May June 1980 - Commodore Computers

112 COMPUU. MAVIJUNE . 1980. ISSUE 4.
112 COMPUTE. MAY/JUNE. 198O ISSUE &.
50 to start outputting MARKS, pokes the lower case
mode for PET screen output, and then calls 52
50 to start outputting MARKS, pokes the lower case
mode for PET screen output, and then calls 52
for the prompt to to check that your cassette is is running
and 54 to initialize variables and the details of
and 54 to initialize variables and the details of
paper size and format that you wish to to use.
71 simply outputs the current S$, then carriage
returns and, if if required, linefeeds to the next page.
80 cleans up: linefeed to to a blank page so that
garbage created by turning off the audio doesn't
spoil the beautiful page that Cheep Print has just
produced, then ding the bell on the printer to announce
that it's done, followed ed by a wait loop for a little
trailer on the tape. Then the shift register at 59467
is reset so that the PET tape read and write will
operate properly and the graphics mode for screen
operate properly and the graphics mode for screen
output is is restored.
98 converts the hex string Q(uery)$ into the
equivalent number A(nswcr). A(nswer).
99 loads (hex machine) C(ode)$ into memory
locations beginning at MA.
Lines 5 and 6 serve only as an index to the
program lines that structure Cheep Print for the
printer you use. I have included the most common
options in the listing.
Line 23: : with the REM, we list the program
with extra space for comments. Delete the REM, and
the program is listed single spaced, with the print
output strung across the whole page. This is useful
during development; not only does it execute more
quickly, but it is an advantage to have as much as
possible of the program in view at once, provided
that the details of what is supposed to happen are
fresh in your mind.
Lines 50 and 63-66 are involved in setting the
baud rate and whether originate or answer mode. In
line 50, only the mode needs to be determined, and the
two possible choices are snuck in following the REM.
Lines 63-66 contain that portion of the machine code
which depends upon the baud rate and the mode.
The four options given are receive and originate
mode at 110 and 300 baud (10 and 30 characters
per second), and which one you use is determined
by which of the initial REM's isn't there. Table 2
gives the details of getting output at different
frequencies and baud rates. The timing parameters
MF and SF determine the mark frequency and the
space frequency appropriately for you to be in
originate or answer mode; the decimal equivalent of
MF also needs to be reflected in the initialization at
line 50. Usually you will want to be in answer
mode when talking to a terminal and in originate
mode when talking to a mainframe. The baud rate is
determined by the timing loop parameters XR and
YR, and the duration of the stop bits by SB. I did
some experimenting with these parameters and found
that the baud rate could be off by 7 or 8 percent
and the mark and space frequencies off by 4 percent
and still get solid copy; SB only needs to to be
sufficiently long. The output can be structured in in
any way you wish, of course; for instance, you
could generate tapes to Kansas City standards if if you
wish. The most important thing is is that your cassette
recorder should run at the same speed at home as
it it does at your printer, but it it doesn't matter what
the speed is. If you are forced to use battery
operation at your printer, then you should use
battery operation at home or else experiment with
the timing parameters to compensate for the speed
difference that many recorders exhibit between battery
and a nd AC line operation.
.
Lines 85-86 contain the formatting data for the
two styles of listing, and lines 87-88 contain two
types of carriage return- line feed sequences. These
are implemented by LISTing 85-88 on the screen,
deleting the line number and REM from the data
you wish to use, usc, and then hitting RETURN;
the PET screen editor picks up the line number
55 or 56 that remains and inserts these statements
into the program. Lines 85-88 remain unchanged for
future use. The carriage return-line feed sequence
that you need will depend on the printer you use. I've
used a DECwriter and a thermal printer in which
carriage return (Control-M) does not also give a
line feed (Control-J), and a Selectric which includes
the line feed with the carriage return whether you
like it or not. Further, the DECwriter requires no
nulls to waste time while the print head returns to
the left side of the paper, the thermal printer sometimes
requires as many as four nulls, and the Selectric
wants a goodly number of nulls. The Anderson­
Anderson-
Jackson Selectric has an 800 character buffer to
deal with transmission faster than the print mechanism
can keep up with, and the nulls plus the program
execution time seem to be enough to keep the buffer
from overflowing. If you have a Selectric, one solution
is to make every other character a null, , which is
included in the REM of line 8; if you use these
nulls, declarations of P will have to go inside the
FOR-NEXT loops of lines 44, 47, 53, , 22, and 17. . I
doubt that the line 8 nulls are a good solution for
a Selectric, because there is a mechanical clutch
that continually engages and disengages, and probably
wears more than it needs to. . This clutch is also the
reason that the listing output is generated line by
line, rather than simply printing individual characters
when ready; ;
As an example of using Cheep Print to serve a
host program, I wrote a quick 6502 disassembler in
Basic so that Cheep Print could list its own machine
code for commenting. The disassembler isn't included
here because there's nothing particularly exciting or
instructive about it. The disassembly shows that the
machine code is only a simple software timing loop,
performed with interrupts inhibited (this is important!)
so that the timing doesn't get messed up. The
machine code outputs the start bit, and then the
contents of memory location 1023/03FF low order bit
first. All the printers I know ignore the highest