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Guard is a logical expression which has Boolean type in the block. Guarded expression in structure is used to control the concurrent signal assignment in the block. Signal assignments are processed as long as guarded condition written on the right side is TRUE. In th case of WRONG, no aciton is taken.

Expression of guard refers to the signal that takes place in the block declaration under the name of group. Guard signal can be seen only in the related block.

Guard signal can be drived by a seperate process. If any of the signals written on the right side of the guard expression is changed, guard expression becomes active then signals are updated as TRUE.

Unless a guard expression is used, the default guard signal value is TRUE.

Guard block is not syntesizable.

     signal<= guarded expresssion;


RISING_EDGE : block (CLK'EVENT and CLK='1')
  OUT_1 <= guarded not IN_1;
end block RISING_EDGE;

In this example, OUT_1 v ariable becomes not IN_1, when both values of CLK'EVENT and CLK are "1",






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