Major Components Definitions
Motorola 6845 CRT Controller
This device provides the necessary interface to drive a raster scan
CRT.
Mode Set And Status Registers
This is a general purpose programmable
I/O
register.
It
has
I/O
points
which may be individually programmed. Its function in this attachment
is to provide mode selection (page 2-49 and 2-50) and color selection
in
the medium resolution color graphics mode (page 2-51.)
Display Buffer
The Display Buffer resides in the
CPU
address space starting at
address
X'
B8000'.
It
provides 16K bytes
of
dynamic read/write
memory.
A dual-ported implementation allows the
CPU
and the
graphics control unit to access this buffer. The
CPU
and the
CRT
control unit have equal access to this buffer during all modes
of
operation except in high resolution alphanumeric mode. In this mode
the
CPU
should access this buffer during the horizontal retrace
intervals. The
CPU
may however, write to the required buffer at any
time, but a small amount
of
display fetches will result if not during
retrace intervals.
Character Generator
This attachment utilizes a
ROM
character generator.
It
consists
of
8K
bytes
of
storage which cannot be read/written under software control.
This is a general purpose
ROM
character generator with three different
character fonts. Two character fonts are used on this card
(a
7x7 double
dot and 5x7 single dot), selected by a card jumper.
No
jumper gives a
7x7 double dot, with a jumper a single'dot font is selected.
Timing Generator
This block generates the timing signals used by the 6845
CRT
controller and by the dynamic memory.
It
also resolves the
CPU/
graphic controller contentions for accessing the Display Buffer.
Composite Color Generator
The logic in this block generates base band video color information.
2-48
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