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Method of Adding Color/Graphics Adapter-Compatible Thirty-One KHZ Graphics Mode Support to Single-Chip CGA Controllers

IP.com Disclosure Number: IPCOM000101948D
Original Publication Date: 1990-Sep-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 6 page(s) / 229K

Publishing Venue

IBM

Related People

Au, EW: AUTHOR [+3]

Abstract

This article describes a method and hardware implementation to produce color/graphics adapter (CGA)-compatible graphics modes on a 31-KHz CRT using a single-chip CGA controller, even though all of the video memory address lines are not brought out of the chip.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 37% of the total text.

Method of Adding Color/Graphics Adapter-Compatible Thirty-One KHZ Graphics Mode Support to Single-Chip CGA Controllers

       This article describes a method and hardware
implementation to produce color/graphics adapter (CGA)-compatible
graphics modes on a 31-KHz CRT using a single-chip CGA controller,
even though all of the video memory address lines are not brought out
of the chip.

      Many low-cost 'CGA-on-a-chip' controllers are available on the
market.  CGA is a video card for a personal computer (PC).  Most are
designed to drive CRTs having a 15 KHz horizontal sweep rate.  In
some applications, it is desirable to drive 31 KHz CRTs because of
the crisper definition of characters in text modes, but still retain
CGA compatibility in all video modes.  Reprogramming the CRT
controller to drive the higher sweep rate in text modes is generally
straightforward because the increased number of horizontal scan lines
required at 31 KHz can be provided by enlarging the character font
from 8 pixels high to 16 pixels high without causing compatibility
problems.  However, graphics modes are not as easy at higher sweep
rates because the additional scan lines require "double scanning" the
available data and not all of the address lines and other timing
information needed to do this may be available outside of the video
chip.  This article describes a way to produce CGA-compatible
graphics modes on a 31 KHz CRT using a single-chip CGA controller,
even though all of the video memory address lines are not brought out
of the chip.

      A CGA based upon the Motorola 6845 CRT controller (CRTC)
provides a number of different text and graphics modes.  Typically,
the CGA card drove a digital CRT with a 15 KHz horizontal sweep rate
and provided a maximum of 640 pixels horizontally and 200 pixels
vertically.  The video function has been enhanced by adding new
modes, driving CRTs with higher sweep rates which translates into
more vertical pixels, and by pulling all of the circuitry required to
perform the video function into a smaller number of chips.

      In programming the 6845 CRTC, it is necessary to state how many
rows of characters are to be displayed and how many scan lines each
row consists of along with other parameters which do not have a
direct bearing on this disclosure.  In text modes, the CRTC is
usually programmed to address 25 rows of characters where each row
consists of 8 to 16 scan lines.  Ideally, in graphics modes the 6845
would be programmed to reflect that there are 200 rows which are each
1 pixel high.  However, the original 6845 CRTC was not capable of
handling more than 128 horizontal rows on the screen, so the CGA was
designed in such a way that the CRTC could be programmed for 100
rows, each consisting of 2 scan lines.  This resulted in a total of
200 scan lines on the screen but required that the least significant
bit of the row address (RAO) be used in computing external video
random-access memory (R...