Editing Acorn BBC Microcomputer (section)

===Operation===
Unlike a protocol such as the [[AT keyboard interface]], the BBC Micro keyboard does not transmit keypresses as they occur. All the matrix rows are connected at one end to a 74LS30 NAND gate,<ref name="AUG-489-90" /> which is in turn connected to CA2 (port A control line 2) of the System VIA; this control line generates the keyboard interrupt. (The VIA chips are MOS 6522 Versatile Interface Adapter ICs found on the computer motherboard.) The matrix columns are strobed continually at 1 MHz; when any key is pressed, the OS receives the interrupt signal, and takes control of the keyboard scanning. During controlled scanning, the matrix rows are sensed individually to identify which keys are pressed.<ref name="Arduino" /> Detection of key ''release'' however is unexplained.

Acorn MOS supports two-key rollover; two locations in zero page (&EC and &ED) hold the non-modifier keys currently depressed, or zero otherwise.<ref name="AUG-142" />  Row zero of the matrix has anti-ghosting diodes fitted<ref name="AUG-489-90" /> which allows contacts in this row to be detected simultaneously with any two keys elsewhere in the matrix.  Row zero contains the modifier keys '''shift''' and '''control''' and the start-up options.