A single descriptor class is used for scalar data and fixed-length strings. Any OpenVMS data type, except data type 34 (unaligned bit string), can be used with this descriptor. Figure 7-2 shows the format of a fixed-length descriptor. Table 7-3 describes the fields of the descriptor.

Figure 7-2 Fixed-Length Descriptor Format

Table 7-3 Contents of the CLASS_S Descriptor

Symbol	Description
DSC$W_LENGTH DSC64$Q_LENGTH	Length of the data item in bytes, unless the DTYPE field contains the value 1 (aligned bit string) or 21 (packed-decimal string). Length of the data item is in bits for bit string. Length of the data item is the number of 4-bit digits (not including the sign) for a packed-decimal string.
DSC64$W_MBO	Must be 1. See Section 7.1.
DSC$B_DTYPE DSC64$B_DTYPE	A data-type code. Data-type codes are listed in Sections 6.1 and 6.2.
DSC$B_CLASS DSC64$B_CLASS	Defines the descriptor class code that must be equal to 1 for CLASS_S.
DSC$A_POINTER DSC64$PQ_POINTER	Address of first byte of data storage.
DSC64$L_MBMO	Must be -1. See Section 7.1.

If the data type is 14 (character string) and the string must be extended in a string comparison or is being copied to a fixed-length string containing a greater length, the space character (hexadecimal 20 if ASCII) is used as the fill character.

7.3 Dynamic String Descriptor (CLASS_D)

A class D descriptor is used for dynamically allocated strings. When a string is written, either the length field, pointer field, or both can be changed. The OpenVMS Run-Time Library provides procedures for changing fields. As an input parameter, this format is interchangeable with class 1 (CLASS_S). Figure 7-3 shows the format of a dynamic string descriptor. Table 7-4 describes the fields of the descriptor.

Figure 7-3 Dynamic String Descriptor Format

Table 7-4 Contents of the CLASS_D Descriptor

Symbol	Description
DSC$W_LENGTH DSC64$Q_LENGTH	Length of the data item in bytes, unless the DTYPE field contains the value 1 (aligned bit string) or 21 (packed-decimal string). Length of the data item is in bits for the bit string. Length of the data item is the number of 4-bit digits (not including the sign) for a packed-decimal string.
DSC64$W_MBO	Must be 1. See Section 7.1.
DSC$B_DTYPE DSC64$B_DTYPE	A data-type code. Data-type codes are listed in Sections 6.1 and 6.2.
DSC$B_CLASS DSC64$B_CLASS	Defines the descriptor class code that must be equal to 2 for CLASS_D.
DSC$A_POINTER DSC64$PQ_POINTER	Address of first byte of data storage.
DSC64$L_MBMO	Must be -1. See Section 7.1.

7.4 Array Descriptor (CLASS_A)

The array descriptor shown in Figure 7-4 is used to describe contiguous arrays of atomic data types or contiguous arrays of fixed-length strings. An array descriptor consists of three contiguous blocks. The first block contains the descriptor prototype information and is part of every array descriptor. The second and third blocks are optional. If the third block is present, so is the second. Table 7-5 describes the fields of the descriptor.

Figure 7-4 Array Descriptor Format

Table 7-5 Contents of the CLASS_A Descriptor

Symbol	Description
DSC$W_LENGTH DSC64$Q_LENGTH	Length of an array element in bytes, unless the DTYPE field contains the value 1 (aligned bit string) or 21 (packed-decimal string). Length of an array element is in bits for the bit string. Length of an array element is the number of 4-bit digits (not including the sign) for a packed-decimal string.
DSC64$W_MBO	Must be 1. See Section 7.1.
DSC$B_DTYPE DSC64$B_DTYPE	A data-type code. Data-type codes are listed in Sections 6.1 and 6.2.
DSC$B_CLASS DSC64$B_CLASS	Defines the descriptor class code that must be equal to 4 for CLASS_A.
DSC$A_POINTER DSC64$PQ_POINTER	Address of the first actual byte of data storage.
DSC64$L_MBMO	Must be -1. See Section 7.1.
DSC$B_SCALE DSC64$B_SCALE	Signed power-of-two or power-of-ten multiplier, as specified by FL_BINSCALE, to convert the internal form to external form. (See Section 7.6.)
DSC$B_DIGITS DSC64$B_DIGITS	If nonzero, the unsigned number of decimal digits in the internal representation. If 0, the number of digits can be computed based on LENGTH. This field should be 0 unless the TYPE field specifies a string data type that could contain numeric values.
DSC$B_AFLAGS DSC64$B_AFLAGS	Array flag bits <23:16>: Bits <18:16> Reserved and must be 0. DSC$V_FL_BINSCALE DSC64$V_FL_BINSCALE If set, the scale factor specified by SCALE is a signed power-of-two multiplier to convert the internal form to external form. If not set, SCALE specifies a signed power-of-ten multiplier. (See Section 7.6.) DSC$V_FL_REDIM DSC64$V_FL_REDIM If set, the array can be redimensioned; that is, A0, M i, L i, and U i can be changed. The redimensioned array cannot exceed the size allocated to the array ARSIZE. DSC$V_FL_COLUMN DSC64$V_FL_COLUMN If set, the elements of the array are stored by columns (FORTRAN). That is, the leftmost subscript (first dimension) is varied most rapidly, and the rightmost subscript ( nth dimension) is varied least rapidly. If not set, the elements are stored by rows (most other languages). That is, the rightmost subscript is varied most rapidly and the leftmost subscript is varied least rapidly. DSC$V_FL_COEFF DSC64$V_FL_COEFF If set, the multiplicative coefficients in block 2 are present. Must be set if FL_BOUNDS is set. DSC$V_FL_BOUNDS DSC64$V_FL_BOUNDS If set, the bounds information in block 3 is present and requires that FL_COEFF be set.
DSC$B_DIMCT DSC64$B_DIMCT	Number of dimensions, n.
DSC$L_ARSIZE DSC64$Q_ARSIZE	Total size of array (in bytes, unless the TYPE field contains the value 21; see the description for LENGTH). A redimensioned array can use less than the total size allocated. For data type 1 (aligned bit string), LENGTH is in bits while ARSIZE is in bytes because the unit of length is bits, while the unit of allocation is aligned bytes.
DSC$A_A0 DSC64$PQ_A0	Address of element A(0,0,...,0). This need not be within the actual array. It is the same as POINTER for zero-origin arrays.
DSC$L_M i DSC64$Q_M i	Addressing coefficients ( M i = U i - L i + 1 ).
DSC$L_L i DSC64$Q_L i	Lower bound (signed) of ith dimension.
DSC$L_U i DSC64$Q_U i	Upper bound (signed) of ith dimension.

The following formulas specify the effective address, E, of an array element.

The effective address, E, for element A(I):

E = A0 + I*LENGTH = POINTER + [I - L1]*LENGTH

The effective address, E, for element A(I₁,I₂) with FL_COLUMN clear:

E = A0 + [I1*M2 + I2]*LENGTH = POINTER + [[I1 - L1]*M2 + I2 - L2]*LENGTH

The effective address, E, for element A(I₁,I₂) with FL_COLUMN set:

E = A0 + [I2*M1 + I1]*LENGTH = POINTER + [[I2 - L2]*M1 + I1 - L1]*LENGTH

The effective address, E, for element A(I₁, . . . ,I_n) with FL_COLUMN clear:

E = A0 + [[[[...[I1]*M2 + ...]*Mn-2 + In-2]*Mn-1 + In-1]*Mn + In]*LENGTH = POINTER + [[[[...[I1 - L1]*M2 + ...]*Mn-2 + In-2 - Ln-2]*Mn-1 + In-1 - Ln-1]*Mn + In - Ln]*LENGTH

The effective address, E, for element A(I₁, . . . ,I_n) with FL_COLUMN set:

E = A0 + [[[[...[In]*Mn-1 + ...]*M3 + I3]*M2 + I2]*M1 + I1]*LENGTH = POINTER + [[[[...[In - Ln]*Mn-1 + ...]*M3 + I3 - L3]*M2 + I2 - L2]*M1 + I1 - L1]*LENGTH

7.5 Procedure Argument Descriptor (CLASS_P)

A descriptor for a procedure argument identifies a procedure and its result data type, if any.

On OpenVMS VAX systems, the descriptor for a procedure argument specifies its entry address and function value data type. On OpenVMS Alpha systems, the procedure argument descriptor is a pointer to the procedure descriptor, which is described in Section 3.4. On OpenVMS I64 systems, the procedure argument descriptor is a pointer to the function descriptor, which is described in Section 4.7.7. Figure 7-5 shows the format of a procedure argument descriptor. Table 7-6 describes the fields of the descriptor.

Figure 7-5 Procedure Argument Descriptor Format

Table 7-6 Contents of the CLASS_P Descriptor

Symbol	Description
DSC$W_LENGTH DSC64$Q_LENGTH	Length associated with the function value, or 0 if no function value is returned.
DSC64$W_MBO	Must be 1. See Section 7.1.
DSC$B_DTYPE DSC64$B_DTYPE	Function value data-type code. Data-type codes are listed in Sections 6.1 and 6.2.
DSC$B_CLASS DSC64$B_CLASS	Defines the descriptor class code that must be equal to 5 for CLASS_P.
DSC$A_POINTER DSC64$PQ_POINTER	Address of entry mask to the procedure for VAX environments. Address of the procedure descriptor of the procedure for Alpha environments. Address of the function descriptor of the procedure for I64 environments.
DSC64$L_MBMO	Must be -1. See Section 7.1.

Procedures return a function value as described in Section 2.5 for VAX systems, Section 3.7.7 for Alpha systems, or Section 4.7.6 for I64 systems.

7.6 Decimal String Descriptor (CLASS_SD)

Figure 7-6 shows the format of a decimal string descriptor. Decimal size and scaling information for both scalar data and simple strings is given in this descriptor form. Table 7-7 describes the fields of the descriptor.

Figure 7-6 Decimal String Descriptor Format

Table 7-7 Contents of the CLASS_SD Descriptor

Symbol	Description
DSC$W_LENGTH DSC64$Q_LENGTH	Length of the data item in bytes, unless the DTYPE field contains the value 1 (aligned bit string) or 21 (packed-decimal string). Length of the data item is in bits for the bit string. Length of the data item is the number of 4-bit digits (not including the sign) for packed-decimal string.
DSC64$W_MBO	Must be 1. See Section 7.1.
DSC$B_DTYPE DSC64$B_DTYPE	A data-type code. Data-type codes are listed in Sections 6.1 and 6.2.
DSC$B_CLASS DSC64$B_CLASS	Defines the descriptor class code that must be equal to 9 for CLASS_SD.
DSC$A_POINTER DSC64$PQ_POINTER	Address of the first byte of data storage.
DSC64$L_MBMO	Must be -1. See Section 7.1.
DSC$B_SCALE DSC64$B_SCALE	Signed power-of-two or power-of-ten multiplier, as specified by FL_BINSCALE, to convert the internal form to external form. (See examples in Table 7-8.)
DSC$B_DIGITS DSC64$B_DIGITS	If nonzero, the unsigned number of decimal digits in the internal representation. If 0, the number of digits can be computed based on LENGTH. This field should be 0 unless the TYPE field specifies a string data type that could contain numeric values.
DSC$B_SFLAGS DSC64$B_SFLAGS	Scalar flag bits <23:16>: Bits <18:16> Reserved and must be 0. DSC$V_FL_BINSCALE DSC64$V_FL_BINSCALE If set, the scale factor specified by SCALE is a signed power-of-two multiplier to convert the internal form to external form. If not set, SCALE specifies a signed power-of-ten multiplier. (See examples in Table 7-8.) Bit <23:20> Reserved and must be 0.

Examples of SCALE and FL_BINSCALE interpretation are presented in Table 7-8.

Table 7-8 Internal-to-External BINSCALE Conversion Examples

Internal Value	SCALE	FL_BINSCALE	External Value
123	+1	0	1230
123	+1	1	246
200	--2	0	2
200	--2	1	50

7.7 Noncontiguous Array Descriptor (CLASS_NCA)

The noncontiguous array descriptor describes an array in which the storage of the array elements can be allocated with a fixed, nonzero number of bytes separating logically adjacent elements. Two elements are said to be logically adjacent if their subscripts differ by 1 in the most rapidly varying dimension only. The difference between the addresses of two adjacent elements is termed the stride. You can align elements by row or column, because the accessing algorithm in the called procedure handles both alignments.

This array descriptor is to be used where the calling program, at its option, can pass a slice of an array that contains noncontiguous allocations. This standard indicates no preference between the noncontiguous array descriptor (NCA) and the contiguous array descriptor (A), as described in Section 7.4, for language processors that always allocate contiguous arrays. Figure 7-7 shows the format of a noncontiguous array descriptor, which consists of three contiguous blocks. Table 7-9 describes the fields of the descriptor.

Figure 7-7 Noncontiguous Array Descriptor Format

Table 7-9 Contents of the CLASS_NCA Descriptor

Symbol	Description
DSC$W_LENGTH DSC64$Q_LENGTH	Length of an array element in bytes, unless the DTYPE field contains the value 1 (aligned bit string) or 21 (packed-decimal string). Length of an array element is in bits for the bit string. Length of an array element is the number of 4-bit digits (not including the sign) for a packed-decimal string.
DSC64$W_MBO	Must be 1. See Section 7.1.
DSC$B_DTYPE DSC64$B_DTYPE	A data-type code. Data-type codes are listed in Sections 6.1 and 6.2.
DSC$B_CLASS	Defines the descriptor class code that must be equal to 10 for CLASS_NCA.
DSC$A_POINTER DSC64$PQ_POINTER	Address of first actual byte of data storage.
DSC64$L_MBMO	Must be -1. See Section 7.1.
DSC$B_SCALE DSC64$B_SCALE	Signed power-of-two or power-of-ten multiplier, as specified by FL_BINSCALE, to convert the internal form to external form. (See Section 7.6.)
DSC$B_DIGITS DSC64$B_DIGITS	If nonzero, the unsigned number of decimal digits in the internal representation. If 0, the number of digits can be computed based on LENGTH. This field should be 0 unless the TYPE field specifies a string data type that could contain numeric values.
DSC$B_AFLAGS DSC64$B_AFLAGS	Array flag bits <23:16>: Bits <18:16> Reserved to Hewlett-Packard. Must be 0. DSC$V_FL_BINSCALE DSC64$V_FL_BINSCALE If set, the scale factor specified by SCALE is a signed power-of-two multiplier to convert the internal form to external form. If not set, SCALE specifies a signed power-of-ten multiplier. (See Section 7.6.) DSC$V_FL_REDIM DSC64$V_FL_REDIM Must be 0. DSC$V_FL_UNALLOC DSC64$V_FL_UNALLOC If set, the storage for the array described by this descriptor has not been allocated; the POINTER field must contain 0. If not set, storage for the array described by this descriptor has been allocated; the POINTER field may or may not be 0, depending on the bounds of the array. (If the POINTER field contains a nonzero value, then this flag must not be set.) DSC$V_FL_NODEALLOC If set, the storage for the array described by this descriptor must not be deallocated. (The POINTER and other fields of this descriptor may be cleared or otherwise set to eliminate access to the described storage, but the storage itself belongs to some other descriptor which must be used to deallocate that storage.) Bit <23:23> Reserved to Hewlett-Packard. Must be 0.
DSC$B_DIMCT DSC64$B_DIMCT	Number of dimensions, n.
DSC$L_ARSIZE DSC64$Q_ARSIZE	If the elements are contiguous, ARSIZE is the total size of the array (in bytes, unless the DTYPE field contains the value 21; see the description of LENGTH). If the elements are not allocated contiguously or if the program unit allocating the descriptor is uncertain whether the array is actually contiguous, the value placed in ARSIZE might be meaningless. For data type 1 (aligned bit string), LENGTH is in bits while ARSIZE is in bytes because the unit of length is in bits while the unit of allocation is in bytes.
DSC$A_A0 DSC64$PQ_A0	Address of element A(0,0,...,0). This need not be within the actual array. It is the same as POINTER for zero-origin arrays. A0 = POINTER - ( S 1*L 1 + S 2*L 2 + ...+ S n *L n )
DSC$L_Si DSC64$Q_Si	Stride of the ith dimension. The difference between the addresses of successive elements of the ith dimension.
DSC$L_L i DSC64$Q_L i	Lower bound (signed) of the ith dimension.
DSC$L_U i DSC64$Q_U i	Upper bound (signed) of the ith dimension.

The following formulas specify the effective address, E, of an array element.