ACT apricot - Initialising the CRTC

CRT CONTROL
Cursor Format (End) Register (RII). This register sets the
end address of the cursor within the character cell and is a
value corresponding
to a raster row address ei ther higher or
the same value as the cursor format start address.
Start Address Register {RI2}. The contents of this register
with the contents of RI3 define the first refresh address of
each
new frame period. RI2 is programmed with the 6 most
significant bits of a I4-bit word. The 8 least significant bits
are provided by R13.
Start Address Register (RI3). See RI2 above.
Cursor
Position Register {RI4}. The contents of this register
with the contents of RlS define the position of the text
cursor on the screen. RI4 is programmed with the 6 most
significant bits of a 14-bit word, corresponding to a refresh
address value.
The 8 least significant bits are provided by
RIS. Both registers
RI3 and R14 can be written to and read
from.
Cursor
Position Register (RI5). See R14 on the previous
page.
Light
Pen Register (RI6). When a positive edge occurs on the
LPSTB input of the CR TC, the current refresh address is
latched into the two registers R16 and R17, to define the
position of the light pen on the screen. RI6 stores the 6
most significant bits of the 14-bit address and R17 the 8
least significant bits.
The position of the light pen is then
determined by reading the contents of these registers.
Light
Pen Register (R17). See R16above.
Initialising
the CRTC
The values supplied to the control registers to initialise
the CRTC to operate with the standard 9 inch monochrome
Display Unit for the two different display modes are
detailed below.
The control registers are programmed using
two separate write operations. The first write is to address
location
68H in the system input/output space (the address
register)
to specify the control register pointer address. The