Screen Memory And Character Generator Signals - Zenith Z-100 Reference Manual

page 7.18
Video Graphics Programming
Screen Memory and Character Generator Si gnals
There are two sets of signals provided by the 6845 to implement screen memory
and character generator logic interface. MAO through MA13 are screen memory
address outputs and RAO through RA4 are the raster address signals sent to the
character generator logic. The fourteen screen memory address lines allow the
6845 to address the 16K of video memory. The raster address lines are outputs
from the 6845's scan line counter required by the character generator logic to
determine which scan line of a character row is in progress.
CRT Monitor Control Signals
HSYNC and VSYNC are standard horizontal and verlical synchronization signals
required by the CRT monitor for display stabilization.
DISPEN is the display enable signal which goes high whenever the video signal
to the CRT is to be active. DISPEN is low during horizontal and vertical retrace
and could also be called the video blanking signal.
CURSOR is the cursor enable signal which allows a steady stream of dots to
be produced on the CRT screen as a cursor symbol.
LPSTB is the light pen strobe input signal which, in conjunction with
ancillary circuitry, may be used to interface a light pen. A high on the LPSTB
line signals the 6845 to save the contents of the screen memory address in one
of the internal register sets so the microprocessor can subsequently determine
the position on the monitor screen at which the light pen was detected.
CRT Controller Registers
The nineteen internal registers of the 6845 CRT controller can be software program-
med to define display and control parameters of a raster scanned monitor. One
of these registers, the address register, is used only as a pointer to the addresses
of the other eighteen registers. The address register is write only and is loaded
from the CPU using an 1/0 OUT instruction to the output port at 3D4. This register
is loaded with the five LSBs of the OUT argument.