Paging The Display - Zenith Data Systems Z-100 Series Installation Manual

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.