Data is reformatted in a memory external to the processing elements of a processing array, relieving the processing array of this task and allowing it to perform its data processing functions more efficiently. Data is transferred to or from the multi-channel memory in a wordstream format, with individual data words mapped into the proper memory channels and the proper addresses within each channel in a matrix format. The invention is applicable to two-dimensional reformatting for data transfers between an input/output port and the memory, and to one-dimensional reformatting for data transfers between the processing array and either the memory or the input/output port. Logic circuitry within each channel selects the proper data words for that channel from a wordstream, and the associated address for each selected word, according to the respective positions of the data words in the wordstream. Data words are assigned to memory channels and to addresses within those channels based upon programmable format specifiers.