Asynchronous Serial Communications Programmer's Reference Manual (32022-90052)

74 Chapter3

Common Device Control Functions

Writing to Asynchronous Devices

When writing to a terminal, a controlcode value of %320 is often used.

This value speciﬁes that no carriage control will be sent to the terminal

and the next character printed by the next write will physically follow

the data written by the current write. You might use a controlcode of

%320 when prompting for user input at a terminal. No carriage return

or linefeed will follow the write and the cursor will be positioned next to

the prompt.

Figure 3-5 illustrates the use of the FWRITE intrinsic. This code

fragment opens a terminal ﬁle as $STDLIST with write access

speciﬁed. The prompt to be written to the terminal is placed into buffer

and the variable cctl is speciﬁed as %320. FWRITE is then called using

these values. The result will be that the prompt “Enter Your Name” will

be written to the device and the cursor will be placed next to the

prompt.

Your program should check condition codes after each FWRITE;

however, you should be aware that this intrinsic is completed logically.

This means that once data is passed to an output buffer the write is

considered to have completed successfully. If the output device has

suspended operation, due to ﬂow control or some other reason, the

actual write may not have occurred. There is no programmatic way to

verify the physical output of data to a device.