Evaluation of Statements:} \] \[ \begin{aligned} \textbf{I: True.} \quad & \text{Hardware control is faster, but hardwired circuits are difficult to modify.} \\[6pt] \textbf{II: True.} \quad & \text{In horizontal microprogramming, each control signal corresponds to one bit in the control word.} \\[6pt] \textbf{III: False.} \quad & \text{Vertical microprogramming uses compact encoding (shorter control words), but less parallelism.} \\[6pt] \textbf{IV: True.} \quad & \text{Microprogrammed control is flexible, easier to implement and modify compared to hardware control.} \end{aligned} \] --- \[ \boxed{\text{Correct Statements: I, II, and IV}} \]