Zenith Z-100 Series, Техническое руководство

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Page 4.50
CIRCUIT DESCRIPTION
How the CRT-C Addresses RAM
As mentioned before, the CRT-C is normally programmed to
emulate the H19 video terminal. That is, the display will con-
tain 25 lines, 80 characters per line, and nine scan lines (rows)
per character line.
MAO-MA11 points to the character location within a character
line, and also points to the current character line. They do
this by incrementing the base address by ten after every ten
scan lines. RAO-RA3 counts the number of scan lines. After
one scan line is complete (MAO-MA11 count to 79), RAO-RA3
reset to 0 and MAO-MA11 reset to their base address. The
count begins again. This procedure continues until nine scan
lines are processed. RAO-RA3 again returns to zero, but MAO-
MA11 increment their base address by ten to point to the
next character line.
For each address, a byte is read from video RAM (VRAM)
and shifted serially out to the video amplifier with the horizontal
and vertical sync pulses. The scan rate is such that each
address row appears beneath the previous one so that the
serial dots form characters on the screen. Once the last char-
acter row is processed, both RAO-RA3 and MAO-MA11 reset
to zero, vertical retrace takes place, and the process repeats.
Incidentally, at vertical retrace a sync pulse is sent through
U366 (lower left on the schematic) to interrupt the CPU. This
permits the CPU to access the CRT-C registers (for example,
to scroll the display) without interferring with the display.
The address lines reach memory by passing through a set
of multiplexers. RAO-RA3 connects to multiplexer U357 while
MAO-MA11 connects to U363, U358, and U359. These ICs
allow coupling the CRT-C address lines to VRAM, or the CPU
address lines to VRAM.