HP Vectra Technical Reference Manual page 36

SECTION 3. VIDEO
The HP MultiMode Video Display Adapter provides a wide variety of display modes, resolution,
character attributes, and other features. The purpose of the video driver is to allow programs to
access these features and control the video display.
3.1 Overview
In the text mode, the MultiMode Video Display Adapter uses an 8 X 16 character cell which
generates high quality characters. Access to the display memory is fully synchronized to eliminate
the "snow" problem present in many color display adapters. (Snow occurs when writing a
character to display memory while the video memory is being accessed by the display refresh
circuitry.) This full synchronization makes the INT 10H video driver faster, since there is no need
, . to wait for a vertical retrace to place characters on the screen.
The MultiMode Video Display Adapter provides seven more display modes than the industry
standard color graphics adapter. Four of the modes allow 27 lines of text on the screen. The
other three modes allow graphics modes that double the graphics resolution of the display
(320 X 400 and 640 X 400 pixels). The standard INT 10H video driver has been extended to allow
the programmer to set these modes. No other support is provided to make use of these modes.
Refer to HP Vectra Technical Reference Manual Volume I: Hardware for more information on the
MultiMode Video Display Adapter.
3.2
Data Structures
The MultiMode Video Display Adapter has 32KB of video memory starting at address OB8000H.
This allows graphics resolutions of 320
X
400 in medium resolution modes and 640
X
400 in
high resolution modes. The following is a discussion of how this memory is organized depending
on the video mode selected.
Video 27