Peritek VCD-V, Руководство пользователя

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Peritek
Programming On-board Devices and Memories 5-61
5.11 Hardware Cursors
A hardware implemented cursor is highly desirable because it allows the
cursor to be moved on the screen simply by changing the cursor
coordinates. The Peritek graphics boards have cursors implemented in one
of three Cases:
Case 1 as a part of the color map chip (in the case of the BT459 and
BT482 on the VCD-V and the BT468 on the VCU-V).
Case 2 as a single modified BT482 color map chip used as a cursor
controller (in the case of some versions of the digital port on
the VCD-V).
Case 3 as a pair of BT431 cursor control chips (in the case of the
VCT-V and some versions of the digital port on the VCD-V).
The underlying principle in all cases is that the cursor is a bitmap whose
contents and X-Y position can be programmed by the user. The bitmap
array size is 64 x 64 for all but the BT482, which is 32 x 32. This is not a
serious limitation since many cursors in X are only 16 x 16. The cursor is
synchronized to the rest of the video timing with HSYNC and VSYNC.
The cursor coordinates are a function of the
color map load clock (which
makes a transition every 4 or 8 pixel times) and horizontal and vertical
syncs (which clear the cursor's X and Y position counters. It then counts
load pulses to position itself along the horizontal axis and counts
horizontal sync pulses to determine vertical position. Since the cursor
clock is referenced to HSYNC and VSYNC, you can derive the relative
0,0 position from the 34020 timing initialization table (see Section 5.4).
Since all video timing changes with zoom factor, the cursors will maintain
their postions.
One complication of the hardware cursor is that interlaced displays
confuse the cursor's vertical timing. The result is that, unless special
measures are taken, it is "zoomed" vertically (by a factor of two) for
interlace displays. The BT482 has an internal control bit which can be set
for interlaced mode which then makes it work correctly. External logic
does the same thing for the BT431 cursors when used on the VCT-V.
Unfortunately, nothing can be done for the BT459 and BT468 cursors, and
so they really are unsuitable for interlaced displays.