Smooth Scroll Or Scrolling Window; Video Generator; Character Font - Facit 4431 Service Instruction

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7.3.4.2 Smooth scroll or scrolling window
The Facit 4431 has two smooth scrolling speeds,
fast and slow. In a slow smooth scrolling ope-
ration the character rows within the scrolling
window are shifted upwards or downwards by one
scan line once during every refresh cycle. (A
refresh cycle is the period from one vertical
sync pulse to the next.) In fast smooth scroll
the character rows are shifted by two scan
lines during each refresh cycle. If the opera-
tor has not explicitly defined a scrolling win-
dow, then the entire screen is, in effect, a
scrolling window.
In the first screen refresh cycle of as smooth
scroll operation the first scan line of the
scrolling window will display the data from the
second line of the chararcter row (slow speed).
Recall that the scan line counter D21 generats
an NMI during the 10th scan line of each cha-
racter row, and that this interrupt allows the
CPU to set the scan line count and row start
address of the next character row. Thus, at the
start of the scrolling window, the processor
sets the scan line counter D21 to display the
second scan line of the character row.
Since the scan line counter D21 only generates
a carry on the 10th scan line of each charac-
ter, and the last scan line of the scrolling
window is now the first scan line of a new cha-
racter row, it is necessary to generate an
extra nonmaskable interrupt. This extra inter-
rupt is needed so that the CPU can set the row
start address and scan line count of the first
character row below the scrolling window. Since
the scan line counter outputs of the CRT Con-
troller are referenced to the start of the
screen and not the start of the scrolling win-
dow, they are used to decode the end of the
window. Thus, the CRTC scan line outputs at D49
-4,8 are ANDed in D6-12,13 to decode scan line
10, and the output at D6-11 is ANDed with the
extra interrupt enable signal at D6-1,2 so that
the output at D6-3 will generate the extra NMI.
(The extra interrupt enable signal comes from
D30-19, and is set by the processor when smooth
scroll is enabled.)
The Q outputs of the F/F D3 controls whether or
not interrupts are let through to the proces-
sor. Since the vertical sync pulse must always
be allowed to generate NMI, the vertical sync
pulse presets F/F D3. As interrupts from the
scan line counter would normally occur at the
very beginning of a scan line, the processor
would be forced to waste a relatively long time
in the WAIT state. To improve efficiency, the-
refore, F/F D3 is used to delay the interrupt
towards the end of the scan line. The F/F is
set to enable the interrupts when the column
count reaches 64 if 80 column mode and 96 if in
132 column mode.
When in 80 column mode D17-13 is high and the
column count of 64 from D49-32 passes through
D34-2 to clock the F/F. When in 132 column mode
D17-13 is low, disabling the D17-11 from cloc-
king the F/F. When the column count reaches 96,
D17-8,9 gate a high at D17-10 which clocks the
F/F D3. Note the the F/F is reset by the twice
delayed blank signal.
7.4 VIDEO GENERATOR
The Video Generator (VG) circuit receives paral-
lel data from the memory (M) and the Sync gene-
rator (SG) circuits and generates both a compo-
site video output signal to drive an external
monitor (optional features) and a direct drive
signal to drive the internal 12" (diagonal) data
display monitor.
7.4 1
CHARACTER FONT
The display may be sectioned into 24 rows of
either 80 or 132 columns of basic character
cells and a 25th status row of either 80 or 132
character cells.
The basic cell is illustrated below in Fig 7.4.
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FIG 7.4 BASIC CHARACTER CELL AND EIGHT INDEPEN-
DENT DOT LOCATIONS
The basic cell is six dots wide by ten dots
high. There are eight horizontal positions where
dots may be placed. Note that three of these
eight positions lie on half-dot boundaries and
the rightmost position extends for two dot
widths into column 0 of the next cell to the
right. Columns 4 and 5 are normally blank to
allow horizontal spacing between characters, but
as will be seen later, these columns may be fil-
led if continuous horizontal lines are to be
drawn. Rows 8 and 9 are used to display decen-
ding lower case or underlined characters and the
line cursor.
Eleven bits are required to address the charac-
ter (seven ASCII data bits and four bits for the
character scan line 0-9) to be displayed. The
octal latches D41 and D46 provide the seven ASC-
II code bits and multiplexer D18 provides the
four scan line number bits. These address lines
to the character generator ROM are summarized in
table below.