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

124 Chapter6

Typeahead Mode

Programming for Typeahead Mode

To programmatically set and control typeahead mode you use the

FDEVICECONTROL intrinsic, specifying one of three control

directives in the parm1 value.

• A control directive of 51 is used to specify whether or not typeahead

mode is on for the device. Use this call with the buffer parameter

set to 1 to enable typeahead mode, or with the buffer parameter set

to 0 to disable typeahead mode (parm2 set to 2 or 3).

• A control directive of 60 allows you to discard any data in the

typeahead buffer and read directly from the device.

• A control directive of 61 allows you to bypass the typeahead buffer

for the next read while saving the data already in the typeahead

buffer.

• A control directive of 63 is used to enable or disable single echo

typeahead. Use this call with the buffer parameter set to 1 to enable

typeahead mode, or with the buffer parameter set to 0 to disable

single echo typeahead mode (parm2 set to 2 or 3).

The code fragments that follow illustrate how these calls might look in

your program.

NOTE

When typeahead is enabled it is enabled for the device and not

speciﬁcally for an application. It is a good practice to make sure that

any program that enables typeahead mode also restores the terminal’s

previous typeahead setting before it terminates. To do so, your program

should save the value of the device’s current typeahead setting. This is

obtained either through a separate call to FDEVICECONTROL with a

control directive of 51 and parm2 set to 1 (Read access), or through the

call which changes the setting, with parm2 set to 3 (Read/Write

access). When your program is ready to end execution, it will be able to

return the device to its original typeahead setting through another

FDEVICECONTROL call, using the value of the previous setting

obtained in the ﬁrst call as the value of buffer.