Commodore PET User Manual page 418

Before forcing an interrecord gap vou must detect when the cassette buffer
has written out a "physical record" or "block" of data to the tape. The buffer holds
191 characters (or 191 bytes). A full buffer is a signal that a block of data was just
written to the tape since the contents of the buffer are dumped only after it has
reached its capacity. By detecting a full buffer. Vou can infer that a block of data
was just written to the tape and an interrecord gap is needed.
How to Detect a Full Buffer
Wh en writing data out to a tape. following each PRINT* statement the
length of each data item is calculated and kept in an accumulator. which is then
compared to the buffer limit (191 characters). When the accumulator equals 191
the writing to the tape is stopped until an interrecord gap is written on the tape.
Below is a sample program:
10 POKE 243. 122:POKE 244.2:0PEN1.l.l
20 FOR X=l TO 100
::::0
PRINT#l .. ;.(
40 A=LEN(STR$(X»+l
50 IF (QT+A»=191 GOSUB 1000 :REM .IF EUFFER FULL CALL SUE.
60 QT=QT+A TO ADVANCE TAPE.
?€1
NE)":T
>' :
::::1' : :1
CLOSE 1
90 Et·m
Line 20 prints a variable. If the variable printed (in this case. X) is numeric it
must be converted to string form so the LEN function may be used to determine
X's length. as shown in line 40:
40 A=LEN(STR$(X))+1
One is added on to the lengths of the strings to include the carriage returns
that are written on the tape following each data item. Line 50 accumulates the
number of characters in the previous strings. (OT). plus
A.
and compares the total
to 191 (the buffer limit). If the number of characters written to the tape (OT
+
A) is
greater than or equal to 191 the entire buffer is written to the tape. and it is time to
force an interrecord gap by calling the subroutine at 1000. However. if OT+A is
less than 191 (OT+A
<
191). the buffer is not yet full. Line 60 increments OT by
A.
and the process keeps repeating until the buffer is full. and ail the data is written
fram the buffer to the tape. interspersed with the interrecord gaps.
Advancing the Cassette Tape
There are three necessary steps in the routine to advance the tape:
1. Turn on the cassette tape motor (POKE 59411.53).
2. Use a wait loop to stail program while tape is advancing.
3. Turn off the cassette tape motor (POKE 59411.61).
405