TM 5-6675-316-14
(4) Incorporated on the character generator board is a 1K x 8-bit RAM memory
known as the display memory. This memory has 640 locations which store the
character data displayed on the screen. The remaining storage space is used by the
main program as work space with 16 of these locations allocated as keyboard buffer.
(5) The character symbols are stored in special ROM called the character
generator. It contains 128 symbols arranged in character blocks, any one of which
can be selected by the code information stored in the display memory. The character
generator presents data for one horizontal slice of the selected symbol. By
successive selection of these slices, the entire character or symbol is built up.
(6) As a function of the character generator logic, each complete character
block is located to a specific X- and Y-coordinate on the screen which designates an
address location in the display memory.
(7) The display memory can, therefore, reach into the character generator ROM,
select-a-specific character or symbol and position it in a precise location on the
monitor screen.
The display memory has three main functions:
(a) Stores, in code form, symbols/characters to be displayed by the
monitor.
(b) Used as a program work area.
(c) Used as a keyboard buffer.
(8) Characters and symbols are stored in this lK x 8-bit memory as if it were
an extra part of the main program. The memory is interfaced with the main program
via the data and address registers. These are located on the keyboard interface III
board.
address multiplexer where it is passed to the display memory. At the same time,
data is received via the read/write control. The read/write control only allows
data to be written into the memory while the horizontal display is blank. This
prevents streaking or smearing of the monitor image. The display memory also
accepts inputs from the X- and Y-field counters. This information determines the
position on the screen and the instant in time that the next character is to be
displayed.
(10) The next character, once selected and its display position is determined,
is fed into and stored by the character buffer. The character keystrokes are held
in the seven-word buffer to be processed as required. As this seven-word buffer is
being used, a second buffer is collecting input information. The cursor control
also receives its signal from the buffer register and, when a command is received,
the cursor control will invert the output of the video register.
(11) The character generator ROM requires three voltages, +5 V, -3 V and +12 V,
in addition to the ground. Two of these, -3 V and +12 V, are not available from the
main power supply so they are produced on the character generator board.
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