Abstract: Information concerning the access to a first-in first-out buffer memory for reading and writing operations is indicated by the use of two supplemental memories which provide availability and validity information based upon unoccupied location information indicating a buffer memory location where data has been read, valid occupied location information indicating a buffer memory location where data has not yet been read, and non-valid occupied location information indicating that the location in the buffer memory has been overwritten. For each character stored in the buffer memory, there is a corresponding bit in the respective supplemental memories. The value of the bits in the respective memories indicates the availability of the memory and whether an overwrite will occur. The information contained in the supplemental memories is updated by a writing operation depending upon the availability status of the buffer memory at the time of writing.