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FORMATS
PACK INITIALIZATION
All 3336 Disk Packs are initialized at the factory and
shipped error free with a home address and eight-byte track
descriptor record (RO) written on all tracks. If the data
areas of the disk pack become defective during normal
use, an IBM utility program is available to flag defective
tracks and assign alternates.
Another IBM utility program is available that writes the
volume label, volume table of contents (VTOC), and initial
program load (IPL) records; it then determines the number
of flagged tracks for entry into the VTOC.
RECORD FORMAT
The basic unit of information recorded by the drive is a
byte consisting of eight bits. A group of bytes separated
by a special gap is called an area. Areas are combined to
make a record, the logical unit of information.
A record consists of three areas: count area, key area
(optional), and data area. The significance of the bytes in
these areas is shown in Figure 3.
Count Area
The count area contains the location of a data record on a
specific track, and defines the size of the key and data
areas of that record. The count area is written when the
record is formatted and is not changed until the record is
reformatted.
Key Area
Use of the key area is at the discretion of the programmer.
When used, the key area of the record can contain the primary
identification of the data portion of the record (such as
social security number, man number, part number).
Once the key area is formatted, the contents -- but not the
length -- may be altered. If the key area is altered, the data
area of the record must also be rewritten.
4 3330 Disk Storage
Data Area
The data area contains the information identified by the
count and key areas or' the record. Data information is
organized and arranged by the programmer.
The length of the data area is defined by the count area.
Once the data area is formatted, the contents-but not the
length-may be altered. The contents of the data area may
be altered without affecting any other area in the record.
TRACK FORMAT
Track format is shown in Figure 3.
All tracks are formatted beginning at index and ending at
the following index. Each track has the same basic format:
home address, track descriptor record, and one or more data
records. The records - and areas within the records - are
separated by gaps.
Home Address
Each track contains one home address, which defines the
physical location of the track (track address) and the con-
dition of the track. Home address is the first recorded
area following index.
Special commands are used for writing and reading the.
home address area: Write Home Address and Read Home
Address. Writing home addresses is normally done only at
the time of manufacture.
Track Descriptor Record (RO)
This record is always the first record on the track following
the home address area.
In
IBM programming systems, the count field of the
defective track provides the address of the alternate track.
If it is an alternate track, the count area provides the
address of the defective track. An eight-byte data field
is used to store the number of records on the track and
the number of bytes remaining on the track.
Specific commands, Write RO and Read RO, are used for
writing and reading the track descriptor record.
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