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Rowdy Characters
Add the following lines:
1 0 7 0 D A T A 4 4 , - 1 2 , 8 , - 3 2 , 1 2 7
1 0 8 0 D A T A 3 9 , - 1 7 , 0 , 6 4 , 9 6 , 1 1 2 , 1 2 0 , 1 2 4 , - 1 2 , 1 2 7 , 1 2 4 ,
and RUN.
Oops! There's trouble in River City. Fortunately, we've seen these same
control code problems before. Hate to keep bringing this subject up, but it does
disturb our use of the printer.
We've seen codes 9 and 13 cause trouble on the Apple, and codes 0, 10, 11, and
12 cause trouble on the TRS-80. If we send them to the printer via CHR$, it
interprets them as control codes instead of pin firing instructions. Unfortunate-
ly, that makes sense.
Code 8 can also cause trouble since it means "delete the previous character
sent to the printer and stored in its print buffer. " In line 1070 we tried to send a
whole sequence of them, and on some computers the program blew up!
For Experts Only
The errors caused by these codes do not always occur, but it is best to avoid
them if possible. Short of rewriting the program to circumvent these numbers,
one way is to bypass the PRINT statement and POKE our graphic codes
directly to the printer driver in the computer's memory. That memory address
is 49296 (CO90 Hex) in the Apple, and 14312 (37E8 Hex) in the TRS-80 Model I
(See appendices for examples of use). Other computer users can check their
computer's technical manual for its printer driver memory address.
The advantage of POKE over PRINT CHR$ in this specific situation is that it
bypasses some of the nasty tricks we keep encountering with BASIC. The
disadvantage is that indiscriminate POKEing can cause unbelievable software
crashes. Unless you really understand POKE
inherent with your computer/printer combination, and avoid troublesome
codes when possible. Model I users will need to use the POKE
particular example.
1 2 0 , 1 1 2 , 9 6 , 6 4
Figure 6-4
, best to live with the limitations
S
shown for this
S
Advanced Graphics
55
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