The operators (the symbols
+-*11
<=>
as
weil
as
the words AND.
OR.
and
NOT)
are given keyword codes (high-order
bit
set) since they
"drive"
the BASIC
interpreter
just
as
reserved words do
(e.g
..
179 for
<).
The standard ASCII codes
for these symbols
(e.g
..
60 for
<)
appear only in the text of a string.
Spaces in the source line are stored except for the space between the line
number and first keyword. This space
is
supplied
on
LISTing when a stored state-
ment
is
expanded to its original form. Vou can conserve memory storage space
by eliminating blanks (but this makes the program harder to read). Vou can
also conserve space by putting more than one statement
on
a line, since the
five
bytes of link, line number, and 0 end byte are stored only once.
The size of each statement
is
variable and is terminated by a byte of
zero to indicate the end of the statement.
(A
value of zero anywhere
within
the
text
is
stored
as
48.) Zero byte flags are used by the BASIC interpreter in execut-
ing a program
when
it
goes through the compressed BASIC text from left to right
picking out keywords and performing the indicated operations. A zero byte indi-
cates the end of the statement; the next four bytes are the link and the line num-
ber of the next statement.
In
contrast
to
searching through the text and using 0
byte indicators to locate the next statement. links are used when searching the
statements for their line numbers. Three consecutive bytes of zero
(the
la
st state-
ment's 0 byte followed by
two
zero link bytes) flag the end of text when executing
the program.
A program is stored onto cassette tape
in
the
sa
me format
as
shown
in
Figure
6-3
for memory storage. Thus.
it
is
basically
"dumped"
ante tape in a
continuous block. including link addresses and 0 end bytes.
The use
of
tokens in place of keywords
is
not
unique to the
PET.
but
there
is
no standard coding from one interpreter to another. Thus. a BASIC source pro-
gram SAVEd
on
tape by PET BASIC is not compatible
with
other BASICs, nor
can BASIC programs generated
on
other (non-PET) machines normally be
loaded by the PET BASIC interpreter.
314
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