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There are several code numbers already reserved for special things. If we look
closely at the printouts from this chapter, we can see that the spacing when the
following numbers are used may not be what we have anticipated:
8 - reserved for (DELETE PREVIOUS CHARACTER)
9 - reserved for (TAB)
10 - reserved for Line Feed
12 - reserved for Form Feed
13 - reserved for Carriage Return
There are no nice clean ways around this problem -just very messy ones. (See
Appendices D and F for ways to POKE these codes to the printer). Best we
restrict our selection to the remaining numbers.
Underlining
Many computers do not have the capability of sending an ASCII character to
temporarily suppress the Line Feed when a Carriage Return is sent. An Apple
running BASIC with the Epson printer card falls in this category. We cannot do
underlining by simply sending CHR$ (13), a simple CR, then sending underlin-
ing dashes in the right places.
TRS-80 zealots can refer to Chapter 2, open up the printer, and move the auto
line feed wire to the red terminal. This means absolutely no line feeds unless we
send a CHR$ (10) each time one is needed. This "technique" does permit
underlining, but for all practical purposes, it's impractical!
Underlining should not be confused with just moving down one line then
printing a series of dashes. True underlining requires that we first print the line
to which we want to add whole or partial underlining. We then Return the
Carriage (print head), suppress the LF so the paper does not advance, and print
the underline character ASCII 95. This is called "overstriking." The underline
character is printed in position 7 of our 12 x 6 character dot matrix. Users with
LF suppression can also do such things as slashing zeros, sevens, etc.
There is another way to suppress the LF using the vertical spacing tricks we
just learned. The method to be demonstrated is practical for such things as
report headings or special emphasis, where the result is deemed worthy of the
extra programming effort.
An Introduction to Dot Matrix Printing
37
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