Ram Loadable Character Generator - IBM RT Series Hardware Reference Manual

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RAM Loadable Character Generator
The character generator on the adapter is RAM load able and can
support characters up to 32 scan lines high. Two character
generators are stored within the BIOS and one is automatically
loaded into the RAM by the BIOS when an alphanumeric mode is
selected. The Character Map Select Register can be programmed
to define the function of bit 3 of the attribute byte to be a
character generator switch. This allows the user to select between
any two character sets residing in bit plane 2. This effectively
gives the user access to 512 characters instead of 256. character
tables may be loaded off line.The adapter must have 128K bytes
of storage to support this function. Up to four tables can be
loaded can be loaded with 256K of graphics memory installed.
The structure of the character tables is described in the following
figure. The character generator is in bit plane 2 and must be
protected using the map mask function.
Bit Plane 2
+OK
+8K
+48K
Character
Generator 0
Character
Generator 1
Character
Generator 2
Character
Generator 3
+64K
~--------------------------~
The following figure illustrates the structure of each character
pattern.
If
the CRT controller is programmed to generate n row
IBM Enhanced Graphics Adapter 69

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