Vector 4 - Page 72

VECTOR 4
TECHNICAL INFORMATION
Alpha- In this mode characters are generated from an address given to a pair
of Character Generator PROMs. Because of this structure one byte of memory
translates into one character position on the sereen. Therefore one complete
secreen (24 character rows by 80 character columns) requires 2 K bytes of
screen memory. NOTE: Due to the requirements of the SBC memory mapping
scheme this mode actually uses a total of 1920 bytes.
Graphies- This mode can be divided into three submodes. Exhibit II-9 shows a
chart which describes these submodes. Generally the Graphics mode can be
defined as a video display mode which maps individual bits rather than entire
- character positions. The various submodes may map anywhere from one bit at a
time to blocks of 4 bits. In all of the graphic modes a 26 K block of memory
is required. This block must have a starting address on any 32 K boundary.
e.g. 00000H, 08000H, 10000H etec.
C. Yideo MUX
"The Video MUX consists of two 7T4LS244 buffers which are enabled by the
decoded signal ALPH|/GRAPH. This signal comes from the Subsystem Port
‘Register (<D11>). This register (described in I/O Section) functions as a Port
which is clocked by port addresses 02H or 03H.
When the ALPH|/GRAPH signal is LOW the U99 buffer is enabled and the U81
buffer is disabled. This indicates the SBC is in the Alphanumeric Video display
mode. To accomplish this the Video Controllers address signals are sent out in
the following format:
Video Controller U99 CPU/VID Address MUX
Output Lines Lines Lines *
MAO-MAT A0-AT
MAS8-MAS A8-A9
MA10-MA13 A10-A13** A10-A16
RAQO-RA3 not used
%x®
These signals represent the logical address‘notation. Actually the lines
are multiplexed resulting in the use of the MAO-MAT terminology.
** U99 also receives a 3-line input from jumper»area C. This jumper area
is described in Part IV.
The Video Controllers RA lines are not used by the U99 buffer since the
indexing of a particular scan line is accomplished at the Character Generators.
-However the Graphies Mode does not use the Character Generators. Therefore
the RA lines are incorporated into the address multiplexing at the U81 buffer.
09-01-82
7200-0001
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