Commodore CBM - Page 417

Chapter 3: DEVELOPING A PROGRAM, Interactive Programming
ln the old ROMs. the system location that enables the cursor ta blink is loca-
tion 548.
Ta enable the cursor. you would use the statement:
80
POKE 548.0 Enable cursor (old ROMs)
instead of
80
POKE 167.0
Chapter 4: RN 0
Enable cursor (new ROMs)
RND(O) is non-functional. An argument of zero returns a value that is con-
stant. or nearly constant. and that may vary from
PET ta PET.
You will have ta use -TI ta generate random seeds. This is the method used
in ail of the examples in Chapter 5 under "Generating Random Numbers."
Chapter 5: FILES
This section is for PET users who are having problems reading data files
using the old ROMs. If your
PET has the old ROMs and you intend ta use data files
frequently. you should seriously consider replacing the old
ROMs with the new. as
the new ROMs ensure greater reliability when reading and writing data files.
If you do plan to use the old
ROMs. you must do a little extra programming
ta get around these problems. Wh
en writing data to the data tape. the old ROMs
neglect ta initialize the pointer ta the start address of the cassette tape buffer. and
also fail ta leave enough blank space
on the tape between physical records. Con-
sequently. when the
PET attempts to read the data back from the data tape. the
problems may result in lost or garbled data. Here
are a few precautions you can
take to overcome these obstacles.
1. Initialize the pointer of the cassette buffer start address. Because
the old ROMs fail
ta initialize the start address ta the cassette tape buffer before a
file
is OPENed. you must be sure ta do sa before opening a file with a series of
POKEs:
cassette *1: POKE 243.122:POKE 244.2:0PEN 1,1.1
cassette *2: POKE 243,58 :POKE 244,3:0PEN 2.2.1
Memory address locations 243 and 244 point ta the start address of the cur-
rent tape buffer.
By POKEing in the above values the pointer will be initialized pro-
perly.
2. Force interrecord gaps. The old ROMs do not leave enough blank
space
on the tape between physical records. When the PET attempts ta read back
the data
with an INPUT* or GET*. if the physical records are tao close together
the data cannot
be read. resulting in read errors and lost data, Ta prevent this. you
can force larger interrecord gaps ta
be written between records by calling a
routine to advance the tape each time the cassette buffer
is emptied.
404