6530 Programmer's Guide

Tandem NonStop Kernel Application Interface
4-2
6530 Programmer’s Guide
To communicate with the 6530, your application makes calls to the
Guardian file system requesting various I/O operations, such as reads and
writes. These calls access a set of generic procedures used to perform I/O
on different types of devices as well as files. The workstation or PC is
treated as a system terminal. The use of generic procedure calls for I/O
makes most of the device-specific details transparent to your application.
To establish an I/O channel between the application process and a
terminal, your application first calls the OPEN procedure with the device
name or logical device number for that terminal. (The device name and
corresponding logical device number are assigned during SYSGEN.) The
file system then accesses the appropriate I/O process to handle that
device. The particular I/O process accessed depends on how the terminal
is connected to the host system. For example, terminals connected to the
host patch panel are controlled by TERMPROCESS.
The file system handles the transfer of data between your application
buffers and the host system buffers. After receiving a request from the
file system (initially from your application), the I/O process then handles
the transfer of data between the host system and the terminal. Most of the
functions of the I/O process are transparent to your application. For
example, the I/O process adds the appropriate communications control
characters to outgoing data and strips these characters from incoming
data. Data is transferred via the I/O controller, which provides the
physical and electrical interface between the host system and the
communications line to the terminal. The I/O process reports the
completion status of an operation to the file system. In the case of an
error or device failure, an indication is returned to your application.
At the terminal end, data is transferred through the I/O port and the
communications control system (CCS). The CCS checks incoming data
for parity and other communications errors and then passes either the
data or a communications error message to the 6530. The 6530 then
processes the data or displays the communications error on the screen.
The 6530 interprets incoming data as either displayable (graphics)
characters, which are stored in display memory or control codes and
sequences, which invoke various terminal functions (as described in
previous sections). For outgoing data, the 6530 calls the CCS to handle
the transfer of data to the host system.