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The secondary address can have values over the range a through 31.
FILE NAMES
In random storage devices where there is more than one file to be accessed, the use of names to identify
files is mandatory. In the case of tapes, a file name is desireable, even if there is only one file on the tape,
since it facilitates the identification of tapes.
For the two cassette tape units of the PET, a file name may be any combination of up to 128 characters.
When a file name is searched for, it is matched on an ascending character basis.
Assume that an eight character file name COUNTING was specified when writing. If desired, this could
be
searched for with an abbreviated name such
as
COU. The first file header that
is
found with these
three consecutive characters wifl then be opened and positioned on. In principle, this could include
unwanted file names such
as
COUNT or COUNTRY,
as
well as COUNTING.
It is, therefore, adviseable to specify the complete file name in order to avoid errors.
For other devices which use named files, the individual description of the device should be consulted in
order to ascertain the specific requirements for file name usage.
TAPE CASSETTE OPERATION FOR FILES
The PET devotes special attention to the two tape cassette units that can be attached to it. The tape units
are specially modified so that the PET has control over the motor movement of the cassette.
It can also sense when the PLAY, REWIND, or FAST FORWARD buttons have been pushed; this is done
by means of a single switch mounted in the tape unit.
Note that the same switch is used to sense all three buttons: if any of the three is pushed, the PET will
think that you pushed the PLAY button and will respond accordingly.
Because of the type of mechanism used in the tape unit, the user must rewind, fast forward, stop, load
and eject tapes. He must also put the unit into the write mode by pushing the record button either
simultaneously with, or before the PLAY button is pressed.
The PET has total control over the movement of the tape once the appropriate buttons have been pushed
to engage the motor.
Messages displayed throughout the program will tell the user when it is necessary for him to initiate the
function of play or record. Logic dictates the times when the user should rewind and fast forward.
The two tape units of the PET are handled independently, and the various control lines permit the reading
of data from cassette #1, the reading of data from cassette #2, motor control of cassette #1. motor
control for cassette #2 and a common write line.
FILE RECORDING TECHNIQUE
The data structure embodied in the tape files will ensure the maximum memory utilization and maximum
reliability of recording.
To accomplish this, the PET records data at two audio frequencies in two consecutive blocks of data. All
of the data is totally repeated and by means of error checking techniques incorporated in the PET
software, it is possible for most audio dropouts to be corrected by the operating system utilizing the
redundant data stored in the second data block.
In order to correct for (a), the fact that tape units record at different speeds, and (b), the normal drag
characteristics of tapes, a speed correlation technique is used during reading. To correct for the
individual start/stop characteristics on the tape and syncronize correctly between each block on tape, a
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