Commodore VIC-20 - Page 198

184 The VIC 20 User Guide
TABU 6-1. ASCII to Screen Code Conversion
ASCII Code
Screen Code
0-31 None
32-63 Same
64-95
0-31
96-127
64-95
128-159
None
160-191
96-127
192-254
64-126
255
94
ters and certain control codes, which are not used by other manufacturers.
Many
of the control codes do not have a displayable character associated
with them. In order to make as many graphic codes as possible available on
the screen, Commodore devised a different code for screen memory. This
code eliminates some ASCII characters and changes the values
of others.
Appendix E contains a table showing the screen display codes.
If your
program
is POKEing characters that are set at the time you write it, you can
simply look them up in the table.
If you do not know in advance what
characters you will be POKEing, your program must convert them from
ASCII to screen display code. This might be necessary if, for example, your
program uses GET to read instructions from the keyboard and you want to
echo the characters in a particular place
on the display.
The conversion to screen code can be done with little effort because the
ASCII codes
that were changed were moved in blocks of 32 characters. The
changes are shown in Table 6-1.
Here
is a simple subroutine that takes a key value, K V$, and converts it
to a screen code, Sc. If the key cannot be translated (a cursor control key,
for example), it returns a value of
-1. The main program can thus tell the
difference between a displayable and a nondisplayable keystroke.
60000 SC- ASC(KV"
60010 IF SC(32 THEN SC-128:RETURN
60020 IF SC(64 THEN RETURN
60030 IF SC{96 THEN SC·SC-64:RETURN
60040 IF SC{128 THEN SC-SC-32:RETURN