Download 4 - Forth Interest Group

1 Synopsis
I was working with a 128x64 graphics display and needed
(mother of all inventions) some fonts to display text. I had
one font which was 8x16, from an old project, bvt this gave
me only four rows of 16 characters. The display came with a
smaller font, 8x8, which gave me eight rows of 16 characters,
but the set was incomplete since it only contained numbers
and upper-case letters. Also, it was a bunch of hex codes and
commas-difficult to visualize and extend. So I set out to
define a font set which would be readable, but very small at
the same time; mostly, it had to be easy to modify and build.
The result is presented in this article. It is a 4x5 font 1 call
MicroFont. It contains all printable ASCII characters, is quite
readable, and the code satisfies the requirement of ease of
use and design. It allows for ten lines of 25 characters on the
tiny graphics display.
/
Figure 0ne.The character A specified with asterisks.
Letter A
**
Letter is a word responsible for parsing the next five lines,
converting them into nibbles, packing them into bytes, and
comma-ing them into the dictionary.
2.2 Code
The definitions in Figure Two are used to lay down the font.
1
2 Details
Figure Two
Each character is four
bits wide
five bits tall. : *>NIBBLE ( - - nibble )
OTIB INPUT 4 BLANKS ( input conditioning )
INPUT-LINE
O=
IF
."
EOF!"
ABORT ENDIF
'This forces a few compromises w h e n d e s i g n i n g
0 INPUT 4 FOR C@+ >R ' * = IF 1 OR ENDIF 2* R> NEXT DROP
2/ 0 SCAN ;
characters such as I%) or #.
l ' h e four bits across fit
nicely i n t o a nibble, so : Character ( - - )
1 e a c h c h a r a c t e r is f i v e
2 FOR *>NIBBLE 4 SHIFT
*>NIBBLE OR C,
NEXT
*>NIBBLE 4 SHIFT C,
nibbles. For processing efficiency, the five nibbles : Letter ( - - ) Character ;
are fit into three bytes. So
each c h a r a c t e r is t h r e e : Number ( - - ) Character ;
bytes long and, since there
are 94 characters, the font (Figure Two continues on next page)
takes u p 2 8 2 bytes. In
theory, two characters FigureThree
fit into five
( Note: nibbles are rendered across and down with msbit as left )
and the font would then be
235 bytes; but the savings
: ASCII>MICRO
( char - - n )
'
- MIN BL - 0 MAX ;
inight be nullified by the
increase in code t o deal
: .NIBBLE
( n -- )
5 column + ! 2* 5
with it.
FOR DUP 1 AND SET-PIXEL 2/ -1 column + ! NEXT DROP 1 row + !
;
;
2.7 Character Coding
: microEMIT
( char - - )
ASCII>MICRO 3 * microfont t
The character descrip3 FOR C@+ >R 16 /MOD 2 FOR .NIBBLE NEXT R> NEXT DROP
tions are done with t h e
-6 row + ! 5 column t ! ;
ASCII character * (star).
Other characters could be
( -- )
' microEMIT emitter ! 6 line-size ! 5 char-size !
: MICRO-FONT
used but this best approxirllated a pixel.
: TESTMICRO
( -- )
MICRO-FONT
BL ' - OVER - FOR DUP FEMIT 1 + NEXT DROP ;
Forth Dimensions XIX.4
;
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