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
0 L2 _& Q3 |8 h2 o$ z请问是什么原因，怎样解决？
6 c6 w1 D0 T3 A9 N. N. m" I0 n* _, r# vsim -vopt work.mul
# ** Note: (vsim-3812) Design is being optimized...
# Loading G:\Modeltech_6.2b\win32/../std.standard
# Loading work.mul(func)
# ** 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
8 L/ R. s) ^3 R( W# Error loading design
" E# h5 u% ^" D# d! V7 B4 w# E
9 R3 s- q1 k. B' c& I请大家帮帮忙，谢谢