Page
4.7
Software Considerations
With
the optional RAM installed, the starting point of the display can
be
moved to a new location
in
the
RAM
without changing the RAM. This
is
done
by
writing
OGH
into I/O address 3B4H to prepare the high-order
register
in
the 6845 to accept data. Then write the high-order location
to
I/O
address 3B5. The next step
is
to write
OOH
to I/O address 3B4H
and
then the low-order location into I/O address 3B5H. The top-left corner
of the display will now be the address specified. The programmer must
exercise caution when addressing starting locations beyond the first page
so
that
no
addresses beyond the capability of the RAM are used at the
end
of the page.
Paging the Display
The
standard RAM contains only enough memory
to
display one page
of
text.
The optional
RAM
can display four pages of standard text or two
pages
in
the interlace mode (the interlace mode cannot
be
used with
standard memory). With the optional
RAM,
the programmer can page
in
one
of two ways: using the GRTG address registers as described for scrol-
ling
(enter
an
address one page higher than the current address instead
of one line higher), or through the video control register.
To
use the video control register to page the text, write a 1 into bit 4
of
the byte at I/O address 3B8H. Be careful not to change the other bits
of this byte when using this procedure.
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