System for transferring frame data by transferring the descriptor index data to identify a specified amount of data to be transferred stored in the host computer

摘要

A network interface device includes a random access memory used as a transmit and receive buffer for transmission and reception of data frames between a host computer bus and a packet switched network. The network interface device includes read and write controllers for each of the transmit and receive buffers, where each write controller operates in a clock domain separate from the corresponding read controller. The memory management unit also includes a synchronization circuit that controls arbitration for accessing the random access memory between the read and write controllers. The synchronization circuit determines the presence of a stored frame in the random access memory by asynchronously comparing write counter and read counter values. The memory management unit also includes a descriptor management unit for controlling DMA transfers between the transmit and receive buffers and the system memory. The descriptor management obtains descriptor lists from system memory based on descriptor index data supplied by the host CPU and verified as valid by the descriptor management, and releases control of the descriptor lists by writing to status locations in system memory.