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3.10
Circuit Description
Screen Memory and Character
Generator Signals
There are two sets of signals provided by the 6845 to implement screen
memory and character generator logic interface. MAO through MA 13 are
screen memory address outputs, and RAO through RA4 are the raster
address signals to character generator logic. The 14 screen memory ad
dress 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 Signals
HSYNC and VSYNC are standard horizontal and vertical synchronization
Signals required by the CRT monitor for display stabilization. OISPEN is
the display enable signal which goes high whenever the video signal to
the CRT is to be active. OISPEN is low during horizontal and vertical
retrace, and also could 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.
LPEN is the light pen strobe input signal which may be used to interface
a light pen. A high on the LPEN 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 19 internal registers of the 6845 CRT controller can be software
programmed 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 18 registers. The address register
is write-only and is loaded from the CPU, using an I/O OUT instruction
to the I/O port at 304. This register is loaded with the five least-significant
bits from the I/O port.
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