Cursor and pen-detectable fields

BMS supports detectable fields, another special hardware feature available on some terminals. There are two hardware mechanisms for detectable fields: the “cursor select” key and the light pen.

A terminal has either the key or a pen, not both. Both work the same way and, as the key succeeded the pen, we talk about the key.

For a field to be detectable, it must have certain field attributes, and the first character of the data, known as the designator character , must contain one of five particular values. You can have other display data after the designator character if you want.

The bits in the field attributes byte that govern detectability also control brightness. High intensity (ATTRB=BRT) fields are detectable if the designator character is one of the detectable values. Normal intensity fields may or may not be detectable; you have to specify ATTRB=DET to make them so; nondisplay (ATTRB=DRK) fields cannot be detectable.

As usual, you can specify attributes and designator characters either in the map definition or by program override. However, DET has a special effect when it appears in an input-only map, as we explain in a moment.

Note that because high-intensity fields have, by definition, the correct field attributes for detectability, the terminal operator can make an unprotected high-intensity field detectable by keying a designator character into the first position of the field.