Editing TRS-80 Color Computer (section)

====Alphanumeric/Semigraphics display====
[[Image:Cocobvdg.png|thumb|320px|Sample MC6847 VDG character set display]]

In alphanumeric mode, each character is a 5 dot wide by 7 dot high character in a box 8 dots wide and 12 lines high. This display mode occupies 512 bytes of memory from $400-$5FF and is a 32 character wide screen with 16 lines. The ROM character generator only holds 64 characters, so no lowercase characters are provided. "Lowercase" characters are rendered as uppercase with inverted color. On most CoCo generations it is green on very dark green.

[[Semigraphics]] is a mode where alphanumerics and low resolution graphics can be mixed together. The 8th bit of a character determines if it is alphanumeric or treated as a 2 × 2 pixel grid. When the 8th bit is set, the next three bits determine the color and last 4 bits specify which quadrants of the character box are either the selected color or black. This allows a 64 × 32 graphics mode with 9 colors, the only mode where it is possible to display all 9 colors simultaneously.

There are two color sets. The default has black characters on a green background. The alternate has black characters on an orange background. The color set selection does not affect semigraphics characters. The border is always black.

The 6847 is capable of a Semigraphics 6 display mode, where two bits select a color and 6 bits determine which 1/6 of the character box is lit. Only 4 colors are possible, but the colorset bit of the VDG selects two groups of 4 colors. Only two colors are available in graphics blocks when using Semigraphics 6 on the CoCo.<ref>[http://www.cs.unc.edu/~yakowenk/coco/text/semigraphics.html Semi-graphics-6 display mode] {{Webarchive|url=https://web.archive.org/web/20191109215817/http://www.cs.unc.edu/~yakowenk/coco/text/semigraphics.html |date=2019-11-09 }}, By William J. Yakowenko, 25-Mar-1996, [[University of North Carolina]] [https://cs.unc.edu Department of Computer Science] {{Webarchive|url=https://web.archive.org/web/20240409234130/https://cs.unc.edu/ |date=2024-04-09 }}</ref>

The default alphanumeric display for the CoCo is Semigraphics 4.