Q. Concurrently read exclusively write?

It's one of the models based on PRAM. In this model, processors access the memory location simultaneously for reading whereas exclusively for writing operations. In the algorithm that uses CREW model of computation n² number of processors have been connected in form of a two dimensional array of size n x n.

Algorithm Matrix Multiplication using CREW

Input// Two Matrices M1 and M2 

For I=1 to n                             //Operation performed in PARALLEL

      For j=1 to n                         //Operation performed in PARALLEL 

        {

               O_ij = 0;

               For k=1 to n

                    O_ij = O_ij + M1_ik * M2_kj

               End For

       } 

      End For 

End For

The complexity of CREW based algorithm is O(n).