Tandy 1000
Technical Reference Manual
Light Pen Latch
Set
30C
Reset
30B
Any output to the port sets or resets the light pen latch (data byte has no
ef-
fect).
Before the 6845 can read the light pen again, the latch (30B) must be
cleared.
CRT
IProcessor Page Register
This
8-bit (write-only) register
is
addressed
at
30F. The descriptions below are
of
the register functions:
Bit Description
o
CRT
Page 0
1
CRT
Page 1
2
CRT
Page 2
3 Processor Page 0
4 Processor Page 1
5 Processor Page 2
6 Video Address Mode 0
7 Video Address Mode 1
CRT / Processor Page Register
CRT
Page 0·2
Bits
0-2
select the 16K page used by the video.
In
32K modes, bit 0
is
ignored.
Processor Page 0-2
These
processor page bits are combined with the CPU address to select the
32K
segment
of
memory accessed
at
B8000.
If
an
odd page number
is
selected
(1,3,5, etc.) the window
is
reduced to 16K.
Video Address Mode
0·1
These bits are used
in
conjunction with the graphics control bit (bit 1
at
308)
to
select the video address supplied to the
RAM.
The following chart lists the
values to be used for each video mode.
Video Address Mode
Mode Description
All
Alpha Modes
Low-Resolution-Graphics Modes
High-Resolution-Graphics Modes
Unused, Reserved
1 (Bit 7)
o
o
1
1
o(Bit
6)
o
1
1
o
CRT / Processor Page Register
57
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