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5.3
THEORY OF OPERATION
•
The internal memory source includes the time clock and the
characteristics you establish during Setup or initialization.
These may or may not involve characters to be placed on
the display.
Regardless of the source of the character or command se-
quence, the CPU receives and processes the code and then
either acts upon it (if it is a command) or routes it to the appropri-
ate memory for use by the Terminal.
ON-SCREEN CHARACTER GENERATION
A complex interrelationship is maintained between the CPU,
video character and attribute RAM (memory), the CRT control-
lers, and display synchronization logic. Part of this relationship
is aided by OMA (Direct Memory Access)
logic.
PLACING THE CHARACTER IN MEMORY
When a new character is to be generated on the screen, the
CPU will route (handled by the bus control circuitry) the charac-
ter code into the appropriate location (which corresponds to a
screen location) in the video character RAM and the attribute
code into the corresponding location in the video attribute RAM.
PLACING CHARACTERS ON THE SCREEN
The CPU, under timing control of the clocking signals, and in
conjunction with the OMA logic, directs the video and attribute
CRT controllers to access the video character and attribute
memories in sequential order with respect to placement on the
screen.
The codes for the individual characters are sent through the
video CRT controller to the character generator, where they are
translated into the proper bit pattern for display. From there, the
bit pattern is sent to a shift register. Here, the information is sent
in
a serial fashion through synchronization logic where
it
is
mixed with the attribute for that character. The resulting signal
is then routed into the video drive circuitry and reproduced on
the screen of the Terminal.
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