HP 3000 Manuals HP 3000 Manuals
General Structures
There are three basic structures used for all files: line sequential,
fixed and variable.
Line sequential is the structure of files with the line sequential
organization. It contains variable length records with trailing spaces
removed.
Fixed is the structure used by fixed format sequential and relative
files, and contains only fixed length records.
Variable is the structure of variable format sequential and relative
files, and fixed and variable format indexed files. Variable structure
files can contain fixed or variable length records.
The following sections describe these file formats.
Fixed and Variable Format
The format of a file is determined by the RECMODE directive and the FD
clauses RECORDING MODE, RECORD CONTAINS n CHARACTERS and RECORD IS
VARYING. The RECORDING MODE clause overrides the RECORD clause which, in
turn, overrides the RECMODE directive.
Hence:
* RECORDING MODE IS V always creates variable format.
* RECORDING MODE IS F always creates fixed format.
* RECORD CONTAINS a TO b CHARACTERS has no effect on file format.
* RECORD IS VARYING creates variable format provided no RECORDING
MODE IS F clause is present.
* RECORD CONTAINS n CHARACTERS creates fixed format provided no
RECORDING MODE IS V clause is present.
* RECMODE"F" creates fixed format for each file where no RECORDING
MODE IS V or RECORD IS VARYING clauses are present.
* RECMODE"V" creates variable format for each file where no
RECORDING MODE IS F or RECORD CONTAINS n CHARACTERS clauses are
present.
* RECMODE"OSVS" causes the LEVEL-01 definitions to determine if the
file is fixed or variable format.
Fixed |Structure
Fixed structure files contain no record or file header information. The
records are all the same length, that length being determined by the
longest record defined in the File Description (FD) in the program's
File-Section.
Variable Structure
Any files containing variable length records, with the exception of line
sequential files and files destined for the printer, contain a block of
128 bytes of header information at the start of the file. Each record in
the file is preceded by a 2- or 4-byte control field. The top 4 bits of
this field indicate the status of the record. A value of 0100 in these
bits means that this record is a user data record. Any other value means
that this record has either been deleted or is used internally. The
remainder of the control field contains the length of the record.
For all files where the maximum record size is less than 4096 (excluding
the prefix), the prefix is 2 bytes long. For all other files, the prefix
is 4 bytes long. Each record always starts on the next 4-byte boundary
in the file.
You must not alter the header information or the control fields in any
way since these are maintained by this COBOL system.
Record Header Types.
First 4 bits Record type
-------------------------------------------------
1 (0001) A system record (IDXFORMAT"4" files
only).
This contains duplicate occurrence
details in the data file.
2 (0010) Deleted record (available for reuse
via the Free Space list).
3 (0011) System record.
4 (0100) User data record.
5 (0101) Reduced user data record (indexed
files only).
The 16-bit word immediately
following the data record, as
indicated by the length in the
header, contains the space between
the end of the data record and the
start of the next record header.
6 (0110) Pointer record (indexed files only).
The first 4 bytes following the
record header contain the offset in
the file to the location of the user
data record.
7 (0111) User data record referenced by a
Pointer record.
8 (1000) Reduced user data record referenced
by a Pointer record.
The first record in every variable structure file is a system record
called the File Header record. This is normally 128 bytes long.
The record header for each record starts on a 4-byte boundary.
Consequently, a record may be followed by up to three padding characters,
usually spaces. These padding characters are not included in the record
length.
Variable structure File Header record description:
Offset Size Description of the field
-------------------------------------------------
0 4 Length of the file header.
The first 4 bits are always set to 3
(0011 in binary) indicating that
this file header record is a system
record.
The remaining bits contain the
length of the file header record.
If the maximum record length is less
than 4095 bytes, the length is 126
and is held in the next 12 bits;
otherwise it is 124 and is held in
the next 28 bits. Hence, in a file
where the maximum record length is
less than 4095 bytes, this field
contains x"30 7E 00 00". Otherwise,
this field contains x"30 00 00 7C".
4 2 Database sequence number, used by
add-on products supplied with this
COBOL system.
6 2 Integrity Flag. Indexed files only.
If this is non-zero when the header
is read, it indicates that the file
is corrupt.
8 14 Creation date and time in
YYMMDDHHMMSSCC format. Indexed
files only.
22 14 Reserved.
36 2 Reserved. Value 62 decimal; x"00
3E".
38 1 Not used. Set to zeros.
39 1 Organization.
1 = Sequential
2 = Indexed
3 = Relative
40 1 Not used. Set to zeros.
Table I-0. (cont.)
Offset Size Description of the field
41 1 Data compression routine number.
0 = No compression
1 = CBLDC001
2-127 = Reserved for internal use
128-255 = User-defined compression
routine
number
42 6 Not used. Set to zeros.
43 1 File format.
0 = Default
1 = C-ISAM
2 = LEVEL II COBOL
3 = Indexed file format used
by this COBOL system
4 = IDXFORMAT"4"
44 4 Reserved.
48 1 Recording mode.
0 = Fixed format
1 = Variable format
For indexed files, the Recording
Mode field of the index file takes
precedence.
49 5 Not used. Set to zeros.
54 2 Not used. Set to zeros.
56 2 Maximum record length.
Example: with a maximum record of
length 80 characters, this field
will contain x"00 50".
58 2 Not used. Set to zeros.
60 2 Minimum record length.
Example: with a maximum record of
length 2 characters, this field will
contain x"00 02".
62 46 Not used. Set to zeros.
Table I-0. (cont.)
Offset Size Description of the field
108 4 Version and build data for the
indexed file handler creating the
file. Indexed files only.
112 16 Not used. Set to zeros.