Information processing system having multiple modules and a memory on a

Abstract

Apparatus and method are provided for preventing access to a memory location while that memory location is being modified, updated, etc. When a peripheral device wishes to accomplish such a change at a memory location, it provides the changed data and its intended memory address to an input/output unit. The input/output unit includes a plurality of separately controlled multiplexers, the number of multiplexers being preferably selected to correspond to the size (in bits) of a memory data word or packet divided by the size (in bits) of a peripheral data word. The input/output unit reads the data at the requested memory location into an input buffer, combines the portions of that data not to be modified with the data provided by the peripheral, and sends the result back to the same memory location. During this Read-Modify-Write operation, the input/output unit also monitors the system bus for any attempts or requests to read data from, or write data to, the memory address for which the Read-Modify-Write operation is being performed. In such event, a signal is sent to the module making such attempt or request, asking or telling that module to wait. That signal is removed when that Read-Modify-Write operation is completed.