HP OpenVMS Systems Documentation

HP OpenVMS Calling Standard

A thread can obtain an invocation handle corresponding to any invocation context block by using the following function format:

Argument:

Function Value Returned:

A thread can obtain an invocation handle of the procedure context preceding that of a specified procedure context by using the following function format:

Argument:

Function Value Returned:

A given procedure invocation context's fields can be updated with new register contents by calling a system library function in following format:

Arguments:

Function Value Returned:

This standard states that a standard call (see Section 1.4) may be accomplished in any way that presents the called routine with the required environment. However, typically, most standard-conforming external calls are implemented with a common sequence of instructions and conventions. Because a common set of call conventions is so pervasive, these conventions are included for reference as part of this standard.

One important feature of the calling standard is that the same instruction sequence can be used to call each of the different types of procedure. Specifically, the caller does not have to know which type of procedure is being called.

3.6.1 Call Conventions

The call conventions describe the rules and methods used to communicate certain information between the caller and the called procedure during invocation and return. For a standard call, these conventions include the following:

• Procedure value
  The calling procedure must pass to the called procedure its procedure value. This value can be a statically or dynamically bound procedure value. This is accomplished by loading R27 with the procedure value before control is transferred to the called procedure.
• Return address
  The calling procedure must pass to the called procedure the address to which control must be returned during a normal return from the called procedure. In most cases, the return address is the address of the instruction following the one that transferred control to the called procedure. For a standard call, this address is passed in the return address register (R26).
• Argument list
  The argument list is an ordered set of zero or more argument items that together constitute a logically contiguous structure known as an argument item sequence. This logically contiguous sequence is typically mapped to registers and memory in a way that produces a physically discontiguous argument list. In a standard call, the first six items are passed in registers R16--21 or registers F16--21. (See Section 3.7.2 for details of argument-to-register correspondence.) The remaining items are collected in a memory argument list that is a naturally aligned array of quadwords. In a standard call, this list (if present) must be passed at 0(SP).
• Argument information
  The calling procedure must pass to the called procedure information about the argument list. This information is passed in the argument information (AI) register (R25). Defined by AI$K_AI_SIZE, the structure is a quadword as shown in Figure 3-10 with the fields described in Table 3-7.

  Figure 3-10 Argument Information Register (R25) Format

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  Table 3-7 Contents of the Argument Information Register (R25)

  Field Name	Contents
  AI$B_ARG_COUNT	Unsigned byte <7:0> that specifies the number of 64-bit argument items in the argument list (known as the "argument count").
  AI$V_ARG_REG_INFO	An 18-bit vector field <25:8> divided into six groups of 3 bits that correspond to the six arguments passed in registers. These groups describe how each of the first six arguments are passed in registers with the first group <10:8> describing the first argument. The encoding for each group for the argument register usage follows: Value Name Meaning 0 AI$K_AR_I64 64-bit or 32-bit sign-extended to 64-bit argument passed in an integer register (including addresses) or Argument is not present 1 AI$K_AR_FF F_floating argument passed in a floating register 2 AI$K_AR_FD D_floating argument passed in a floating register 3 AI$K_AR_FG G_floating argument passed in a floating register 4 AI$K_AR_FS S_floating argument passed in a floating register 5 AI$K_AR_FT T_floating argument passed in a floating register 6, 7   Reserved
  Bits 26--63	Reserved and must be 0.

• Function result
  If a standard-conforming procedure is a function and the function result is returned in a register, then the result is returned in R0, F0, or F0 and F1. Otherwise, the function result is returned via the first argument item or dynamically as defined in Section 3.7.7.
• Stack usage
  Whenever control is transferred to another procedure, the stack pointer (SP) must be octaword aligned; at other times there is no stack alignment requrement. (A side effect of this is that the in-memory portion of the argument list will start on an octaword boundary.) During a procedure invocation, the SP (R30) can never be set to a value higher than the SP at entry to that procedure invocation.
  The contents of the stack located above the portion of the argument list that is passed in memory (if any) belongs to the calling procedure and is, therefore, not to be read or written by the called procedure, except as specified by indirect arguments or language-controlled up-level references.
  Because SP is used by the hardware in raising exceptions and asynchronous interrupts, the contents of the next 2048 bytes below the current SP value are continually and unpredictably modified. Software that conforms to this standard must not depend on the contents of the 2048 stack locations below 0(SP).

  Note One implication of the stack alignment requirement is that low-level interrupt and exception-fielding software must be prepared to handle and correct the alignment before calling handler routines, in case the stack pointer is not octaword aligned at the time of an interrupt or exception.

Because the Alpha hardware architecture has the property of instructions that cannot contain full virtual addresses, it is sometimes referred to as a base register architecture. In a base register architecture, normal memory references within a limited range from a given address are expressed by using displacements relative to the contents of a register containing that address (base register). Base registers for external program segments, either data or code, are usually loaded indirectly through a program segment of address constants.

The fundamental program section containing address constants that a procedure uses to access other static storage, external procedures, and variables is termed a linkage section. Any register used to access the contents of the linkage section is termed a linkage pointer.

A procedure's linkage section includes the procedure descriptor for the procedure, addresses of all external variables and procedures referenced by the procedure, and other constants a compiler may choose to reference using a linkage pointer.

When a standard procedure is called, the caller must provide the procedure value for that procedure in R27. Static procedure values are defined to be the address of the procedure's descriptor. Because the procedure descriptor is part of the linkage section, calling this type of procedure value provides a pointer into the linkage section for that procedure in R27. This linkage pointer can then be used by the called procedure as a base register to address locations in its linkage section. For this reason, most compilers generate references to items in the linkage section as offsets from a pointer to the procedure's descriptor.

Compilers usually arrange (as part of the environment setup) to have the environment setup code (for bound procedures) load R27 with the address of the procedure's descriptor so it can be used as a linkage pointer as previously described. For an example, see Section 3.6.4.

Although not required, linkages to external procedures are typically represented in the calling procedure's linkage section as a linkage pair. As shown in Figure 3-11 and described in Table 3-8, a linkage pair (LKP) block with two fields should be octaword aligned and defined by LKP$K_SIZE as 16 bytes.

Figure 3-11 Linkage Pair Block Format

In general, an object module contains a procedure descriptor for each entry point in the module. The descriptors are allocated in a linkage section. For each external procedure Q that is referenced in a module, the module's linkage section also contains a linkage pair denoting Q (which is a pointer to Q's procedure descriptor and entry code address).

The following code example calls an external procedure Q as represented by a linkage pair. In this example, R4 is the register that currently contains the address of the current procedure's descriptor.

        LDQ     R26,Q_DESC-MY_DESC(R4)   ;Q's entry address into R26
        LDQ     R27,Q_DESC-MY_DESC+8(R4) ;Q's procedure value into R27
        MOVQ    #AI_LITERAL,R25          ;Load Argument Information register
        JSR     R26,(R26)                ;Call to Q. Return address in R26

Because Q's procedure descriptor (statically defined procedure value) is in Q's linkage section, Q can use the value in R27 as a base address for accessing data in its linkage section. Q accesses external procedures and data in other program sections through pointers in its linkage section. Therefore, R27 serves as the root pointer through which all data can be referenced.

3.6.3 Calling Computed Addresses

Most calls are made to a fixed address whose value is determined by the time the program starts execution. However, certain cases are possible that cause the exact address to be unknown until the code is finally executed. In this case, the procedure value representing the procedure to be called is computed in a register.

The following code example illustrates a call to a computed procedure value (simple or bound) that is contained in R4:

        LDQ     R26,8(R4)           ;Entry address to scratch register
        MOV     R4,R27              ;Procedure value to R27
        MOV     #AI_LITERAL,R25     ;Load Argument Information register
        JSR     R26,(R26)           ;Call entry address.

For interoperation with translated images, see Chapter 5

3.6.4 Simple and Bound Procedures

There are two distinct classes of procedures:

• Simple procedure
• Bound procedure

A simple procedure is a procedure that does not need direct access to the stack of its execution environment. A bound procedure is a procedure that does need direct access to the stack of its execution environment, typically to reference an up-level variable or to perform a nonlocal GOTO operation. Both a simple procedure and a bound procedure have an associated procedure descriptor, as described in previous sections.

When a bound procedure is called, the caller must pass some kind of pointer to the called code that allows it to reference its up-level environment. Typically, this pointer is the frame pointer for that environment, but many variations are possible. When the caller is executing its program within that outer environment, it can usually make such a call directly to the code for the nested procedure without recourse to any additional procedure descriptors. However, when a procedure value for the nested procedure must be passed outside of that environment to a call site that has no knowledge of the target procedure, a bound procedure descriptor is created so that the nested procedure can be called just like a simple procedure.

Bound procedure values, as defined by this standard, are designed for multilanguage use and utilize the properties of procedure descriptors to allow callers of procedures to use common code to call both bound and simple procedures.

3.6.4.1 Bound Procedure Descriptors

Bound procedure descriptors provide a mechanism to interpose special processing between a call and the called routine without modifying either. The descriptor may contain (or reference) data used as part of that processing. Between native and translated images, the OpenVMS Alpha operating system uses linker and image-activator created bound procedure descriptors to mediate the handling of parameter and result passing (see Section 5.2). Language processors on OpenVMS Alpha systems use bound procedure descriptors to implement bound procedure values (see Section 3.6.4.2). Other uses are possible.

The minimum size of the descriptor is 24 bytes (defined by PDSC$K_MIN_BOUND_SIZE). An optional PDSC extension in 8-byte increments provides the specific environment values as defined by the implementation.

The fields defined in the bound procedure descriptor are illustrated in Figure 3-12 and described in Table 3-9.

Figure 3-12 Bound Procedure Descriptor (PDSC)