Locked exchange fifo

Abstract

A FIFO with locked exchange capability is disclosed. The FIFO has a memory for storing and retrieving data submissions, a write address generator and a read address generator for sequentially addressing the memory. A difference counter maintains the difference between the number of writes to the queue and reads from the queue. The net difference, as tracked by the counter is a measure of the FIFO utilization. To detect the queue full condition, a comparator compares the maximum FIFO stack depth against the counter output. The result of this comparison is latched and provided to a write strobe generator so that, in a subsequent write operation, if the FIFO is full, the write strobe from the producer is blocked and the data will not be written to the FIFO. Otherwise, the write strobe from the producer is passed to the memory. Additionally, a remaining space count is maintained in a status register. During operation, a bus master performing the read-modify-write cycle to the FIFO reads the status register to find the available space in the FIFO and immediately writes the data to the FIFO. If the read returns a zero, indicating that the FIFO is full, the bus master requeues the data for another read- modify-write cycle as it knows that the data has not been stored in the FIFO.