Basic Operations - IBM RT Series Hardware Reference Manual

Basic Operations
Alphanumeric Modes
The data format for alphanumeric modes on the Enhanced
Graphics Adapter is the same as the data format on the IBM
Color / Graphics Monitor Adapter and the IBM Monochrome
Display Adapter. As an added function, bit three of the attribute
byte may be redefined by the Character Map Select register to act
as a switch between character sets. This gives the programmer
access to 512 characters at one time. This function is valid only
when memory has been expanded to 128K bytes or more.
When an alphanumeric mode is selected, the BIOS transfers
character patterns from the ROM to bit plane 2. The processor
stores the character data in bit plane 0, and the attribute data in
bit plane 1. The programmer can view bit planes 0 and 1 as a
single buffer in alphanumeric modes. The CRTC generates
sequential addresses, and fetches one character code byte and one
attribute byte at a time. The character code and row scan count
address bit plane 2, which contains the character generators. The
appropriate dot patterns are then sent to the palette in the
attribute chip, where color is assigned according to the attribute
data.
Graphics Modes
320x200 Two and Four Color Graphics (Modes 4 and 5)
Addressing, mapping and data format are the same as the
320x200 pel mode of the Color/Graphics Monitor Adapter. The
display buffer is configured at hex B8000. Bit image data is
stored in bit planes 0 and 1.
640x200 Two Color Graphics (Mode 6)
Addressing, mapping and data format are the same as the
640x200 pel black and white mode of the Color/Graphics
8 IBM Enhanced Graphics Adapter