HP OpenVMS Systems Documentation

HP OpenVMS Calling Standard

The effective address, E, of A(I):

   E = A0 + S1*I
     = POINTER + S1*[I - L1]

The effective address, E, of A(I₁,I₂):

   E = A0 + S1*I1 + S2*I2
     = POINTER + S1*[I1 - L1] + S2*[I2 - L2]

The effective address, E, of A(I₁, . . . ,I_n):

   E = A0 + S1*I1 + . . . + Sn*In
     = POINTER + S1*[I1 - L1] + . . . + Sn*[In - Ln]

A class VS descriptor is used for varying string data types (see Section 6.5).

As an input parameter, this format is not interchangeable with class 1 (CLASS_S) or with class 2 (CLASS_D). When a called procedure modifies a varying string passed by reference or by descriptor, it writes the new length, n, into CURLEN and can modify all bytes of BODY. Figure 7-8 shows the format of a varying string descriptor. Table 7-10 describes the fields of the descriptor.

Figure 7-8 Varying String Descriptor Format

Figure 7-9 illustrates the use of a 32-bit varying string descriptor to present a variable that is capable of holding a string value of up to five characters in length and that is currently holding the string value ABCD. As shown in the figure, MAXSTRLEN contains five, CURLEN contains four, string is currently ABCD, and the remaining byte is currently undefined.

Figure 7-9 Varying String Descriptor with Character String Data Type

A variant of the noncontiguous array descriptor is used to specify an array of varying strings where each varying string has the same maximum length. Each array element is of the varying string data type (see Section 6.5).

When a called procedure modifies a varying string in an array of varying strings passed to it by reference or by descriptor, it writes the new length, n, into CURLEN and can modify all bytes of BODY. The format of this descriptor is the same as the noncontiguous array descriptor except for the first two longwords. Figure 7-10 shows the format of a varying string array descriptor. Table 7-11 describes the fields of the descriptor.

Figure 7-10 Varying String Array Descriptor Format

The remaining fields in the descriptor are identical to those in the noncontiguous array descriptor (NCA). The effective address computation of an array element produces the address of CURLEN of the desired element.

7.10 Unaligned Bit String Descriptor (CLASS_UBS)

A descriptor is used to pass an unaligned bit string (DSC$K_DTYPE_VU) that starts and ends on an arbitrary bit boundary. The descriptor provides two components: a base address and a signed relative bit position. Figure 7-11 shows the format of an unaligned bit string descriptor. Table 7-12 describes the fields of the descriptor.

Figure 7-11 Unaligned Bit String Descriptor Format

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 7-12 shows the format of an unaligned bit array descriptor. Table 7-13 describes the fields of the descriptor.

Figure 7-12 Unaligned Bit Array Descriptor Format

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

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 7-13 shows the format of a string with bounds descriptor. Table 7-14 describes the fields of the descriptor.

Figure 7-13 String with Bounds Descriptor Format

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.

7.13 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 7-14 shows the format of an unaligned bit string with bounds descriptor. Table 7-15 describes the fields of the descriptor.

Figure 7-14 Unaligned Bit String with Bounds Descriptor Format

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

   EB = POS + [I - UBSB_L1]

All descriptor class codes from 0 through 191 not otherwise defined in this standard are reserved to Hewlett-Packard. Classes 192 through 255 are reserved for Hewlett-Packard custom systems and for customers for their own use.

Table 7-16 lists some specific descriptor classes and codes that are obsolete or reserved to Hewlett-Packard.