Alto Hardware Manual Section 3: Emulator 19
RCLK
SIO
FONTBASE+400B+
EXTCNT
to end:
Contains a number
of
character descriptor blocks
of
the form:
word
0 to word xw-I:
The bit map for the character and surrounding spaces.
The
bit
map
does not include
O's
at
the top and bottom
of
the character,
as
the
character
will
be vertically positioned by
CONVERT.
The
upper left-hand
bit
of
the character
is
in the
MSB
of
word
O.
word xw:
If
the character
is
~
16
bits wide, this word contains (2*width)+1.
If
the character
is
>
16
bits wide, this word contains
2*
a pseudo-character
which
is
used
as
a character code to index an extension character in the
font.
If
this
is
the last extension block
of
a character, this word contains
(2*
the width
of
the fmal extension), rather than the total width.
The
pointer indexed
by
the character code points to this word.
word
xw+I:
In
the left byte,
HD.
In the right byte, XH.
HD
is the number
of
scan
lines to skip before displaying the character,
XH
is
the height
of
the
bit
map for this character.
The
CONVERT
instruction
ORS
the character bitmap into the display area.
If
the character does
not require an extension,
CONVERT
skips, with the following information
in
the
AC'S:
ACO:
unchanged
ACI:
DBA
AND
17B
Ac2:
unchanged
Ac3:
the width
of
the character in bits
If
the character requires an extension,
CONVERT
returns does not skip.
Ac3
contains the pseudo-
character code for the extension, and
AC'S
0-2 are
as
above.
6I003B
Read Clock:
The microcode maintains a
26
bit real time clock which
is
incremented by the memory refresh
task
at
38.08
microsecond intervals (more precisely, once every 224 ticks
of
the system clock,
whose nominal frequency
is
5.880000
MHz). The high-order 16 bits
of
this clock are maintained
in location
RTC
(430B)
in page 1 The low-order 10 bits are kept in
R37.
The remaining 6 bits
of
R37
contain state information unrelated to the time.
RCLK
loads
ACO
with the contents
of
location
RTC,
and loads AcI with the contents
of
R37.
The period
of
the full 26-bit clock is
about
40
minutes.
The contents
of
R37
are slightly different on Alto I and Alto
II:
on
Alto
I,
R37[0-9]
contain.
the
low order clock bits; on Alto
II,
R37[4-13]
are used. Consequently, on the Alto I, the contents
of
ACO
and AcI returned by
RCLK
may
be
viewed as a 32-bit clock in units
of
.595
microseconds,
provided AcI[IO-I5]
is
first zeroed.
61004B
Start 110:
Start I/O
is
included to facilitate 110 control.
It
places the contents
of
ACO
on
the processor
bus
and executes the
STARTF
function
(Fl=
17B).
By
convention, bits
of
ACO
must
be
"1" in order to
signal
. devices. See Appendix C for
a·
summary
of
assigned bits.
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