Commodore Plus 4 - Page 373

The Datassette Tape Recorder 361
Usually, it is best to simply give every file a name and always access a file using
its name. Then, always rewind the tape completely before you ask for a file. It may
sometimes be convenient to use the tape counter on the Datassette to position the
tape. Remember to reset the counter to 000 after you rewind the tape. Remember,
too, that the counter is only approximate. Fast forward to a position before the
exact position where the file is expected to start and then access the file by name.
Whenever the Datassette is being accessed, the screen is blanked. This is done
to speed up tape access by eliminating the time taken by the computer to manage
the screen display.
Using a Datassette with BASIC
The Datassette can be used to save BASIC programs or to create and retrieve
data files through BASIC. Errors, end-of-files, and end-of-tape marks can be
detected by examining the status variable, ST. The status variable does NOT
detect hardware problems such as failure to press the record button when you
attempt to create a file, reaching the physical end of a tape prior to completing the
file, and so on. For this reason, it is a good idea to check that programs are saved
properly by using the VERIFY command and to include verifying code in
programs that create data files.
ST for the Datassette
Bit
0
1
2
3
4
5
6
7
Meaning
Unused
Unused
Short block
Long block
Read or verify error
Checksum error
End-of-file mark encountered
End-of-tape mark encountered
An additional location of interest when working with the Datassette is 64784
($FD10). Bit 2 of this location can be used to detect whether or not a button is
pushed down on the Datassette. It does not distinguish between the buttons. The
statement
WAIT 64784,4
waits until no button is down. The statement