May June 1980 - Commodore Computers

MAY/JUNE, MAV/JUNE. 1980 , ISSUE 4, A COMPUTE. JJ 33
VARIABLE NAMES AND CONSTANTS
VARIABLE NAMES AND CONSTANTS
As each new variablea blc is encountered, , it is assigned
a number. These numbers begin with 128 and are
a ss igned sequentia ll y u p to 255. Notice these numbers
assigned sequentially up to 255. Notice these numbers
will fit illlo into onc one byte. b Also, , ass each new variable
is encounlered encountered, , the variable name m e is aadded d ded to LO a
variable able name list, , aand 8 bytesb olr memory are reserved
for ror that variable. In the case o[ of undimensioned
ned
variables, these 8 b ytes will co nta in the value o[
variables, these 8 bytes will contain the value of
the e variable. For r strings and arrays, , these 8 bytes
will contain parameters, m e rs, wid1 with the actuall values and
characters rs slOreci stored elsewhere.
.
This method elhod of oi handling n variables has somee
advantages. tages, One advantage is thatt it keeps usage to a
minimum. , The variable name is onlyy stored once, aand
each time that name m e is referenced renced in a BASIC state tc­
ment, , it occupies onlyy onee byte in thc the stored
program. program, Another r advantage is thatt the address
where the value forr a variable e is stored slO can be
computed [rom from the assigned number. u m This isn't true
of the BASIC found in somem e other microcomputersm m where values muSt must be searched [or. for.
There are also some disadvantages as well, well. First,
it limits its you to 128 different variable names. . How­
ever, , the great majority of programsp m s won't ' t need more
a rc
tha n 128 variable na m es. O ne o th e r ci isacivaJ1l agc is
than 128 variable names. One other disadvantage is
that a t should a variablea name m e be no longer needed,
or accidental)' ly entered due to a typo, there is no quick
way to remove that variable from the variable name
list and reuse the 8 bytes reserved forr it.
Apparently, the only wayy lO to gel get rid off unwanted
variablesa is to LIST the program m to cassen cassettee or
disk. For example, LIST ""CC will lisl list the program
to cassette. , Once the program is saved, , use the NEW
command to clear the old program. Then useu the
ENTER ER command to lO reload the ,he program. ra m . For
cassette ll e this lh would be ENTER ""C. Using g the LIST
command saves the program in character form. .
ENTERingE the program m then lhen causes each line lO to be
converted again as was done when n you first typed
it in. . Now onlyy the variables found in the programp m
will be pla ced in the vari a ble name list, a nd spa ce
will be placed in the variable name list, and space
reserved forr lhe their value. Using CSAVE aand CLOAD
won't ' t do this because these save aand load a copy of
the memory o where the program ram is stored. red, UnwantedU variables are rc saved aand loaded with (he the rest off
the program. p .
Constants nts are stored in the BASIC statements
along with the restt or of the line. The constant will
be proceeded precced ed by a ""14"' tokenn as mentionedm previously.
Explaining how ATARI A R I BASIC: rc represents presc nls
the numbers used lI ass constants nt s and as variable
,'a values will require some explanation about BCD
((Binary Coded Decimal) numbers. I will save this lhis
information n for a later r article.
e ,
Too give aann example off using g the information
n
in this section, , let's t take a look at the variable
name list. Ii Sl. Fortunately a lely bytes bYles 130 aand 1311 contain
the address dress of the beginning off the variable e namem e
list. st, The list will consist eonsist off a string off characters,
each ch a racte r occupying o ne byte o r m e mory, T o
each character occupying one byte of memory. To
indicate the (he last character of a name, m ATARI
BASIC addss 128 to lo the value representingg lha thatl
character. Since the values representing ng the characc­
te rs won' t exceed 127 , the new val ue will still
ters won't exceed 127, the new value will still
fit into one byte. b Too indicate ica the eend of the list, ,
a 0 is placed in the byte following g the last
character a off the last name. m e , The program m which
prints the variable a e name list is given in Listing 4.
.
Notice whcn when you u run this program, m , thatt the "C' "("
is saved ass part of a array y name, m e , and the "$"
as pan of a stringg name.
m e ,
MEMORY ORGANIZATION
Finally, let'ss look at how the memory m storage is
organized lo forr a BASIC program. program, The order in which
the various parts off a program m are found in
memory is shown in Figure 1. 'The only part whose
beginning is fixed is the variable namem e list which
begins a t address 2048. T'he begin ning of the mhe r
begins ai address 2048. The beginning of the other
parts pans will move appropriately, pria ass the program m grows.
There are addresses in page zero which can be used
to iind find each off the lhe parts shown in Figure 1. These
addresses, usually y called pointers, are shown in Table
I, I. This table includes the two pointers which were
used in the previousp s programs. p m Figure !:,» ,' End onlf Array Storage IOra,L:t: An:a Area
",'n' Bl'.I.:ill Beginning ll ing off Array StorageS !Ura ,~'c An:a Area
n F,' ' End of oi' Program
ll1
'nn' Bq.{i Beginning nilln g- off Program P rogT