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Data Formats
All 3350 drives are initialized at manufacture and
shipped with a home address and an eight-byte track
descriptor record (RO) written on all tracks.
If
the
data areas become defective during normal use, an
IBM utility program is available to flag defective
tracks and assign alternates.
DATA GROUPS
The basic information unit stored by the drive is a byte
consisting of eight bits (binary digits). A group of
bytes separated by a gap is called an area. Areas and
gaps are combined to form a record, the basic informa-
tion unit.
RECORD FORMAT
A record consists of three specific areas: the count
area, the key area, and the data area (plus the neces-
sary gaps to separate them). The process of writing an
entire record is called formatting.
Count Area
The count area contains the physical address of a data
record and defines the size of key and data areas of
the record. The count area is written when the record
is formatted and is not changed until the record
is
reformatted.
Key Area
The key area can be used to access data when organ-
ized in a keyed sequence. Primarily, it serves as an
identifier of the data area that follows. Examples of a
key identifier are: a part number, an employee num-
ber, or a Social Security number.
After formatting, the key area length is fixed. The
contents of the area can be changed as required.
If
the
key area content is changed, the data area (which
follows) must also be rewritten.
Data Area
The record's data area contains the information
addressed and identified by the count and key areas.
This data is organized and arranged by the program-
mer.
6 IBM 3350 Reference Manual
The data area length is defined by the count area.
Once formatted, the contents, but not the length, of
the data area can be changed without affecting the
count and key areas in the record.
TRACK FORMAT
Each track has the same format: a home address, a
track descriptor record, and one or more data records.
The records, and the areas in the records, are separated
by gaps. See Figure 3.
Home Address
Each track contains one home address (HA) defining
the physical location of the track (track address) and
the track condition. The home address is the first
recorded area following the index point.
Special commands are required to read and write in the
home address area.
Track Descriptor Record
The track descriptor record is the first record following
the home add.ress area. In IBM programming systems,
the RO count area provides the address of an alternate
track if the track is defective. If the track is an alternate,
the address of the defective track is provided.
Special commands are required to read and write data
in the RO area.
Data Records
One or more data records follow the track descriptor
record. The data record format is determined when
the count, key, and data areas are originally written.
The number
of
data records per track
is
determined by
the number
of
bytes
in
each record and track capacity.
Data records, and track descriptor records, can be
formatted with or without keys depending on the
programmer's file organization.
OVERFLOW RECORDS
The record overflow function allows records exceeding
the track capacity to be continued on the next track
following the
HA
and
RO
areas. These record seg-
ments can continue on successive tracks up to the
cylinder boundary.
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