Might suggest to change the title to specify this is about the Assembly language.
The modem will perform the local analog loopback test if &T1 is selected. The test can be run only when in an asynchronous operation in non-error-correction mode (normal). To terminate the test in progress, the escape sequence must be entered first (see Section 3.1.1). If S18 is non-zero, the test will terminate automatically after the time specified by S18 and the OK result code will be reported.
https://web.archive.org/web/20151028101531/http://www.zoomte...
AT vs AT&T phone company and childhood memories.
(Clearly this was wrong after clicking into the article)