Alto Hardware Manual Section 3: Emulator
18
CONVERT
67000B
Scan convert a font character:
The
CONVERT
instruction does scan conversion
of
characters, i.e.,
it
transfers data between
an
area
of
main memory containing a font and
an
area
of
memory containing a bit map to
be
displayed
on
the
TV
monitor.
CONVERT
takes a number
of
arguments:
ACO
contains the address
of
the destination' word into which the
upper
left corner
of
the
character is to be placed, offset by
NWRDS,
the number
of
words to
be
displayed on each scan
line
(ACO=DWA-NWRDS).
Ac3
points to a character pointer in the font for the character to be displayed
(Ac3
=
FONTBASE
+
CHARACTER
CODE).
Ac2+SignExtend(DISP) is the address
of
a two-word table:
word
0:
word
1:
NWRDS
(number
of
words per to scan line);
NWRDS
< 128.
DBA,
the destination bit address corresponding to the left hand edge
of
the
character.
CONVERT
interprets this bit address reversed from the normal
convention,
i.e.,
0 is the least significant bit,
15
the most significant bit.
CONVERT
requires that a
16
word mask table
be
set up starting
at
MASKTAB
(460B)
in page
l.
rv(MAsKTAB+n)=
(21'(n
+ 1»-1
(~ns...15).
The format
of
an Alto font designed for use with
CONVERT
is
given below; names
of
font files
in
this format conventionally have an extension
".AL".
The
CONVERT
instruction does not examine
the words at
FONTBASE-2
and
FONTBASE-l;
these are provided solely for convenience
of
software.
FONTBASE-2:
The height
of
a line
of
text in scan lines. This
number
incorporates the effects
of
the highest and lowest character in the font, i.e.
it
is
max(HD+xH)-min(HD)
where the max and min are taken independently
and
HD
and
XH
are defined
below.
FONTBASE-l:
Bit
0:
0 = Fixed width font.
1 = Proportional width font.
Bits 1-7: Baseline
--
number
of
scan-lines from top
of
highest character in
font
to
the baseline.
Bits
8-15: The width
of
the widest character
in
raster points.
FONTBASE
to
FONTBASE+
377B:
Self-relative pointers to word xw
of
the character descriptor block for the
character codes
0-
377B.
FONTBASE+400B
to
FONTBASE+400B+EXTCNT-l:
These locations contain self-relative pointers to word
xw
of
the character
descriptor blocks for extensions, i.e., portions
of
characters which are wider than
16
bits.
EXTCNT
is
the total number
of
character extensions.
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