ACT apricot - Page 258

SCREEN DRIVER
The II-bit font cell pointer can point to anywhere within
the first 32k (on a WORD only boundary) in the bottom
segment of memory. (for example, if the value contained
here were to be 0, the character font cell pointed to would be
at 0000:0000 in memory) A font cell consists of 16 words or
32 bytes, 1 word for each line of the character:
0
1
2
3
1
4
6 5
6
w
7
0 8
r
9
d
A
s B
C
D
E
F
A
character font-cell
o 1 2 3 4 5 6 7 8 9 ABC D E F
1 1 1 l' 1
11.
1 1 1 1
1 1 1 1
1 1 1 1
1 1
.11
1 1
1111.
1 1 1 1
1 1
.11
1 1 .11
1 1 .11
III III 1
x -
strikethrough
not used
in the
text mode
x - underline bit
The default character set table starts at 800H in the lower
64k memory segment. The first word's Least significant bit
denotes the status of the pixel in the top left-hand corner of
the font cell on the screen: if a bit is set, the pixel will be on,
if a bit is not set, the pixel will be off.
Here is a program
to demonstrate direct screen accessing
from Basic:
10 PRINT CHR$(27);"z";CHR$(27);"x5" 'Clear & cursor off
20 DEF SEG=&HFOOO:FOR P=50 TO 255 'Character loop
30 FOR M=I TO 4000 STEP P 'Screen loop
40 POKE M*2,P 'Put character on screen
50 NEXT M,P:END 'Loop till done