Adding Toa Sequential File - Commodore 1541 User Manual

"channel #" is a secondary address, giving further instructions to the selecteii
device about how further commands are to be obeyed . ln disk files, the channel number
selects a particular channel along which communications for this file can take place .
The
possible range of disk chanrel numbers is 0-15, but 0 is reserved for program
Loa~s
• I for
program Saves, and 15 for the disk command channel. Also be sure that
n~ two disk
files
have the same channel number unless they will never be open at the same ume . (One
wa~
to do this is to make the channel number for each .file the same as its file number.)
"drive#" is the drive number, always 0 on the 1541. Do not omit it, or you WilJ
only be able to use two channels at the same time instead of the normal
m~imum
of
thr~e
If any pre-existing file of the
same name is to be replaced, precede the drive number w1t1i
the "at" sign(@) to request Open-with-replace. .
"file name" is the file name, maximum length 16 characters. Pattern matching
characters are allowed in the name when accessing existing files, but not when creating
new ones .
"file type" is the file type desired: S=sequential , P=program, U=user, and
L =length of a relative file. . ...
"direction" is the type of access desired . There are three poss1b1h11es:
R
=read,
w
=write
and M
=modify . When creating a file, use ''W' ' to write the data to diskette
When
vie~ing
a completed file, use " R" to read the data from
disk~tte.
Only use the
"M" (modify) option as a last ditch way of reading back data from an 1mpro. p erltclos. e d
(Splat) file. (If you try this, check every byte as it is read to be sure the data 1s sttll vahd,
as such files always include some erroneous data, and have no proper end .)
"file type" and
"dir~ction" don't have to be
abbreviated. They can be spelled out
in
full
for clarity in printed listings . .
"file#" "device#" and "channel#" must be valid numeric constants, var1able1
or
expression~.
The rest of the command mus! be a valid string literal, variable or
expression .
The maximum number of files that may be open simultaneously is 10, including a ll
files to all devices . The maximum number of sequential disk files that can be open at once
is 3 (or 2 if you neglect to include the drive number in your Open statement), plus tht
command channel .
EXAMPLES OF OPENING SEQUENTIAL FILES:
To create a sequential file of phone numbers , you could use :
OPEN 2,8,2, "O:PHONES,SEQUENTIAL,WRITE"
or save yourself some typing with:
OPEN 2,8,2, "O:PHONES ,S,W"
of
course, this
~rases
al. I our old phone
numb~rs ,
so make sure that any information that
rnaY
be deleted
1s of no importance . After wnllng our phone file, we remove our diskeue
and
tum off the system. Later, to recall the data in the file , we would reopen it with
s0inething like
OPEN 8,8,8, "O: PHONES,S ,R"
It doesn't matter whether the file and channel numbers match the ones we used before, but
the file name does have to match . However, it is possible to use an abbreviation form of
the file name , if there are no other files that would have the same abbreviation :
OPEN 10,8 ,6, "O:PH* ,S,R"
If we have too many phone numbers, they might not
fit in one file . In that case, we
might use several similar file names , and let a program choose the correct file.
100 INPUT
" WHICH PHONE FILE ( l-3) ";PH
110 IF PH <> I AND PH <>2 AND PH <>3 THEN 100
120 OPEN 4,8 ,2, "PHONE"+ STR$(PH) + ",S,R"
You can omit the drive number on an Open command to read a file . Doing so allows those
with dual drives to search both diskettes for the file.
Note: Basic 2 and Basic 3.5 use the same file handling commands and the same
direct access commands (chapters 7-8) . Unless otherwise noted , you may use the
same commands for both throughout the remainder of this book .
ADDING TO
A SEQUENTIAL FILE
On Commodore's PET and CBM models, an Append command allows you to reopen
an
existing sequential file and add more information to the end of it. The same thing can
be done another way on the 1541. In place of the "type" and "direction" parameters in
Your Open statement, substitute ",A" for Append . This will reopen your file, and
Position the disk head at the end of the existing data in your file, ready to add to it.
FoRMA T FOR THE APPEND OPTION
OPEN file #,device #,channel #,"drive # :file name,A"
oid '
1 ~here.
everything is as on the previous page except for the ending "A" replacing the
On the off-chance we've already got a "PHONES" file on our diskette , we can av 'type" and "direction" parameters .
"FILE EXISTS" error message by doing an @OPEN
OPEN 2,8,2, "(U,O: PHONES ,S,W"
44 45