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HP OpenVMS Systems Documentation |
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OpenVMS Programming Concepts Manual
Chapter 12
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SYS$EXPREG (pagcnt ,[retadr] ,[acmode] ,[region]) |
Specifying the Number of Pages
On Alpha systems, use the pagcnt argument to specify the number of pagelets to add to the end of the region. The Alpha system rounds the specified pagelet value to the next integral number of Alpha pages for the system where it is executing. To check the exact boundaries of the memory allocated by the system, specify the optional retadr argument. The retadr argument contains the start address and the end address of the memory allocated by the system service.
Use the acmode argument to specify the access to be assigned to the newly created pages.
Use the region argument to specify whether to add the pages to the end of the P0 or P1 region.
To deallocate pages allocated with SYS$EXPREG and SYS$CRETVA, use SYS$DELTVA.
For Alpha systems, the following example illustrates the addition of 4 pagelets to the program region of a process by writing a call to the SYS$EXPREG system service.
#include <stdio.h>
#include <ssdef.h>
main() {
unsigned int status, retadr[2],pagcnt=4, region=0;
/* Add 4 pages to P0 space */
status = SYS$EXPREG( pagcnt, &retadr, 0, region);
if (( status & 1) != 1)
LIB$SIGNAL( status );
else
printf("Starting address: %d Ending address: %d\n",
retadr[0],retadr[1];
}
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The value 0 is passed in the region argument to specify that the pages are to be added to the program region. To add the same number of pages to the control region, you would specify REGION=1.
Note that the region argument to the SYS$EXPREG service is optional; if it is not specified, the pages are added to or deleted from the program region by default.
On Alpha systems, the SYS$EXPREG service can add pagelets only in the direction of the growth of a particular region. When you need to add pages to the middle of these regions, you can use the Create Virtual Address Space (SYS$CRETVA) system service. Likewise, when you need to delete pages created by either SYS$EXPREG or SYS$CRETVA, you can use the Delete Virtual Address Space (SYS$DELTVA) system service. For example, if you have used the SYS$EXPREG service twice to add pages to the program region and want to delete the first range of pages but not the second, you could use the SYS$DELTVA system service, as shown in the following example:
#include <stdio.h>
#include <ssdef.h>
struct {
unsigned int lower, upper;
}retadr1, retadr2, retadr3;
main() {
unsigned int status, pagcnt=4, region=0;
/* Add 4 pages to P0 space */
status = SYS$EXPREG( pagcnt, &retadr1, 0, region);
if (( status & 1) != 1)
LIB$SIGNAL( status );
else
printf("Starting address: %d ending address: %d\n",
retadr1.lower,retadr1.upper);
/* Add 3 more pages to P0 space */
pagcnt = 3;
status = SYS$EXPREG( pagcnt, &retadr2, 0, region);
if (( status & 1) != 1)
LIB$SIGNAL( status );
else
printf("Starting address: %d ending address: %d\n",
retadr2.lower,retadr2.upper);
/* Delete original allocation */
status = SYS$DELTVA( &retadr1, &retadr3, 0 );
if (( status & 1) != 1)
LIB$SIGNAL( status );
else
printf("Starting address: %d ending address: %d\n",
retadr1.lower,retadr1.upper);
}
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In this example, the first call to SYS$EXPREG rounds up the requested pagelet count to an integral number of CPU-specific pages and adds that number of pages to the program region; the virtual addresses of the created pages are returned in the 2-longword array at retadr1. The second request converts the pagelet count to pages, adds them to the program region, and returns the addresses at retadr2. The call to SYS$DELTVA deletes the area created by the first SYS$EXPREG call.
Be aware that using SYS$CRETVA presents some risk because it can delete pages that already exist if those pages are not owned by a more privileged access mode. Further, if those pages are deleted, notification is not sent. Therefore, unless you have complete control over an entire system, use SYS$EXPREG or the RTL routines to allocate address space. |
Section 12.3.3 mentions some other possible risks in using SYS$CRETVA
for allocating memory.
12.3.2 Input Address Arrays and Return Address Arrays
When the SYS$EXPREG system service adds pages to a region, it adds them in the normal direction of growth for the region. The return address array, if requested, indicates the order in which the pages were added. For example:
The addresses returned indicate the first byte in the first page that was added or deleted and the last byte in the last page that was added or deleted, respectively.
When input address arrays are specified for the Create and Delete Virtual Address Space (SYS$CRETVA and SYS$DELTVA, respectively) system services, these services add or delete pages beginning with the address specified in the first longword and ending with the address specified in the second longword.
On Alpha systems, the order in which the pages are added or deleted does not have to be in the normal direction of growth for the region. Moreover, because these services add or delete only whole pages, they ignore the low-order bits of the specified virtual address (the low-order bits contain the byte offset within the page). Table 12-1 shows the page size and byte offset.
| Page Size (Bytes) |
Byte Within Page (Bits) |
|---|---|
| 8K | 13 |
| 16K | 14 |
| 32K | 15 |
| 64K | 16 |
Table 12-2 shows some sample virtual addresses in hexadecimal that may be specified as input to SYS$CRETVA or SYS$DELTVA and shows the return address arrays if all pages are successfully added or deleted. Table 12-2 assumes a page size of 8 KB = 2000 hex.
| Input Array | Output Array | ||||
|---|---|---|---|---|---|
| Start | End | Region | Start | End | Number of Pages |
| 1010 | 1670 | P0 | 0 | 1FFF | 1 |
| 2450 | 2451 | P0 | 2000 | 3FFF | 1 |
| 4200 | A500 | P0 | 4000 | BFFF | 5 |
| 9450 | 9450 | P0 | 8000 | 9FFF | 1 |
| 7FFEC010 | 7FFEC010 | P1 | 7FFEDFFF | 7FFEC000 | 1 |
| 7FFEC010 | 7FFEBCA0 | P1 | 7FFEDFFF | 7FFEA000 | 2 |
For SYS$CRETVA and SYS$DELTVA, note that if the input virtual addresses are the same, as in the fourth and fifth items in Table 12-2, a single page is added or deleted. The return address array indicates that the page was added or deleted in the normal direction of growth for the region.
Note that for SYS$CRMPSC and SYS$MGBLSC, which are discussed in Section 12.3.7, the sample virtual address arrays in Table 12-2 do not apply. The reason is that the lower address value has to be an even multiple of the machine page size; that is, it must be rounded down to an even multiple page size. In addition, the higher address value must be one less than the even multiple page size, representing the last byte on the last page. That is, it must be rounded up to an even multiple page size, minus 1.
The procedure for determining start and end virtual addresses is as follows:
On Alpha systems, if you reallocate memory that is already in its virtual address space by using the SYS$CRETVA system service, you may need to modify the values of the following arguments to SYS$CRETVA:
To determine whether you must modify the address as specified in
inadr, specify the optional retadr
argument to determine the exact boundaries of the memory allocated by
the call to SYS$CRETVA.
12.3.4 Page Ownership and Protection
Each page in the virtual address space of a process is owned by the access mode that created the page. For example, pages in the program region that initially provided for the execution of an image are owned by user mode. Pages that the image creates dynamically are also owned by user mode. Pages in the control region, except for the pages containing the user stack, are normally owned by more privileged access modes.
Only the owner access mode or a more privileged access mode can delete the page or otherwise affect it. The owner of a page can also indicate, by means of a protection code, the type of access that each access mode will be allowed.
The Set Protection on Pages (SYS$SETPRT) system service changes the protection assigned to a page or group of pages. The protection is expressed as a code that indicates the specific type of access (none, read-only, read/write) for each of the four access modes (kernel, executive, supervisor, user). Only the owner access mode or a more privileged access mode can change the protection for a page.
When an image attempts to access a page that is protected against the access attempted, a hardware exception called an access violation occurs. When an image calls a memory management system service, the service probes the pages to be used to determine whether an access violation would occur if the image attempts to read or write one of the pages. If an access violation occurs, the service exits with the status code SS$_ACCVIO.
Because the memory management services add, delete, or modify a single page at a time, one or more pages can be successfully changed before an access violation is detected. If the retadr argument is specified in the service call, the service returns the addresses of pages changed (added, deleted, or modified) before the error. If no pages are affected, that is, if an access violation occurs on the first page specified, the service returns a -1 in both longwords of the return address array.
If the retadr argument is not specified, no
information is returned.
12.3.5 Working Set Paging (Alpha Only)
On Alpha systems, when a process is executing an image, a subset of its pages resides in physical memory; these pages are called the working set of the process. The working set includes pages in both the program region and the control region. The initial size of a process's working set is defined by the process's working set default (WSDEFAULT) quota, which is specified in pagelets. When ample physical memory is available, a process's working-set upper growth limit can be expanded to its working set extent (WSEXTENT).
When the image refers to a page that is not in memory, a page fault occurs, and the page is brought into memory, possibly replacing an existing page in the working set. If the page that is going to be replaced is modified during the execution of the image, that page is written into a paging file on disk. When this page is needed again, it is brought back into memory, again replacing a current page from the working set. This exchange of pages between physical memory and secondary storage is called paging.
The paging of a process's working set is transparent to the process. However, if a program is very large or if pages in the program image that are used often are being paged in and out frequently, the overhead required for paging may decrease the program's efficiency. The SYS$ADJWSL, SYS$PURGWS, and SYS$LKWSET system services allow a process, within limits, to counteract these potential problems.
The Adjust Working Set Limit (SYS$ADJWSL) system service increases or decreases the maximum number of pages that a process can have in its working set. The format for this routine is as follows:
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SYS$ADJWSL ([pagcnt],[wsetlm]) |
On Alpha systems, use the pagcnt argument to specify the number of pagelets to add or subtract from the current working set size. The Alpha system rounds the specified number of pagelets to a multiple of the system's page size. The new working set size is returned in wsetlm in units of pagelets.
The Purge Working Set (SYS$PURGWS) system service removes one or more pages from the working set.
The Lock Pages in Working Set (SYS$LKWSET) system service makes one or more pages in the working set ineligible for paging by locking them in the working set. Once locked into the working set, those pages remain in the working set until they are unlocked explicitly with the Unlock Pages in Working Set (SYS$ULWSET) system service, or program execution ends. The format is as follows:
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SYS$LKWSET (inadr ,[retadr] ,[acmode]) |
Specifying a Range of Addresses
On Alpha systems, use the inadr argument to specify the range of addresses to be locked. SYS$LKWSET rounds the addresses to CPU-specific page boundaries, if necessary. The range of addresses of the pages actually locked are returned in the retadr argument.
However, because the Alpha system's instructions cannot contain full virtual addresses, the Alpha system's images must reference procedures and data indirectly through a pointer to a procedure descriptor. The procedure descriptor contains information about the procedure, including the actual code address. These pointers to procedure descriptors and data are collected into a program section called a linkage section. Therefore, it is not sufficient simply to lock a section of code into memory to improve performance. You must also lock the associated linkage section into the working set.
To lock the linkage section into memory, you must determine the start and end addresses that encompass the linkage section and pass these addresses as values in the inadr argument to a call to SYS$LKWSET. For more information about linking, see Migrating to an OpenVMS AXP System: Recompiling and Relinking Applications. Note that this manual has been archived but is available on the OpenVMS Documentation CD-ROM.
Use the acmode argument to specify the access mode to be associated with the pages you want locked.
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