* Screen Display
** Bitmap
Memory associated with graphic display. It is divided into bit-planes, which
determine the number of simultaneous colors available. For a terminal with /n/
bit-planes, /2^n/ colors or shades of gray may be used at once. For each /x,y/
position, one bit is drawn from each bit-plane at that position to form a binary
integer. This integer is the index of the /color register/ that will determine
the display color of the pixel at /x,y/ on the screen. The number of colors that
may be chosen for each color register may be larger than the number of color
registers.

For example, the VT340 has 4 bit-planes. This means that a 4-bit number indexes
the color registers, so there are 2^4 or 16 color registers. But each register
can be set to one of 4096 colors. The VT330, on the other hand, has 2 bit-planes
(4 grayscale registers) and a selection of 64 shades of gray available.

** Graphics Pages
The terminal may have more bitmap memory than is displayable on the screen at
once. In this case, each screen-size bitmap is called a page. The application
can select which page is being drawn on and which is being displayed.

** Color Register
The terminal can usually generate more distinct colors or gray shades than it
has memory available to distinguish on a per-pixel basis. The color registers
therefore select from all available colors the smaller pallete that is currently
usable. The bit-planes values at a pixel position select a color register, and
the register selects the actual color value.

Color terminals have two sets of registers, one for the color pallete and one
for the grayscale pallete.

* ReGIS

** Syntax

*** Commands are case-insensitive single characters, mostly letters

**** S: Screen

***** A: Addressing

***** H: Printing

***** M: Output Map (color registers)

***** I: Background Intensity

****** E: Screen Erase (selected background)

***** T: Time Delay

***** E: Screen Erase (current background)

***** W: Write

****** M: Pixel Vector Multiplier

***** C: Cursor Control/Selection

****** H: Graphics Output Cursor Selection

****** I: Graphics Input Cursor Selection

***** P: Display Graphics Page

**** W: Write

***** M: Pixel Vector Multiplier

***** P: Pattern Control/Selection

****** M: Pattern Multiplication

***** I: Foreground Intensity

***** F: Plane Select

***** V: Overlay Writing

***** R: Replace Writing

***** C: Complement Writing

***** E: Erase Writing

***** N: Negative Pattern Control

***** S: Shading on/off Control

**** P: Position

***** W: Write

****** M: Pixel Vector Multiplier

***** B: Begin Bounded Position Stack

**** V: Vector

**** C: Curve

**** T: Text

**** L: Load

**** @: Macrograph

**** R: Report

**** F: Polygon Fill

**** ;: Resynchronization