The extended hardware buffering (FIFO-mode) capabilities available in supported systems apply to the NS-16550A UART device and other fully compatible devices. These serial devices are installed on many IBM PS/2 system boards, and on various ASYNC communications adapter options. Refer to Hardware Support for Extended Hardware Buffering.

On those systems that incorporate serial devices that fully and compatibly support Extended Hardware Buffering, the OS/2 ASYNC device driver provides three modes for exploiting this feature:

• Enabled
  
• Disabled
  
• Automatic Protocol Override

The default is to enable Automatic Protocol Override on that COM port. Automatic Protocol Override is an asynchronous device driver mode of operation that automatically controls various parameters of Extended Hardware Buffering, such as Receive Trigger Level and Transmit Buffer Load Count.

Automatic Protocol Override causes the Receive Trigger Level and Transmit Buffer Load Count to be adjusted according to the device driver handshaking protocols in effect. When Automatic Protocol Override mode is ON and the handshaking protocols are set to their default settings, the physical device driver partially exploits only the performance advantages of Extended Hardware Buffering. The default handshaking protocols are:

• Enabled for Input Sensitivity Using DSR 
  Enabled for Output Handshaking Using CTS and DSR
  
  Disabled for Output Handshaking Using DCD
  
  Disabled for Automatic Transmit Flow Control

  If both Input Sensitivity Using DSR and Output Handshaking Using CTS and DSR are disabled, the Automatic Protocol Override causes the asynchronous device driver to automatically reset internal parameters (fully exploiting the Extended Hardware Buffering capabilities to the maximum extent possible).

  The physical device driver's initialization default is to set Extended Hardware Buffering capabilities to the Automatic Protocol Override mode. An application or subsystem can alternatively set Extended Hardware Buffering to DISABLED, which causes the hardware to service transmit and receive interrupts one character at a time. It can also set Extended Hardware Buffering to ENABLED, which causes the physical device driver to set certain Extended Hardware Buffering parameters to specified levels, so the serial device fully exploits its Extended Hardware Buffering capabilities to the maximum extent possible.

  When Extended Hardware Buffering is set to ENABLED, the following serial device hardware capabilities are exploited for maximum performance benefit and increased receive data integrity:

  • By setting the Transmit Buffer Load Count to 16, up to 16 characters are placed in the transmit hardware buffer (FIFO) during the processing of one Transmit Holding Register Empty (THRE) interrupt.

  • A 16-character receive hardware buffer (FIFO) is available with the Receive Trigger Level set to 1, 4, 8, or 14 characters. The Receive Trigger Level determines when the receive hardware generates a Receive Data Available hardware interrupt.

  If the physical device driver receives an Open request for a COM port that is not already open, it does not alter the Extended Hardware Buffering setting that is in effect at the time. The ASYNC physical device driver preserves this state across multiple Open and Close requests.

  Initial Value

  Automatic Protocol Override mode is on
  First Level Open
  No effect
  Mode Utility
  User interface to set the Extended Hardware Buffering mode to ENABLED, DISABLED, or to Automatic Protocol Override

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