Compatibility Issues - IBM RT Series Hardware Reference Manual

defined by the start address high and low registers (OCH and
ODH) of the CRTC. Information displayed on screen B always
begins at address OOOOH.
OOOOH
Screen B
Buffer Storage Area
OFFFH
1 - - - - - - - - - \
1000H
Screen A
Buffer Storage Area
7FFFH '---_ _ _ _ _ ---'
Screen Mapping Within the Display Buffer Address Space
The Line Compare Register (18H) of the CRT Controller is
utilized to perform the split screen function. The CRTC has an
internal horizontal scan counter, and logic ·which compares the
horizontal scan counter value to the Line Compare Register value
and clears the memory address generator when a compare occurs.
The linear address generator then sequentially addresses the
display buffer starting at location zero, and each subsequent row
address is is determined by the 16 bit addition of the start of line
latch and the offset register.
Screen B can be smoothly scrolled onto the CRT screen by
updating the Line compare in synchronization with the vertical
retrace signal. The information on screen B is immune from
scrolling operations which utilize the Start Address High and Low
registers to scroll through the Screen A address map.
Compatibility Issues
The CRT Controller on the IBM Enhanced Graphics Adapter is a
custom design, and is different than the 6845 controller used on
the IBM Monochrome Monitor Adapter and the IBM
Color/Graphics Monitor Adapter. It should be noted that several
CR TC register addresses differ between the adapters. The
following figure illustrates the registers which do not map directly
across the two controllers.
74 IBM Enhanced Graphics Adapter