HP 9000 Series 300 Tutorials Manual page 200

Display Data Handling
Output Data Structure
curses uses data structures called windows to collect display text, then
transfers the data structures to the terminal display screen during execution
of refresh routines. Each window contains a two-dimensional data array
for storing text and character highlighting attributes. Other data structures
associated with the window contain the current cursor position and various
pointers, and fill other curses needs.
Two windows are always present when curses is active. Current screen is
named curser for programming purposes, and represents the current screen. It
is used as a reference when optimizing output operations to the CRT screen.
The standard screen window, named stdscr, is the default destination for
all text output operations that are not directed to a window specified in the
function. Both curser and stdscr have the same row and column dimensions
as the physical display screen.
Additional program-definable windows can be created and dimensioned as
programming needs dictate. Such windows can be any size, provided they do
not exceed the row and/or column capacity of the physical display screen.
When a program requires a window that is larger than the available display
screen, pads are used. Pads have the same structure and characteristics as
a window, but they can be any size within the limits of reasonable memory
usage (each pad requires two bytes per character position plus data structure
overhead).
Text and Highlighting Data Format
Every window data structure contains, among other things, a two-dimensional
array of 32-bit data words, each word corresponding to a displayable character
in the window. The lower eight bits in each 32-bit word contain 8-bit character
code of the character associated with the corresponding screen display position.
The middle eight bits contain NLS attributes. The remaining sixteen bits
specify which highlighting attributes, if any, are to be used when the character
is displayed. The window data structure also contains a set of current
attributes that are used to form the attribute bits as each word is placed in
1-2 Using Curses and Terminfo