Blocks and Guards

Blocks partition the concurrent statements in an architecture such that conditional activities unique to each block can occur
A guarded signal assignment statement generates a value only if the block guard expression is true. If false, the assignment is disconnected
Example

ARCHITECTURE guarded_assignments OF n_1_mux IS
BEGIN
  bi: FOR j IN i'range GENERATE
    bj: BLOCK (s(j) = '1' or s(j) = 'z')
        BEGIN
         x<= guarded i(j);
        END BLOCK;
  END GENERATE;
END guarded_assignments;