HP OpenVMS Systems Documentation

OpenVMS Programming Concepts Manual

Write an exit handler as a subroutine, because no function value can be returned. The dummy arguments of the exit subroutine should agree in number, order, and data type with the arguments you specified in the call to SYS$DCLEXH.

In the following example, assume that two or more programs are cooperating with each other. To keep track of which programs are executing, each has been assigned a common event flag (the common event flag cluster is named ALIVE). When a program begins, it sets its flag; when the program terminates, it clears its flag. Because it is important that each program clear its flag before exiting, you create an exit handler to perform the action. The exit handler accepts two arguments, the final status of the program and the number of the event flag to be cleared. In this example, since the cleanup operation is to be performed regardless of whether the program completes successfully, the final status is not examined in the exit routine. (This subroutine would not be used with the exit handler declaration in the previous example.)

CLEAR_FLAG.FOR

SUBROUTINE CLEAR_FLAG (EXIT_STATUS,
2                      FLAG)
! Exit handler clears the event flag

! Declare dummy argument
INTEGER EXIT_STATUS,
2       FLAG
! Declare status variable and system routine
INTEGER STATUS,
2       SYS$ASCEFC,
2       SYS$CLREF
! Associate with the common event flag
! cluster and clear the flag
STATUS = SYS$ASCEFC (%VAL(FLAG),
2                    'ALIVE',,)
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS))
STATUS = SYS$CLREF (%VAL(FLAG))
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL (STATUS))

END

If for any reason you must perform terminal I/O from an exit handler, use appropriate RTL routines. Trying to access the terminal from an exit handler using language I/O statements may cause a redundant I/O error.

9.15.3 Debugging an Exit Handler

To debug an exit handler, you must set a breakpoint in the handler and wait for the operating system to invoke that handler; you cannot use the DEBUG command STEP/INTO to enter an exit handler. In addition, when the debugger is invoked, it establishes an exit handler that exits using the SYS$EXIT system service. If you invoke the debugger when you invoke your image, the debugger's exit handler does not affect your program's handlers because the debugger's handler is established first and so executes last. However, if you invoke the debugger after your program begins executing (the user presses Ctrl/Y and then types DEBUG), the debugger's handler may affect the execution of your program's exit handlers, because one or more of your handlers may have been established before the debugger's handler and so is not executed.

9.15.4 Example of Exit Handler

As in the example in Section 9.15.2, write the exit handler as a subroutine because no function value can be returned. The dummy arguments of the exit subroutine should agree in number, order, and data type with the arguments you specify in the call to SYS$DCLEXH.

In the following example, assume that two or more programs are cooperating. To keep track of which programs are executing, each has been assigned a common event flag (the common event flag cluster is named ALIVE). When a program begins, it sets its flag; when the program terminates, it clears its flag. Because each program must clear its flag before exiting, you create an exit handler to perform the action. The exit handler accepts two arguments: the final status of the program and the number of the event flag to be cleared.

In the following example, because the cleanup operation is to be performed regardless of whether the program completes successfully, the final status is not examined in the exit routine.

! Arguments for exit handler
INTEGER*4 EXIT_STATUS       ! Status
INTEGER*4 FLAG /64/
! Setup for exit handler
STRUCTURE /EXIT_DESCRIPTOR/
 INTEGER LINK,
2        ADDR,
2        ARGS /2/,
2        STATUS_ADDR,
2        FLAG_ADDR
END STRUCTURE
RECORD /EXIT_DESCRIPTOR/ HANDLER

! Exit handler
EXTERNAL CLEAR_FLAG

INTEGER*4 STATUS,
2         SYS$ASCEFC,
2         SYS$SETEF

! Associate with the common event flag
! cluster and set the flag.
STATUS = SYS$ASCEFC (%VAL(FLAG),
2                    'ALIVE',,)
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS))
STATUS = SYS$SETEF (%VAL(FLAG))
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL (STATUS))
! Do not exit until cooperating program has a chance to
! associate with the common event flag cluster.

! Enter the handler and argument addresses
! into the exit handler description.
HANDLER.ADDR = %LOC(CLEAR_FLAG)
HANDLER.STATUS_ADDR = %LOC(EXIT_STATUS)
HANDLER.FLAG_ADDR = %LOC(FLAG)
! Establish the exit handler.
CALL SYS$DCLEXH (HANDLER)

! Continue with program
               .
               .
               .
END

! Exit Subroutine

SUBROUTINE CLEAR_FLAG (EXIT_STATUS,
2                      FLAG)
! Exit handler clears the event flag

! Declare dummy argument
INTEGER EXIT_STATUS,
2       FLAG

! Declare status variable and system routine
INTEGER STATUS,
2       SYS$ASCEFC,
2       SYS$CLREF

! Associate with the common event flag
! cluster and clear the flag
STATUS = SYS$ASCEFC (%VAL(FLAG),
2                    'ALIVE',,)
2                    'ALIVE',,)
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL(STATUS))
STATUS = SYS$CLREF (%VAL(FLAG))
IF (.NOT. STATUS) CALL LIB$SIGNAL (%VAL (STATUS))

As of Version 7.0, the OpenVMS Alpha operating system provides support for 64-bit virtual memory addressing. This capability makes the 64-bit virtual address space, defined by the Alpha architecture, available to the OpenVMS Alpha operating system and to application programs. OpenVMS Alpha Version 7.1 provided extended, additional memory management Very Large Memory (VLM) features. For information about Very Large Memory, see Chapter 16.

10.1 Using 64-Bit Addresses

Many OpenVMS Alpha tools and languages (including the Debugger, run-time library routines, and Compaq C) support 64-bit virtual addressing. Input and output operations can be performed directly to and from the 64-bit addressable space by means of RMS services, the $QIO system service, and most of the device drivers supplied with OpenVMS Alpha systems.

Underlying this are system services that allow an application to allocate and manage the 64-bit virtual address space, which is available for process-private use.

By using the OpenVMS Alpha tools and languages that support 64-bit addressing, programmers can create images that map and access data beyond the limits of 32-bit virtual addresses. The 64-bit virtual address space design ensures upward compatibility of programs that execute under versions of OpenVMS Alpha prior to Version 7.0, while providing a flexible framework that allows 64-bit addresses to be used in many different ways to solve new problems.

Nonprivileged programs can optionally be modified to take advantage of 64-bit addressing features. OpenVMS Alpha 64-bit virtual addressing does not affect nonprivileged programs that are not explicitly modified to exploit 64-bit support. Binary and source compatibility of existing 32-bit nonprivileged programs is guaranteed.

By using 64-bit addressing capabilities, application programs can map large amounts of data into memory to provide high levels of performance and make use of very large memory (VLM) systems. In addition, 64-bit addressing allows for more efficient use of system resources, allowing for larger user processes, as well as higher numbers of users and client/server processes for virtually unlimited scalability.

This chapter describes the layout and components of the OpenVMS Alpha 64-bit virtual memory address space.

For more information about the OpenVMS Alpha programming tools and languages that support 64-bit addressing and recommendations for enhancing applications to support 64-bit addressing and VLM, refer to the subsequent chapters in this guide.

10.2 Traditional OpenVMS 32-Bit Virtual Address Space Layout

Figure 10-1 32-Bit Virtual Address Space Layout

The lower half of the OpenVMS VAX virtual address space (addresses between 0 and 7FFFFFFF₁₆) is called process-private space. This space is further divided into two equal pieces called P0 space and P1 space. Each space is 1 GB long. The P0 space range is from 0 to 3FFFFFFF₁₆. P0 space starts at location 0 and expands toward increasing addresses. The P1 space range is from 40000000₁₆ to 7FFFFFFF₁₆. P1 space starts at location 7FFFFFFF₁₆ and expands toward decreasing addresses.

The upper half of the VAX virtual address space is called system space. The lower half of system space (the addresses between 80000000₁₆ and BFFFFFFF₁₆) is called S0 space. S0 space begins at 80000000₁₆ and expands toward increasing addresses.

The VAX architecture associates a page table with each region of virtual address space. The processor translates system space addresses using the system page table. Each process has its own P0 and P1 page tables. A VAX page table does not map the full virtual address space possible; instead, it maps only the part of its region that has been created.

10.3 OpenVMS Alpha 64-Bit Virtual Address Space Layout

Figure 10-2 illustrates the 64-bit virtual address space layout design.

Figure 10-2 64-Bit Virtual Address Space Layout

The 64-bit virtual address space layout is designed to accommodate the current and future needs of the OpenVMS Alpha operating system and its users. The new address space consists of the following fundamental areas:

• Process-private space
• System space
• Page table space

Supporting process-private address space is a focus of much of the memory management design within the OpenVMS operating system.

Process-private space, or process space, contains all virtual addresses below PT space. As shown in Figure 10-2, the layout of process space is further divided into the P0, P1, and P2 spaces. P0 space refers to the program region. P1 space refers to the control region. P2 space refers to the 64-bit program region.

The P0 and P1 spaces are defined to equate to the P0 and P1 regions defined by the VAX architecture. Together, they encompass the traditional 32-bit process-private region that ranges from 0.00000000₁₆ to 0.7FFFFFFF₁₆. P2 space encompasses all remaining process space that begins just above P1 space, 0.80000000₁₆, and ends just below the lowest address of PT space.

Note that P2 space addresses can be positive or negative when interpreted as signed 64-bit integers.

10.3.2 System Space

64-bit system space refers to the portion of the entire 64-bit virtual address range that is higher than that which contains PT space. As shown in Figure 10-2, system space is further divided into the S0, S1, and S2 spaces.

The S0 and S1 spaces are defined to equate to the S0 and S1 regions defined by the VAX architecture. Together they encompass the traditional 32-bit system space region that ranges from FFFFFFFF.80000000₁₆ to FFFFFFFF.FFFFFFFF₁₆. S2 space encompasses all remaining system spaces between the highest address of PT space and the lowest address of the combined S0/S1 space.

S0, S1, and S2 are fully shared by all processes. S0/S1 space expands toward increasing virtual addresses. S2 space generally expands toward lower virtual addresses.

Addresses within system space can be created and deleted only from code that is executing in kernel mode. However, page protection for system space pages can be set up to allow any less privileged access mode read and/or write access.

System space base is controlled by the S2_SIZE system parameter. S2_SIZE is the number of megabytes to reserve for S2 space. The default value is based on the sizes required by expected consumers of 64-bit (S2) system space. Consumers set up by OpenVMS at boot time are the page frame number (PFN) database and the global page table. (For more information about setting system parameters with SYSGEN, see the OpenVMS System Management Utilities Reference Manual: M--Z.)

The global page table, also known as the GPT, and the PFN database reside in the lowest-addressed portion of S2 space. By moving the GPT and PFN database to S2 space, the size of these areas is no longer constrained to a small portion of S0/S1 space. This allows OpenVMS to support much larger physical memories and much larger global sections.

10.3.3 Page Table Space

In versions of OpenVMS Alpha prior to Version 7.0, page table space (also known as PT space) was addressable in more than one way. The PALcode translation buffer miss handler used addresses starting at 2.00000000₁₆ to read PTEs, while memory management code addressed the page tables primarily within the traditional 32-bit system space. The process page tables were within the process header (PHD), and the system space page tables were located in the highest virtual addresses, all within the traditional 32-bit system space.

As of OpenVMS Alpha Version 7.0, page tables are addressed primarily within 64-bit PT space. Page tables refer to this virtual address range; they are no longer in 32-bit shared system address space.

The dotted line in Figure 10-2 marks the boundary between process-private space and shared space. This boundary is in PT space and further serves as the boundary between the process-private page table entries and the shared page table entries. Together, these sets of entries map the entire address space available to a given process. PT space is mapped to the same virtual address for each process, typically a very high address such as FFFFFFFC.00000000₁₆.

10.3.4 Virtual Address Space Size

The Alpha architecture supports 64-bit addresses. OpenVMS Alpha Version 7.0 dramatically increased the total amount of virtual address space from 4 GB (gigabytes) to the 8 TB (terabytes) supported by the current Alpha architecture implementations.

The Alpha architecture requires that all implementations must use or check all 64 bits of a virtual address during the translation of a virtual address into a physical memory address. However, implementations of the Alpha architecture are allowed to materialize a subset of the virtual address space. Current Alpha hardware implementations support 43 significant bits within a 64-bit virtual address. This results in an 8-TB address space.

On current Alpha architecture implementations, bit 42 within a virtual address must be sign-extended or propagated through bit 63 (the least significant bit is numbered from 0). Virtual addresses where bits 42 through 63 are not all zeros or all ones result in an access violation when referenced. Therefore, the valid 8-TB address space is partitioned into two disjoint 4-TB ranges separated by a no access range in the middle.

The layout of the OpenVMS Alpha address space transparently places this no access range within P2 space. (The OpenVMS Alpha memory management system services always return virtually contiguous address ranges.) The result of the OpenVMS Alpha address space layout design is that valid addresses in P2 space can be positive or negative values when interpreted as signed 64-bit integers.

Note that to preserve 32-bit nonprivileged code compatibility, bit 31 in a valid 32-bit virtual address can still be used to distinguish an address in P0/P1 space from an address in S0/S1 space.

10.4 Virtual Regions

A virtual region is a reserved range of process-private virtual addresses. It may be either a user-defined virtual region reserved by the user program at run time or a process-permanent virtual region reserved by the system on behalf of the process during process creation.

Three process-permanent virtual regions are defined by OpenVMS at the time the process is created:

• Program region (in P0 space)
• Control region (in P1 space)
• 64-bit program region (in P2 space)

These three process-permanent virtual regions exist so that programmers do not have to create virtual regions if their application does not need to reserve additional ranges of address space.

Virtual regions promote modularity within applications by allowing different components of the application to manipulate data in different virtual regions. When a virtual region is created, the caller of the service is returned a region ID to identify that virtual region. The region ID is used when creating, manipulating, and deleting virtual addresses within that region. Different components within an application can create separate virtual regions so that their use of virtual memory does not conflict.

Virtual regions exhibit the following characteristics.

• A virtual region is a light-weight object. That is, it does not consume pagefile quota or working set quota for the virtual addresses specified. Creating a user-defined virtual region by calling a new OpenVMS system service merely defines a virtual address range as a distinct address object within which address space can be created, manipulated, and deleted.
• Virtual regions do not overlap. When creating address space within a virtual region, the programmer must specify a region ID to the OpenVMS system service. The programmer must specify the virtual region in which the address space is to be created.
• The programmer cannot create, manipulate, or delete address space that does not lie entirely within the bounds of a defined virtual region.
• Each user-defined virtual region's size is fixed at the time it is created. Given the large range of virtual addresses in P2 space and the light-weight nature of virtual regions, it is not costly to reserve more address space than the application component immediately needs within that virtual region.
  Note the exception of process-permanent regions, which have no fixed size.
  The 64-bit program virtual region is the only virtual region whose size is not fixed when it is created. At process creation, the 64-bit program region encompasses all of P2 space. When a user-defined virtual region is created in P2 space, OpenVMS memory management shrinks the 64-bit program region so that no two regions overlap. When a user-defined virtual region is deleted, the 64-bit program region expands to encompass the virtual addresses within the deleted virtual region if no other user-defined virtual region exists at lower virtual addresses.
• Each virtual region has an owner mode and a create mode associated with it. Access modes that are less privileged than the owner of the virtual region cannot delete the virtual region. Access modes that are less privileged than the create mode set for the virtual region cannot create virtual addresses within the virtual region. Owner and create modes are set at the time the virtual region is created and cannot be changed. The create mode for a virtual region cannot be more privileged than the owner mode.
• When virtual address space is created within a virtual region, allocation generally occurs within the virtual region in a densely expanding manner, as is done within the program (P0 space) and control (P1 space) regions. At the time it is created, each virtual region is set up for the virtual addresses within that virtual region to expand toward either increasing virtual addresses, like P0 space, or decreasing virtual addresses, like P1 space. Users can override this allocation algorithm by explicitly specifying starting addresses.
• All user-defined virtual regions are deleted along with the pages created within each virtual region at image rundown.

There is one process-permanent virtual region for all of P0 space that starts at virtual address 0 and ends at virtual address 0.3FFFFFFF₁₆. This is called the program region. There is also one process-permanent region for all of P1 space that starts at virtual address 0.40000000₁₆ and ends at virtual address 0.7FFFFFFF₁₆. This is called the control region.

The program and control regions are considered to be owned by kernel mode and have a create mode of user, because user mode callers can create virtual address space within these virtual regions. This is upwardly compatible with OpenVMS Alpha releases prior to Version 7.0.

These program and control regions cannot be deleted. They are considered to be process-permanent.

10.4.2 64-Bit Program Region

P2 space has a densely expandable virtual region starting at the lowest virtual address of P2 space, 0.80000000₁₆. This region is called the 64-bit program region. Having a 64-bit program region in P2 space allows an application that does not need to take advantage of explicit virtual regions to avoid incurring the overhead of creating a virtual region in P2 space. This virtual region always exists, so addresses can be created within P2 space immediately.

As described in Section 10.4.3, a user can create a virtual region in otherwise unoccupied P2 space. If the user-defined virtual region is specified to start at the lowest address of the 64-bit program region, then any subsequent attempt to allocate virtual memory within the region will fail.

The region has a user create mode associated with it, that is, any access mode can create virtual address space within it.

The 64-bit program region cannot be deleted. It is considered to be process-permanent and survives image rundown. Note that all created address space within the 64-bit program region is deleted and the region is reset to encompass all of P2 space as a result of image rundown.

10.4.3 User-Defined Virtual Regions

A user-defined virtual region is a virtual region created by calling the new OpenVMS SYS$CREATE_REGION_64 system service. The location at which a user-defined virtual region is created is generally unpredictable. In order to maximize the expansion room for the 64-bit program region, OpenVMS memory management allocates virtual regions starting at the highest available virtual address in P2 space that is lower than any existing user-defined virtual region.

For maximum control of the process-private address space, the application programmer can specify the starting virtual address when creating a virtual region. This is useful in situations when it is imperative that the user be able to specify exact virtual memory layout.

Virtual regions can be created so that allocation occurs with either increasing addresses or decreasing virtual addresses. This allows applications with stacklike structures to create virtual address space and expand naturally.

Virtual region creation gives OpenVMS subsystems and the application programmer the ability to reserve virtual address space for expansion. For example, an application can create a large virtual region and then create some virtual addresses within that virtual region. Later, when the application requires more virtual address space, it can expand within the virtual region and create more address space in a virtually contiguous manner to the previous addresses allocated within that virtual region.

Virtual regions can also be created within P0 and P1 space by specifying VA$M_P0_SPACE or VA$M_P1_SPACE in the flags argument to the SYS$CREATE_REGION_64 service.

If you do not explicitly delete a virtual region with the SYS$DELETE_REGION_64 system service, the user-defined virtual region along with all created address space is deleted when the image exits.