The Video Display Generator (VDG) creates a 32 character wide by 16 character tall display using 512 bytes of RAM and built-in character bitmaps from internal mask ROM or stored externally drawn in hardware to a 256 x 192 display.

The mask ROM only held 64 bitmaps of 6 by 8 bits for the core CoCo ASCII character set.

Using an external ROM allowed the use of the full 128 bitmaps of 8 by 12 bits for each character.

The VDG's most noticeable characteristic is the … interesting … choice of color palette for text screens.

This means you have a combination of eight text screen modes available from the VDG:

• Blackish characters on bright green background using internal mask ROM character bitmaps
• Bright green characters on blackish background using internal mask ROM character bitmaps
• Blackish characters on bright orange background using internal mask ROM character bitmaps
• Bright orange characters on blackish background using internal mask ROM character bitmaps
• Blackish characters on bright green background using external ROM character bitmaps
• Bright green characters on blackish background using external ROM character bitmaps
• Blackish characters on bright orange background using external ROM character bitmaps
• Bright orange characters on blackish background using external ROM character bitmaps

Example black text on green using internal mask ROM characters:

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