Document revision date: 30 March 2001

A variant of the noncontiguous array descriptor is used to specify an array of unaligned bit strings. Each array element is an unaligned bit string data type (DSC$K_DTYPE_VU) that starts and ends on an arbitrary bit boundary. The length of each element is the same and is 0 to 2¹⁶ - 1 bits. You can access elements of the array directly by using the VAX variable bit field instructions. Therefore, the descriptor provides two components: a byte address, BASE, and a means to compute the signed bit offset, EB, with respect to BASE of an array element.

The unaligned bit array descriptor consists of four contiguous blocks that are always present. The first block contains the descriptor prototype information. Figure 5-12 shows the format of an unaligned bit array descriptor. Table 5-13 describes the fields of the descriptor.

Figure 5-12 Unaligned Bit Array Descriptor Format

Table 5-13 Contents of the CLASS_UBA Descriptor

Symbol	Description
DSC$W_LENGTH DSC64$Q_LENGTH	Length of an array element in bits.
DSC64$W_MBO	Must be 1. See Section 5.1.
DSC$B_DTYPE DSC64$B_DTYPE	A data-type code that must have the value 34, which specifies the unaligned bit string data type (see Sections 4.1 and 4.2). The use of other data types is reserved to Compaq.
DSC$B_CLASS DSC64$B_CLASS	Defines the descriptor class code that must be equal to 14 for CLASS_UBA.
DSC$A_BASE DSC64$PQ_BASE	Base address relative to the effective bit offset, EB, that is used to locate elements of the array. The base address need not be the first actual byte of data storage.
DSC64$L_MBMO	Must be -1. See Section 5.1.
DSC$B_SCALE DSC64$B_SCALE	Reserved to Compaq. Must be 0.
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 Compaq. Must be 0. DSC$V_FL_BINSCALE DSC64$V_FL_BINSCALE Must be 0. DSC$V_FL_REDIM DSC64$V_FL_REDIM Must be 0. Bits <23:21> Reserved to Compaq. 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 bits. 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.
DSC$L_V0 DSC64$Q_V0	Signed bit offset of element A(0,...,0) with respect to BASE. V 0 = POS - [S 1*L 1 + ... + S n *L n ].
DSC$L_Si DSC64$Q_Si	Stride of the ith dimension. The difference between the bit (not byte) 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.
DSC$L_POS DSC64$Q_POS	Relative bit position with respect to BASE of the first actual bit of the array, that is, element A(L 1,...,L n ).

The following formulas specify the signed effective bit offset, EB, of an array element:

The signed effective bit offset, EB, of A(I₁):

EB = V0 + S1*I1 = POS + S1*[I1 - L1]

The signed effective bit offset, EB, of A(I₁,I₂):

EB = V0 + S1*I1 + S2*I2 = POS + S1*[I1 - L1] + S2*[I2 - L2]

The signed effective bit offset, EB, of A(I₁, ... , I_n):

EB = V0 + S1*I1 + ... + Sn*In = POS + S1*[I1 - L1] + ... + Sn*[In - Ln]

Note that EB is computed ignoring integer overflow.

On VAX systems, EB is used as the position operand, and the content of BASE is used as the base address operand in the VAX variable-length bit field instructions. Therefore, BASE must specify a byte within 2²⁸ bytes of all bytes of storage in the bit array.

For example, consider a single-origin, one-dimensional, five-element array consisting of 3-bit elements allocated adjacently (therefore, S1 = 3). Assume BASE is byte 1000 and the first actual element, A(1), starts at bit <4> of byte 1001.

The following dependent field values occur in the descriptor:

POS = 12 V0 = 12 - 3*1 = 9

5.12 String with Bounds Descriptor (CLASS_SB)

A variant of the fixed-length string descriptor is used to specify strings where the string is viewed as a one-dimensional array with user-specified bounds. Figure 5-13 shows the format of a string with bounds descriptor. Table 5-14 describes the fields of the descriptor.

Figure 5-13 String with Bounds Descriptor Format

Table 5-14 Contents of the CLASS_SB Descriptor

Symbol	Description
DSC$W_LENGTH DSC64$Q_LENGTH	Length of the string in bytes.
DSC64$W_MBO	Must be 1. See Section 5.1.
DSC$B_DTYPE DSC64$B_DTYPE	A data-type code that must have the value 14, which specifies the character string data type (see Sections 4.1 and 4.2). The use of other data types is reserved to Compaq.
DSC$B_CLASS DSC64$B_CLASS	Defines the descriptor class code that must be equal to 15 for CLASS_SB.
DSC$A_POINTER DSC64$PQ_POINTER	Address of the first byte of data storage.
DSC64$L_MBMO	Must be -1. See Section 5.1.
DSC$L_SB_L1 DSC64$Q_SB_L1	Lower bound (signed) of the first (and only) dimension.
DSC$L_SB_U1 DSC64$Q_SB_U1	Upper bound (signed) of the first (and only) dimension.

The following formula specifies the effective address, E, of a string element A(I):

E = POINTER + [I - SB_L1]

If the string must be extended in a string comparison or assignment, the space character (hexadecimal 20 if ASCII) is used as the fill character.

5.13 Unaligned Bit String with Bounds Descriptor (CLASS_UBSB)

A variant of the unaligned bit string descriptor is used to specify bit strings where the string is viewed as a one-dimensional bit array with user-specified bounds. Figure 5-14 shows the format of an unaligned bit string with bounds descriptor. Table 5-15 describes the fields of the descriptor.

Figure 5-14 Unaligned Bit String with Bounds Descriptor Format

Table 5-15 Contents of the CLASS_UBSB Descriptor

Symbol	Description
DSC$W_LENGTH DSC64$Q_LENGTH	Length of the data item in bits.
DSC64$W_MBO	Must be 1. See Section 5.1.
DSC$B_DTYPE DSC64$B_DTYPE	A data-type code that must have the value 34, which specifies the unaligned bit string data type (see Sections 4.1 and 4.2). The use of other data types is reserved to Compaq.
DSC$B_CLASS DSC64$B_CLASS	Defines the descriptor class code that must be equal to 16 for CLASS_UBSB.
DSC$A_BASE DSC64$PQ_BASE	Base address relative to the signed relative bit position, POS, used to locate the bit string. The base address need not be the first actual byte of data storage.
DSC64$L_MBMO	Must be -1. See Section 5.1.
DSC$L_POS DSC64$Q_POS	Signed longword that defines the relative bit position of the first bit of the unaligned bit string to the BASE address.
DSC$L_UBSB_L1 DSC64$Q_UBSB_L1	Lower bound (signed) of the first (and only) dimension.
DSC$L_UBSB_U1 DSC64$Q_UBSB_U1	Upper bound (signed) of the first (and only) dimension.

The following formula specifies the effective bit offset, EB, of a bit element A(I):

EB = POS + [I - UBSB_L1]

5.14 Reserved Descriptor Class Codes

All descriptor class codes from 0 through 191 not otherwise defined in this standard are reserved to Compaq. Classes 192 through 255 are reserved to Compaq's Computer Special Systems Group and customers.

Table 5-16 lists some specific descriptor classes and codes that are obsolete or reserved to Compaq.

Table 5-16 Specific OpenVMS VAX Descriptors Reserved to Compaq

Descriptor	Code	Class
DSC$K_CLASS_V	3	Obsolete (variable buffer)
DSC$K_CLASS_PI	6	Obsolete (procedure incarnation)
DSC$K_CLASS_J	7	Reserved to DEBUG (label)
DSC$K_CLASS_JI	8	Obsolete (label incarnation)
DSC$K_CLASS_CT	17	Reserved to ACMS (compressed text)
DSC$K_CLASS_BFA	191	Reserved to BASIC (file array)

5.14.1 Facility-Specific Descriptor Class Codes

Descriptor class codes 160 through 191 are reserved to Compaq for facility-specific purposes. These codes must not be passed between facilities, because different facilities might use the same code for different purposes. These codes can be used by compiler-generated code to pass parameters to the language-specific, run-time support procedures associated with that language or to the OpenVMS Debugger.

An OpenVMS condition is a hardware-generated synchronous exception or a software event that is to be processed in a manner similar to a VAX or an Alpha hardware exception.

Floating-point overflow exception, memory access violation exception, and reserved operation exception are examples of hardware-generated conditions. An output conversion error, an end of file, and the filling of an output buffer are examples of software events that might be treated as conditions.

Depending on the condition and on the program, you can exercise any of four types of action when a condition occurs:

• Ignore the condition.
  For example, if an underflow occurs in a floating-point operation, continuing from the point of the exception with a zero result might be sufficient.
• Take some special action and continue from the point at which the condition occurred.
  For example, if the end of a buffer is reached while a series of data items are being written, the special action is to start a new buffer.
• End the operation and branch from the sequential flow of control.
  For example, if the end of an input file is reached, the branch exits from a loop that is processing the input data.
• Treat the condition as an unrecoverable error.
  For example, when the floating divide-by-zero exception condition occurs, the program exits after writing (optionally) an appropriate error message.

When an unusual event or error occurs in a called procedure, the procedure can return a condition value to the caller indicating what has happened (see Section 6.1). The caller tests the condition value and takes the appropriate action.

When an exception is generated by the hardware, a branch out of the program's flow of control occurs automatically. In this case, and for certain software-generated events, it is more convenient to handle the condition as soon as it is detected rather than to program explicit tests.

6.1 Condition Values

Condition values are used in the OpenVMS operating system to provide the following functions:

• Indicate the success or failure of a called procedure as a function value
• Describe an exception condition when an exception is signaled
• Identify system messages
• Report program success or failure to the command language level

A condition value is a longword that includes fields to describe the software component that generates the value, the reason the value was generated, and severity status of the condition value. Figure 6-1 shows the format of a condition value. Table 6-1 describes the fields of a condition value.

Figure 6-1 Format of a Condition Value

Table 6-1 Contents of the Condition Value

Symbol	Description
Severity	Indicates success or failure. The severity code bit <0> is set for success (logical true) and is clear for failure (logical false); bits <1> and <2> distinguish degrees of success or failure. Bits <2:0>, when taken as an unsigned integer, are interpreted as shown in the following table: Symbol Value Description STS$K_WARNING 0 Warning STS$K_SUCCESS 1 Success STS$K_ERROR 2 Error STS$K_INFO 3 Information STS$K_SEVERE 4 Severe error   5 Reserved to Compaq   6 Reserved to Compaq   7 Reserved to Compaq Section 6.1.1 more fully describes severity codes.
Condition identification	Identifies the condition uniquely on a systemwide basis.
Message number	Describes the status, which can be a hardware exception that occurred or a software-defined value. Message numbers with bit <15> set are specific to a single facility. Message numbers with bit <15> clear are systemwide status codes.
Facility number	Identifies the software component generating the condition value. Bit <27> is set for customer facilities and is clear for Compaq facilities.
Control	Controls the printing of the message associated with the condition value. Bit <28> inhibits the message associated with the condition value from being printed by the SYS$EXIT system service. This bit is set by the system default handler after it has output an error message using the SYS$PUTMSG system service. It should also be set in the condition value returned by a procedure as a function value, if the procedure has also signaled the condition (so the condition has been printed or suppressed). Bits <31:29> must be 0; they are reserved to Compaq for future use. Table 6-2 lists the possible software symbols that are defined for the various fields of the condition-value longword.

Table 6-2 Value Symbols for the Condition Value Longword

Symbol	Value	Meaning	Field
STS$V_COND_ID	3	Position of 27:3	Condition identification
STS$S_COND_ID	25	Size of 27:3	Condition identification
STS$M_COND_ID	Mask	Mask for 27:3	Condition identification
STS$V_INHIB_MSG	1@28	Position for 28	Inhibit message on image exit
STS$S_INHIB_MSG	1	Size for 28	Inhibit message on image exit
STS$M_INHIB_MSG	Mask	Mask for 28	Inhibit message on image exit
STS$V_FAC_NO	16	Position of 27:16	Facility number
STS$S_FAC_NO	12	Size of 27:16	Facility number
STS$M_FAC_NO	Mask	Mask for 27:16	Facility number
STS$V_CUST_DEF	27	Position for 27	Customer facility
STS$S_CUST_DEF	1	Size for 27	Customer facility
STS$M_CUST_DEF	1@27	Mask for 27	Customer facility
STS$V_MSG_NO	3	Position of 15:3	Message number
STS$S_MSG_NO	13	Size of 15:3	Message number
STS$M_MSG_NO	Mask	Mask for 15:3	Message number
STS$V_FAC_SP	15	Position of 15	Facility-specific
STS$S_FAC_SP	1	Size for 15	Facility-specific
STS$M_FAC_SP	1@15	Mask for 15	Facility-specific
STS$V_CODE	3	Position of 14:3	Message code
STS$S_CODE	12	Size of 14:3	Message code
STS$M_CODE	Mask	Mask for 14:3	Message code
STS$V_SEVERITY	0	Position of 2:0	Severity
STS$S_SEVERITY	3	Size of 2:0	Severity
STS$M_SEVERITY	7	Mask for 2:0	Severity
STS$V_SUCCESS	0	Position of 0	Success
STS$S_SUCCESS	1	Size of 0	Success
STS$M_SUCCESS	1	Mask for 0	Success

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