There
are 256 ASCII codes,
numbered
0 to 255.
The
first 128
include the codes for the letters
of
the alphabet (both
lower
and
upper case), the digits 0 to
9,
and punctuation marks. For example,
the letter A
is
ASCII 65; the digit 6
is
ASCII 54.
The
first 128 ASCII
codes also contain a
number
of
non
printing codes., called control
codes. These codes, ASCII 0
through
31, are the ones that
control
the
MPS
1200 functions. We will have
more
to
say about
them
later
in this manual.
The
second 128 codes, ASCII 128 to 255, are sometimes called the
"high-bit"
or
"8-bit"
ASCII characters.
They
are less standardized
than the first 128 and their meaning depends
somewhat
on
the
particular
equipment
and application involved.
The·
MPS 1200
interprets the
high-bit
ASCII codes
as
uppercase characters, graphic
characters,
and
special symbols.
One
of
the
most
confusing things
about
ASCII codes
is
that there
are
two
numbering
systems for the codes.
Some
books and
pro-
grams refer to
them
by
their decimal value
(0
to 255) and others use
their hexadecimal value.
Hexadecimal
Numbers
The
numbers
we
usua-lly use are called
decimal
numbers
because
they are based
on
ten-the
ten digits 0
through
9.
Hexadecimal
numbers,
or
hex
numbers
for short, are based
on
16--the
ten digits
o
through
9 and the six letters A
through
F.
To
distinguish
them
from
decimal numbers,
hex
numbers
are usually
written
in one
of
three ways:
1.
preceded
by
a dollar sign (e.g. $13
or
$6A).
2.
followed
by
an h
(e.
g. 13h
or
6Ah)
3. preceded
by
an ampersand and H (e.g. &H13
or
&H6A)
All have the same meaning; it depends
on
which
book
you are
reading. In this
book,
we
use the second style; hex numbers are
followed
with
an h,
as
in 6Ah.
The
ASCII codes in hexadecimal are
OOh
to
FFh. Whether
you
use
decimal
or
hexadecimal ASCII codes depends
on
your
computer
system
and
which
software
program
you
are using. Appendix B
contains a complete list
of
all the ASCII codes
with
their decimal
and
hex
values.
2-3
To Next Page
Loading...
