Memory-Mapped Access to Serial Ports

Memory-Mapped Access to Serial Ports

The Audio/Serial Option (ASO) board for the Challenge and Onyx series provides six high-performance serial ports, each of which can be set to run at speeds as high as 115,200 bits per second. The features and administration of the Audio/Serial Option board are described in the Audio/Serial Option User's Guide (document 007-2645-001).

The serial ports of the ASO board can be accessed in the usual way, by opening a file to a device in the /dev/tty* group of names. However, for the minimum of latency and overhead, a program can open a device in the /dev/aso_mmap directory. These device files are managed by a device driver that permits the input and output ring buffers for the port to be mapped directly into the user process address space. The user-level program can spin on the input ring buffer pointers and detect the arrival of a byte of data in microseconds after the device driver stores it.

The details of the memory-mapping driver for ASO ports are spelled out in the asoserns(7) reference page (available only when the ASO feature has been installed).