The only documentation I know of, on Internet, is for subscribers only at:
Anyway here are answers to your questions.
These statements are used in a Tandem language called SCOBOL (Screen COBOL). SCOBOL uses COBOL syntax to build screens using the Tandem 6530 emulation - much like the IBM 3270 (which SCOBOL also supports).
Screens are constructed in the Screen Section. Rows, columns and their attributes and values are defined in this section.
Screens are anchored into screen bases - typically a BASE is defined as starting on line 1, colum 2 and ending in line 24, column 79 for a 24X80 display screen.
Inside a screen base, you can define areas to hold one or more OVERLAY screens. These are the same as a screen base, except they exist only within the context of their parent BASE screen.
It is possible to have more than one BASE. It is also possible to have several OVERLAY screens alternatively share a single overlay area on a BASE. Example: the same BASE for the company and different overlays for departments within the company.
DISPLAY BASE
DISPLAY OVERLAY
Before data can be displayed or manipulated on the screen, DISPLAY BASE establishes the current screen, which serves as the foundation for all other screen operations.
Similarly, DISPLAY OVERLAY establishes the current overlay screen, and must execute before other screen operations using the overlay screen.
Example:
DISPLAY BASE main-menu
DISPLAY main-menu
DISPLAY OVERLAY employees-only-menu
DISPLAY employees-only-menu
ACCEPT main-menu, employees-only-menu
LENGTH [MUST BE]
The LENGTH clause specifies the acceptable number of characters that can be entered into a screen input field. MUST BE refers to the absolute number of characters that must be entered.
Example:
02 input-field-1
AT 6, 4
USING ws-input-field-1
PIC X(08)
LENGTH MUST BE 8
Dimandja
