modelsim仿真出现错误FATAL ERROR while loading design 该怎么解决，谢谢

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初学modelsim，仿真时出现问题，FATAL ERROR while loading design
' m1 N7 I! m4 {  e) h请问是什么原因，怎样解决？
) V$ a  \& e& v3 q# vsim -vopt work.mul
  n' G* q3 o$ W- b6 f6 f. T9 N. R# ** Note: (vsim-3812) Design is being optimized...
) b0 @1 T+ Y. G2 \& s5 s# w+ n9 ?9 V# Loading G:\Modeltech_6.2b\win32/../std.standard
/ f9 C6 J8 b8 y0 t5 r" w# Loading work.mul(func)
& b) d% g8 u; K( C8 d* J# ** Fatal: (vsim-3347) Port 'in1' is not constrained.
#    Time: 0 ns  Iteration: 0  Instance: /mul File: G:/Modeltech_6.2b/examples/mul.vhd Line: 3
# FATAL ERROR while loading design
# Error loading design

$ M8 g9 ~5 @" {, F7 W请大家帮帮忙，谢谢