copied from "Mapping the ATARI" as an example only! Needs to be re-written or completely written from scratch!
17 $0011 BRKKEY
Zero means the BREAK key is pressed; any other number means it's not. A BREAK during I/O returns 128 ($80). Monitored by both keyboard, display, cassette and screen handlers. See POKMSK location 16 ($A) for hints on disabling the BREAK key. The latest editions of OS provide for a proper vector for BREAK interrupts. The BREAK key abort status code is stored in STATUS (48; $30).
It is also checked during all I/O and scroll/draw routines. During the keyboard handler routine, the status code is stored in DSTAT (76; $4C). BRKKEY is turned off at powerup. BREAK key abort status is flagged by setting BIT 7 of 53774 ($D20E). See the note on the BREAK key vector, above.