96
10.1.4 Descriptors
Each descriptor listed in the “
variable definition structure
” defines a file variable. The structure of a generic
descriptor is as follows:
Generic descriptor
Filed Size
Field Name
Field Type
Field Description
1 byte
Descriptor size
Unsigned Integer
Descriptor size (in bytes)
1 bit
External allocation
flag
Flag
flag=0 indicates that the variable is stored within the descriptor;
If flag=1 the variable is stored externally.
1 bit
Single allocation flag
Flag
This flag is significant only if the variable is allocated externally to
the descriptor;
flag=0 indicates that the file contains only one copy of the variable;
If flag=1 means the file contains multiple copies.
6 bits
Variable type ID
Unsigned Integer
Numeric index: identifies the data type of the variable defined by
this descriptor.
Varies with the
value of the field
“Variable type ID”
Variable Identification
Varies with the value
of the field
“Variable
type ID”
A parameter (which may be simple or a structure) identifying the
defined variable.
Varies with the
value of the field
“Variable type ID”
Variable
Varies with the value
of the field
“Variable
type ID”
Variable defined by this descriptor (which may be simple or a
structure).
This field is present only if the external allocation flag is set to
0
.
Descriptors – and the variables they define - are further classified, as follows, according to the values of the
external allocation and single allocation flags:
Variable Descriptor Types in structured files
External Allocation
Flag
Single Allocation Flag
Descriptor Type
0 0
0 1
Internal allocation
1
0
External multiple allocation
1
1
External single allocation
