OpenVMS System Management Utilities Reference Manual

Default values for system parameters allow booting on any supported OpenVMS configuration. SYSGEN displays these default values under the heading default when you enter the SYSGEN command SHOW [parameter-name] for one of the parameter categories or attributes. Reset the default parameter values with the USE DEFAULT command.

The computed, installed value referred to in this section is the value derived by the AUTOGEN command procedure. (See the OpenVMS System Manager's Manual.)

J.2 Parameter Descriptions

This section describes system parameters and provides guidelines to help you decide whether you should consider modifying the parameters. The following attributes are indicated for the parameters:

AUTOGEN---A
DYNAMIC---D
FEEDBACK---F
GEN---G
MAJOR---M

Section J.2.2 describes the SPECIAL system parameters.

Note
In versions of the operating system before Version 4.0, a separate process, the ancillary control process (ACP), performed file operations such as file opens, closes, and window turns. Version 4.0 introduced the XQP (extended QIO procedure), which allows every process on the system to perform these operations. Consequently, many ACP parameters are applicable only when Files-11 On-Disk Structure Level 1 disks are mounted or when an ACP is specifically requested during a mount command. For compatibility reasons, the names of the parameters have not changed.

J.2.1 System Parameters

This section describes the parameters in all of the categories except the SPECIAL system parameters, which are described in Section J.2.2.

ACP_BASEPRIO (D)
ACP_BASEPRIO sets the base priority for all ACPs. The DCL command SET PROCESS/PRIORITY can be used to reset the base priorities of individual ACPs. ACP_BASEPRIO is not applicable for XQPs.
ACP_DATACHECK (D)
ACP_DATACHECK is a dynamic system parameter. It controls the consistency checks that are performed on internal file system metadata such as file headers.
ACP_DATACHECK is a bit-mask. The following table shows the bits that are currently defined:

Bit Description

0 Set this bit to perform consistency checks on read operations.
When this bit is set, the IO$M_DATACHECK function modifier is automatically set on all subsequent IO$_READLBLK operations that read file system metadata (see the OpenVMS I/O User's Reference Manual).

1 Set this bit to perform consistency checks on write operations.
When this bit is set, the IO$M_DATACHECK function modifier is automatically set on all subsequent IO$_WRITELBLK operations that read file system metadata (see the OpenVMS I/O User's Reference Manual).

2 Set this bit to perform read-after-write consistency checks.
This is similar to setting bit 1, except that in this case it is the file system that does the checks, not the lower level device or disk driver.
Note that read-after-write consistency checks are not allowed on deferred writes. Deferred writes are turned off if this bit is set.

3 Reserved for Digital; must be zero.

4 Reserved for Digital; must be zero.

5 and 6 These two bits control the checks that are performed on reads and writes of directory blocks. You can select one of four different levels:

To check that the block is a valid directory block (reads only), select level 0 by setting bit 6 to 0 and bit 5 to 0.
To check that the block is a valid directory block (reads and writes), select level 1 by setting bit 6 to 0 and bit 5 to 1.
To check that the block is a valid directory block, and it contains valid entries (reads and writes), select level 2 by setting bit 6 to 1 and bit 5 to 0.
To check that the block is a valid directory block, and that it contains valid entries in correct alphanumeric order (reads and writes), select level 3 by setting bit 6 to 1 and bit 5 to 1.

When you set the SYSTEM_CHECK system parameter to 1, you enable level 3 checking of directory blocks.
Write errors result in BUGCHECK and crash your system, read errors exit with error status SS$_BADDIRECTORY.

7 Reserved for Digital; must be zero.

ACP_DINDXCACHE (A,D,F)
ACP_DINDXCACHE controls the size of the directory index cache and the number of buffers used on a cachewide basis. In addition, ACP_DINDXCACHE builds a temporary index into the directory file, thereby reducing search time and directory header lookup operations. This parameter replaces the ACP_SYSACC parameter for Files-11 ODS-2 disks.
ACP_DIRCACHE (A,D,F)
ACP_DIRCACHE sets the number of pages for caching directory blocks. Too small a value causes excessive XQP I/O operations, while too large a value causes excessive physical memory to be consumed by the directory data block cache.
ACP_EXTCACHE (D,F)
ACP_EXTCACHE sets the number of entries in the extent cache. Each entry points to one contiguous area of free space on disk. A specification of 0 means no cache. Too small a value causes excessive XQP I/O operations, while too large a value causes excessive physical memory to be consumed by the extent cache.
ACP_EXTLIMIT (D)
ACP_EXTLIMIT specifies the maximum amount of free space to which the extent cache can point, expressed in thousandths of the currently available free blocks on the disk. For example, if available free space on the disk is 20,000 blocks, a specification of 10 limits the extent cache to 200 blocks.
The computed, installed value is usually adequate. Users with four or more VAXcluster node systems may want to adjust this parameter.
ACP_FIDCACHE (D,F)
ACP_FIDCACHE sets the number of file identification slots cached. A specification of 1 means no cache. Too small a value causes excessive XQP I/O operations, while too large a value causes excessive physical memory to be consumed by the FID caches.
ACP_HDRCACHE (A,D,F)
ACP_HDRCACHE sets the number of pages for caching file header blocks. Too small a value causes excessive XQP I/O operations, while too large a value causes excessive physical memory to be consumed by the file header caches.
ACP_MAPCACHE (A,D,F)
ACP_MAPCACHE sets the number of pages for caching index file bitmap blocks. Too small a value causes excessive XQP I/O operations, while too large a value causes excessive physical memory to be consumed by the bitmap cache.
ACP_MAXREAD (D)
ACP_MAXREAD sets the maximum number of directory blocks read in one I/O operation.
ACP_MULTIPLE (A,D)
ACP_MULTIPLE enables (1) or disables (0) the default creation of a separate disk XQP cache for each volume mounted on a different device type. Prior to Version 4.0, a separate ACP process was created for each device type if this parameter was enabled. Because ACP operations are now handled by the per process XQP, such separate processes are no longer created. In general, having multiple caches is unnecessary. One large cache is more efficient than several small ones. ACP_MULTIPLE can be overridden on an individual-volume basis with the DCL command MOUNT.
ACP_QUOCACHE (A,D,F)
ACP_QUOCACHE sets the number of quota file entries cached. A specification of 0 means no cache. Too small a value causes excessive XQP I/O operations, while too large a value causes excessive physical memory to be consumed by the quota caches.
ACP_REBLDSYSD
ACP_REBLDSYSD specifies whether the system disk should be rebuilt if it was improperly dismounted with extent caching, file number caching, or disk quota caching enabled. The ACP_REBLDSYSD default value (1) ensures that the system disk is rebuilt. Setting the value to 0 means the disk is not rebuilt.
Depending on the amount of caching enabled on the volume before it was dismounted, the rebuild operation may consume a considerable amount of time. Setting the value of ACP_REBLDSYSD to 0 specifies that the disk should be returned to active service immediately. If you set ACP_REBLDSYSD to 0, you can enter the DCL command SET VOLUME/REBUILD at any time to rebuild the disk.
ACP_SHARE (D)
ACP_SHARE enables (0) or disables (1) the creation of a global section for the first ACP used, enabling succeeding ACPs to share its code. This parameter should be set to 0 when ACP_MULTIPLE is on.
ACP_SWAPFLGS (A,D)
ACP_SWAPFLGS enables or disables swap through the value of a 4-bit number for the following four classes of ACPs:

Bit Class of ACP

0 Disks mounted by MOUNT/SYSTEM

1 Disks mounted by MOUNT/GROUP

2 Private disks

3 Magnetic tape ACP

If the value of the bit is 1, the corresponding class of ACPs can be swapped. The value of decimal 15 (hexadecimal F---all bits on) enables swap for all classes of ACP. A value of decimal 14 disables swap for ACPs for volumes mounted with the /SYSTEM qualifier but leaves swap enabled for all other ACPs. Note that one will only have disk ACPs present if they are specifically requested at mount time or if a Files-11 On-Disk Structure Level 1 disk is mounted. In general, only bit 3 is significant because usually no file ACPs exist.
ACP_SYSACC (A,D)
ACP_SYSACC sets the number of directory file control blocks (FCBs) that will be cached for disks mounted with the /SYSTEM qualifier. Each directory FCB contains a 16-byte array containing the first letter of the last entry in each block of the directory (or group of blocks if the directory exceeds 16 blocks). Since entries in a directory are alphabetical, the cached FCB provides quick access to a required directory block. This parameter value should be roughly equivalent to the number of directories that will be in use concurrently on each system volume. It may be overridden on a per-volume basis with the /ACCESSED qualifier to the DCL command MOUNT. The value should be kept low in systems with small physical memory and little file activity, because the FCBs require a significant amount of space in the nonpaged dynamic pool.
Too small a value causes excessive XQP I/O operations, while too large a value causes excessive physical memory to be consumed by the FCB caches.
ACP_WINDOW (D)
ACP_WINDOW sets the default number of window pointers to be allocated in a window for a default file access, for disks mounted with the /SYSTEM qualifier.
ACP_WORKSET (D)
ACP_WORKSET sets the default size of a working set for an ACP. A value of 0 permits the ACP to calculate the size. This value should be nonzero only on small systems where memory is tight. Too small a value causes excessive ACP page, while too large a value causes excessive physical memory to be consumed by the ACP. Note that this parameter has no effect on the per-process XQP.
ACP_WRITEBACK (D)
ACP_WRITEBACK enables writeback caching. The default value of ACP_WRITEBACK is 1, which enables writeback caching. To disable writeback caching, set ACP_WRITEBACK to 0.
On ODS--2 disks, only PATHWORKS servers can use writeback caching. All other applications use writethrough caching.
ACP_XQP_RES
ACP_XQP_RES controls whether the XQP is currently in memory. The default value (1) specifies that the XQP is permanently in memory. Change the default only on restricted memory systems with a small number of users and little or no file activity that would require XQP intervention. Such activity includes file opens, closes, directory lookups, and window turns.
ALLOCLASS
ALLOCLASS determines the device allocation class for the system. The device allocation class is used to derive a common lock resource name for multiple access paths to the same device.
AWSMIN (D)
On VAX systems, AWSMIN establishes the lowest number of pages to which a working set limit can be decreased by automatic working set adjustment.
On Alpha systems, AWSMIN establishes the lowest number of pagelets to which a working set limit can be decreased by automatic working set adjustment.
AWSTIME (D)
AWSTIME specifies the minimum amount of processor time that must elapse for the system to collect a significant sample of a working set's page fault rate. The time is expressed in units of 10 milliseconds. The default value of 20, for example, is 200 milliseconds.
Some application configurations that have a large number of memory-intensive processes may benefit if the value is reduced. The value can be as low as 4.
AWSTIME expiration is checked only at quantum end. Reducing its value and not reducing QUANTUM effectively sets the value of AWSTIME equal to the value of QUANTUM.
BALSETCNT (A,G,M)
BALSETCNT sets the number of balance set slots in the system page table. Each memory-resident working set requires one balance set slot.
You can monitor the active system with the DCL command SHOW MEMORY or the MONITOR PROCESSES command of the Monitor utility to determine the actual maximum number of working sets in memory. If this number is significantly lower than the value of BALSETCNT, this parameter value may be lowered. If all balance set slots are being used, raise the value of BALSETCNT.
BALSETCNT should never be set to a value higher than 2 less than MAXPROCESSCNT. If physical memory is a significant system constraint, you should consider lowering this value even further. However, if your system runs with a number of processes nearly equal to MAXPROCESSCNT, lowering BALSETCNT will force swapping to occur, which can affect system performance. Note that virtual balance slots (VBS) can affect the values of BALSETCNT and MAXPROCESSCNT.
BORROWLIM (A,D,M)
BORROWLIM defines the minimum number of pages required on the free-page list before the system will permit process growth beyond the working set quota (WSQUOTA) for the process. This parameter should always be greater than FREELIM.
This parameter allows a process to grow beyond the value set by the working set quota (WSQUOTA) to the working set quota extent (WSEXTENT) on a system that has a substantial memory on the free-page list. This automatic working set adjustment also depends upon the values of parameters WSINC, PFRATH, and AWSTIME.
Working set growth attempts to alleviate heavy page faulting. To make use of this growth, you must also set the user's WSEXTENT authorization quota to a larger number than the WSQUOTA value.
BUGCHECKFATAL (D)
BUGCHECKFATAL enables or disables the conversion of nonfatal bugchecks into fatal bugchecks. The system must be rebooted on a fatal bugcheck. A nonfatal bugcheck places an entry only in the error log and deletes the corresponding process.
This parameter should normally be off (0); you should set it on (1) only when the executive is being debugged.
BUGREBOOT (D)
BUGREBOOT enables or disables automatic rebooting of the system if a fatal bugcheck occurs. This parameter should normally be on (1); set it off (0) only when the executive is being debugged.
CHECK_CLUSTER
On VAX systems, CHECK_CLUSTER is the VAXCLUSTER parameter sanity check. When CHECK_CLUSTER is set to 1, SYSBOOT will output a warning message and force a conversational boot if it detects the VAXCLUSTER parameter is set to 0.
CLASS_PROT (D)
CLASS_PROT performs the nondiscretionary classification checks. CLASS_PROT is also checked by XQP to determine if a classification block should be added to the header of any created files.
CLISYMTBL (D)
CLISYMTBL sets the size of the command interpreter symbol table, which controls the number of DCL or MCR symbols that can be created.
CLUSTER_CREDITS
CLUSTER_CREDITS specifies the number of per-connection buffers a node allocates to receiving VMS$VAXcluster communications.
If the SHOW CLUSTER command displays a high number of credit waits for the VMS$VAXcluster connection, you might consider increasing the value of CLUSTER_CREDITS on the other node. However, in large cluster configurations, setting this value unnecessarily high will consume a large quantity of nonpaged pool. Each receive buffer is at least SCSMAXMSG bytes in size but might be substantially larger depending on the underlying transport.
It is not required for all nodes in the cluster to have the same value for CLUSTER_CREDITS. For small or memory-constrained systems, the default value of CLUSTER_CREDITS should be adequate.
CRDENABLE
CRDENABLE enables or disables detection and logging of memory-corrected read data (ECC) errors. This parameter should normally be on (1).
CWCREPRC_ENABLE
CWCREPRC_ENABLE controls whether an unprivileged user can create a process on another OpenVMS Cluster node. The default value of 1 allows an unprivileged user to create a detached process with the same UIC on another node. A value of 0 requires that a user have DETACH or CMKRNL privilege to create a process on another node.
DBGTK_SCRATCH
On Alpha systems, DBGTK_SCRATCH specifies how many pages of memory are allocated for the remote debugger. This memory is allocated only if remote debugging is enabled with the 8000 boot flag. Normally, the default value is adequate, but if the remote debugger issues an error message, you should increase this value. See the Writing OpenVMS Alpha Device Drivers in Cfor more information. (This manual has been archived but is available in PostScript and DECW$BOOK (Bookreader) formats on the OpenVMS Documentation CD-ROM. A printed book can be ordered through DECdirect (800-354-4825).)
DEADLOCK_WAIT (D)
DEADLOCK_WAIT defines the number of seconds that a lock request must wait before the system initiates a deadlock search on behalf of that lock. Setting DEADLOCK_WAIT to 0 disables deadlock checking. Setting DEADLOCK_WAIT to a value greater than 0 but still less than the default setting provides faster detection of deadlocks but requires more CPU usage.
DEFMBXBUFQUO (D)
DEFMBXBUFQUO sets the default for the mailbox buffer quota size in bytes when this value is not specified in a Create Mailbox ($CREMBX) system service call.
DEFMBXMXMSG (D)
DEFMBXMXMSG sets the default for the mailbox maximum message size in bytes when this value is not specified in a Create Mailbox ($CREMBX) system service call.
DEFPRI (D)
DEFPRI sets the base default priority for processes.
DEFQUEPRI (D)
DEFQUEPRI establishes the scheduling priority for jobs entered in batch and output (printer, server, and terminal) queues when no explicit scheduling priority is specified by the submitter. The value of this parameter can range from 0 to 255; the default value is 100.
The value of DEFQUEPRI should be less than or equal to MAXQUEPRI.

Note
DEFQUEPRI refers to relative queue scheduling priority, not the execution priority of the job.

DEVICE_NAMING
On Alpha systems, DEVICE_NAMING is a bitmask indicating whether port allocation classes are used in forming SCSI device names.
Following is the bit definition:

Bit Definition

0 If 1, enable new naming.

For more information about port allocation classes, see OpenVMS Cluster Systems.
DISK_QUORUM (D)
The DISK_QUORUM parameter is the name of an optional quorum disk in ASCII. ASCII spaces indicate that no quorum disk is being used.
DORMANTWAIT (D)
DORMANTWAIT specifies, in seconds, the amount of time that can elapse without a significant event before the system treats a low-priority computable process as a DORMANT process for scheduling purposes. (A low-priority process is a non real-time process whose current priority is equal to or less than the value specified by the system parameter DEFPRI [default=4].) After SUSP (suspended) processes, DORMANT processes are the most likely candidates for memory reclamation by the swapper.
Increasing the value of DORMANTWAIT can increase the interval that a low priority process blocks a high priority process if that low priority process is holding a lock or resource that the higher priority process is waiting for.
DR_UNIT_BASE (G)
DR_UNIT_BASE specifies the base value from which unit numbers for DR devices (Digital StorageWorks RAID Array 200 Family logical RAID drives) are counted.
DR_UNIT_BASE provides a way for unique RAID device numbers to be generated. DR devices are numbered starting with the value of DR_UNIT_BASE and then counting from there. For example, setting DR_UNIT_BASE to 10 will produce device names such as $1$DRA10, $1$DRA11, and so on.
Setting DR_UNIT_BASE to appropriate, nonoverlapping values on all cluster members that share the same (nonzero) allocation class will ensure that no two RAID devices are given the same name.
DUMPBUG
DUMPBUG enables (1) or disables (0) the writing of error log buffers and memory contents to SYS$SYSTEM:SYSDUMP.DMP when a fatal bugcheck occurs. This parameter should be off (0) only when the executive is being debugged.
DUMPSTYLE (A,D)
DUMPSTYLE specifies the method of writing system dumps.
DUMPSTYLE is a 32-bit mask, with the following bits defined. Each bit can be set independently. The value of the system parameter is the sum of the values of the bits that have been set. Remaining or undefined values are reserved to Digital.

Bit Mask Description

0 00000001 0 = Full dump (SYSGEN default). The entire contents of physical memory will be written to the dump file.

1 = Selective dump. The contents of memory will be written to the dump file selectively to maximize the usefulness of the dump file while conserving disk space.

1 00000002 0 = Minimal console output.

1 = Full console output (includes stack dump, register contents, and so on).

2 00000004 0 = Dump to system disk.

1 = Dump off system disk (DOSD) to an alternate disk. (Refer to the OpenVMS System Manager's Manual for details.)

3 (Alpha only) 00000008 0 = Do not compress.

1 = Compress. (See note below.)

4 - 14 Reserved to Digital.

15 (VAX 7000s only) 00008000 0 = Disable use of bits 16 - 27.

1 = Enable use of bits 16 - 27.

16 - 27 (VAX only)² 0FFF0000 Range of DOSD unit numbers.

28 - 31 Reserved to Digital.

Note
VAX systems do not support dump compression.

If you plan to enable the Volume Shadowing minimerge feature on an Alpha system disk, be sure to specify DOSD to an alternate disk.

Note
On Alpha systems, you can save space on the system disk and, in the event of a crash, save time recording the system memory, by using the OpenVMS Alpha dump compression feature. Unless you override the default AUTOGEN calculations (by setting DUMPSTYLE in MODPARAMS.DAT), AUTOGEN uses the following algorithm:

On a system with less than 128 MB of memory, the system sets the DUMPSTYLE to 1 (a raw selective dump) and sizes the dump file appropriately.
On a system with 128 MB of memory or greater, the system sets the DUMPSTYLE to 9 (a compressed selective dump), and creates the dump file at two-thirds the value of the corresponding raw dump.

Examples:

The mask of 00000006 directs the system to send a full dump, with full console output, off the system disk (to the alternate disk).
For a VAX 7000, a mask of 00098006 directs the system to send a full dump with full console output to the DOSD whose unit number is 9.
On Alpha systems, the mask of 00000009 directs the system to compress a selective dump with minimal console output.
ERLBUFFERPAGES (A on Alpha)
ERLBUFFERPAGES specifies the amount of memory to allocate for each buffer requested by the ERRORLOGBUFFERS parameter.
On VAX systems, ERLBUFFERPAGES has a default value of 2 pages and a maximum value of 32 pages.
On Alpha systems, ERLBUFFERPAGES has a default value of 4 pagelets and a maximum value of 32 pagelets. ERLBUFFERPAGES is an AUTOGEN-altered parameter.
ERRORLOGBUFFERS
ERRORLOGBUFFERS specifies the number of buffers reserved for system error log entries. Each buffer is ERLBUFFERPAGES in length. If ERRORLOGBUFFERS is too low, messages might not be written to the error log file. If it is too high, unneeded physical pages can be consumed by the error log buffers.
If you increase ERRORLOGBUFFERS, you must also increase the size of the system dump file.
EXPECTED_VOTES (A)
EXPECTED_VOTES specifies the maximum number of votes that may be present in a cluster at any given time. Set it to a value that is equal to the sum of the vote parameters of all VAXcluster members, plus any votes that are contributed by the quorum disk. This value is used to automatically derive the number of votes that must be present for the cluster to function (quorum).
EXTRACPU (D)
EXTRACPU sets the time, in units of 10 milliseconds, allotted to each of a process's exit handlers (for each access mode) after the process times out (that is, reaches its CPU time limit).
FREEGOAL (A,D,M)
FREEGOAL establishes the number of pages that you want to reestablish on the free-page list following a system memory shortage. Memory shortages occur when the system drops below the minimum number of pages required on the free-page list (FREELIM). The value of FREEGOAL must always be greater than or equal to the value of FREELIM.
FREELIM (A,M)
FREELIM sets the minimum number of pages that must be on the free-page list.
The system will write pages from the modified-page list, swap out working sets, or reduce the size of the working sets to maintain the minimum count.
While the larger free-page list generally means less page I/O, it also means less space for the balance set, which tends to result in more swap I/O. You can monitor the size of the free-page list, the amount of page, and the amount of swap with the MONITOR IO command of the Monitor utility.
GBLPAGES (A,D,F,G,M)
GBLPAGES sets the number of global page table entries allocated at bootstrap time. Each global section requires 1 global page table entry per section page, plus 2 entries, with the total rounded up to an even number.
Users with CMKRNL privilege can change this parameter on a running system. Increasing the value of this parameter will allow the global page table to expand, on demand, up to the maximum size.
The default value is sufficient for the images normally installed as shared in the system startup command procedures. Once the system is running and all global sections are created, you can examine the actual requirements with the /GLOBAL qualifier of the Install utility (INSTALL) and reduce the value of GBLPAGES accordingly. However, do not set the value of this parameter too low, because the page table entries use little permanently resident memory. If you plan to install many user images as shared, or if user programs are likely to create many global sections, you must increase the value of this parameter.
GBLPAGFIL (A,D)
GBLPAGFIL defines the maximum number of systemwide pages allowed for global page-file sections (scratch global sections that can be used without being mapped to a file). These global page-file sections can be temporary, permanent, system, or group, and are allocated from the page file specified in the system process header at bootstrap time. When you allow pages for global page-file sections, you must increase the size of the page file accordingly. Users with CMKRNL privilege can change this parameter value on a running system.

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Bit	Description
0	Set this bit to perform consistency checks on read operations. When this bit is set, the IO$M_DATACHECK function modifier is automatically set on all subsequent IO$_READLBLK operations that read file system metadata (see the OpenVMS I/O User's Reference Manual).
1	Set this bit to perform consistency checks on write operations. When this bit is set, the IO$M_DATACHECK function modifier is automatically set on all subsequent IO$_WRITELBLK operations that read file system metadata (see the OpenVMS I/O User's Reference Manual).
2	Set this bit to perform read-after-write consistency checks. This is similar to setting bit 1, except that in this case it is the file system that does the checks, not the lower level device or disk driver. Note that read-after-write consistency checks are not allowed on deferred writes. Deferred writes are turned off if this bit is set.
3	Reserved for Digital; must be zero.
4	Reserved for Digital; must be zero.
5 and 6	These two bits control the checks that are performed on reads and writes of directory blocks. You can select one of four different levels: To check that the block is a valid directory block (reads only), select level 0 by setting bit 6 to 0 and bit 5 to 0. To check that the block is a valid directory block (reads and writes), select level 1 by setting bit 6 to 0 and bit 5 to 1. To check that the block is a valid directory block, and it contains valid entries (reads and writes), select level 2 by setting bit 6 to 1 and bit 5 to 0. To check that the block is a valid directory block, and that it contains valid entries in correct alphanumeric order (reads and writes), select level 3 by setting bit 6 to 1 and bit 5 to 1. When you set the SYSTEM_CHECK system parameter to 1, you enable level 3 checking of directory blocks. Write errors result in BUGCHECK and crash your system, read errors exit with error status SS$_BADDIRECTORY.
7	Reserved for Digital; must be zero.

Bit	Class of ACP
0	Disks mounted by MOUNT/SYSTEM
1	Disks mounted by MOUNT/GROUP
2	Private disks
3	Magnetic tape ACP

Bit	Mask	Description
0	00000001	0 =	Full dump (SYSGEN default). The entire contents of physical memory will be written to the dump file.
		1 =	Selective dump. The contents of memory will be written to the dump file selectively to maximize the usefulness of the dump file while conserving disk space.
1	00000002	0 =	Minimal console output.
		1 =	Full console output (includes stack dump, register contents, and so on).
2	00000004	0 =	Dump to system disk.
		1 =	Dump off system disk (DOSD) to an alternate disk. (Refer to the OpenVMS System Manager's Manual for details.)
3 (Alpha only)	00000008	0 =	Do not compress.
		1 =	Compress. (See note below.)

4 - 14			Reserved to Digital.

15 (VAX 7000s only)	00008000	0 =	Disable use of bits 16 - 27.
		1 =	Enable use of bits 16 - 27.
16 - 27 (VAX only)²	0FFF0000		Range of DOSD unit numbers.
28 - 31			Reserved to Digital.