Table of Contents
Advertisement
FORMATS
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 inform-
ation. A record consists of count, key, and data areas.
Count Area
The count area contains the location of a data record
on a specific track and defines the size of tlhe 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
Use of the key area is at the discretion of the program-
mer. When used, the key area of the record contains
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.
Data Area
The data area contains the information identified by
the count and key areas of the record. Data informa-
tion 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
All tracks are initially formatted beginning at an index
point (see Figure 6). 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 condition of the track. Home address is the first
recorded area following an index point.
Specific commands are used for writing and reading a
home address area: Write Home Address and Read
Home Address. Home address is normally rewritten to
flag the defective track. Writing home addresses is
usually done at the IBM plant.
Track Descriptor Record (R0)
This record is always the first record on the track
following the home address area. In IBM program-
ming systems, the R0 count field of the defective track
provides the address of the alternate track. If it is an
alternate track, the R0 count area proyides the address
of the defective track. An 8-byte data field is used to
store the number of bytes remaining on the track.
Specific commands, Write R0 and Read R0, are used
for writing and reading the track descriptor record.
Data Records
One or more data records may follow the track
descriptor record (R0) on a track. Record format is
determined at the time the count, key, and data areas
of the record are originally written by execution of a
Format Write command. The format of the record is
rewritten by another Format Write command.
Data records, as well as track descriptor records, can
be formatted with or without keys. Generally, file
organization determines whether keys are used.
RECORD OVERFLOW
The record overflow function provides a means of
processing logical records that exceed the capacity of a
track. When using overflow records, the cylinder
boundarv limits the size of the record.
8 IBM 3340144 Reference Manual
Advertisement
Table of Contents
