Table
7-5:
Contents of the Beam Position Counter
VPOSR
Read-only
Read the high bit of the vertical
position (V8) and the frame-type bit.
Bit 15
LOF (Long-frame bit). Used
to
initialize interlaced displays.
Bits
14-1
Unused
Bit 0
High
bit
of the vertical position
(V8). Allows PAL line counts (313)
to
appear in PAL versions of the Amiga.
VHPOSR
Read-only
Read vertical and horizon
tal
position of the counter
that
is
producing the beam on the screen
(also reads the light pen).
Bits 15-8 Low bits of the vertical
position, bits
V7-
VO
Bits 7-0
The
horizontal position, bits H8-Hl.
Horizontal resolution is 1/160th
of the screen width.
VPOSW
Write only
Bits same as VPOSR above.
VHPOSW
Write only
Bits same as VHPOSR above.
Used for counter synchronization
with chip test patterns.
As usual, the address pairs VPOSR,VHPOSR and VPOSW,VHPOSW can be read from
and written
to
as long words, with the most significant addresses being VPOSR and
VPOSW.
206 System Control Hardware
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