# The Grid subsystem

The grid subsystem at B401 supplies a set of words on top of
the Grid protocol (see doc/protocol.txt) that facilitates the
development of programs presenting a complex text interface,
for example, the Visual Editor.

It create the concept of a cursor, always being a some position
on screen. That position is in the variable XYPOS, which is a
simple integer following the same "pos" logic as in the Grid
protocol.

It implements (emit), which sets the cell under the cursor to
the specified character, then moves the cursor right. If the
cursor is at the last column of the screen, it overflows to the
next line. If it's on the last line, it overflows to the first
line.

Grid's (emit) handles 0xd by moving the cursor to the next line
and 0x8 by moving the cursor left.

AT-XY ( x y -- ) moves the cursor to the specified position. It
is equivalent to setting XYPOS directly, but uses separate X
and y numbers.

When the grid's cursor enters a new line, it clears its
contents through a repeated call to CELL!. That implementation
is in its world named NEWLN ( ln -- ). This word can be over-
ridden. If it exists when the grid subsystem is loaded, the ex-
isting NEWLN will be used.

The clearing of the newly entered line is usually only desirable
when in "shell" mode. In "graphical" mode, we usually don't
want this to happen. XYMODE is a flag to indicate whether the
grid system is in "graphical" mode. When its value is nonzero,
NEWLN is not called when entering a new line.

At build time, the Grid subsystem needs 3 bytes of system me-
mory through the GRID_MEM constant. At run time, GRID$ needs to
be called to initialize the system.