Acorn BBC Master 128, Service Manual

Page: 29 / 87

Master Series Service Manual
5.4 External I/O
1MHz External I/O:-
Screen Output:- There are two chips primarily responsible for
providing the screen output:-
a) 6845
b) Acorn proprietary VIDPROC (IC42)
High Resolution Modes:- The 6845 generates a linear memory address
sequence which increments every 0.5 us or lus, depending on the video
bandwidth selected and video data format.
The amount of memory reserved for screen use is also varied. The
available options are shown in figure 7
Figure 7
All modes except 7 and 135 display a bit mapped image of the reserved
memory. The 6845 may be re-programmed to display any arbitrary section
of memory. If this is done, however, the hardware scrolling will not
work correctly, as it assumes that the screen memory is in its usual
location.
The screen always ends at &7FFF and starts 1,8,10 or 20K further down
than that, depending on the selected mode.
The selection of video bandwidth and data format is performed by
programming the VIDPROC. The cursor size and position is also
controllable by VIDPROC. Special measures have been taken to ensure
correct cursor operation in the Teletext modes.
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