P Rimary Operating Registers - Zenith Z-100 Reference Manual

The scan line spedfied in RIO is the first scan in which the CURSOR signal is
to be set and it will remain set until the scan line specified in Rll has been
completed. Accordingly, if you want the cursor symbol to occupy a single scan
line, the same value must be loaded into both registers. If different values are
loaded, a block-type cursor will be formed.
In interlaced sync and video mode both registers must be loaded with either
odd or even values. Bits 5 and 6 determine whether or not the cursor is to blink,
and if so, at either I/16th or I/32th of the field rate.
Primary Operating Registers
The remaining six registers, R12 through R17, are considered the primary operating
registers, since, in the course of display programming, you will undoubtedly change
the contents of them on an ongoing basis rather than loading them one time
when the system is tumed on. The six registers are arranged as three 14-bit register
pairs (bits 6 and 7 of each most-significant byte are not used).
R12 and R13 comprise the top of page register which specifies the screen memory
address containing the first character from the top left corner to be displayed.
At the end of each vertical retrace, the first screen memory address generated
will be the one contained in these registers. Since the 6845 addresses memory
serially rather than on a row/column basis, scrolling can be performed on either
a character by character or row by row basis.
The cursor position registers, R14 and R15, generate a cursor signal when the
6845 generates a screen memory address on MAO through MA13 that matches
the contents of this register pair, and when the scan line counter (RAO through
RA4) outputs fall within the limits established by registers RIO and Rll. It may
be necessary to delay the cursor signal with external logic circuitry to achieve
display at the desired position.
Cursor movement on the screen is accomplished by loading new values into R14
and R15. These registers are the 6845's only read/write registers so they can also
be used to keep track of the cursor position rather than copying this information
from another memory location.
R16 and R17 are the light pen register pair that will be loaded with the screen
memory address corresponding to the screen position at which the LPSTB signal
was detected. Since there are several critical timing parameters involved with this
signal, be sure to carefully study the documentation supplied with the light pen
you are using.
Video Graphics Programming
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