Specifications

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OpenVMS Operating System for Alpha and VAX (Versions 7.1, 7.1–1H1, 7.1–1H2, and 7.1–2) SPD 41.87.06

OPERATING SYSTEM ENVIRONMENT

OpenVMS VAX Processes and Scheduling

The basic unit of execution in OpenVMS is the process.

A process consists of individual address space and reg-

isters known as context, and code called an executable

image. The context identiﬁes the process and describes

its current state. Executable images consist of system

programs and user programs that have been compiled

and linked.

The maximum number of concurrent processes is 8,192

per OpenVMS VAX system.

Processes receive time to execute their images based

on the priority. Thirty-two priorities are recognized on

OpenVMS VAX, and 64 priorities are recognized on

OpenVMS Alpha. Priorities 0 to 15 are for time-sharing

processes and applications that are not time critical (four

is the typical default for timesharing processes). Prior-

ities 16 to 32 on VAX and 16 to 63 on Alpha are for

real-time processes.

Each time an event such as an I/O interrupt occurs, the

system services the event ﬁrst and then passes control

to the highest priority process ready to execute. The

system automatically adjusts the priorities of processes

in the range of 0 to 15 to favor I/O-bound and interactive

processes. However, the system does not adjust the

priority of a process in the range of 16 to 31 for VAX or

16 to 63 for Alpha.

Real-time processes can be assigned higher priorities to

ensure that they receive processor time whenever they

are ready to execute. Real-time processes are sched-

uled preemptively; that is, if a real-time process is ready

to execute, it is given to the processor immediately, un-

less a process with a higher priority is ready to execute.

OpenVMS uses paging and swapping to provide suf-

ﬁcient virtual memory for concurrently executing pro-

cesses. Also, paging and swapping are provided for

processes whose memory requirements exceed avail-

able physical memory. The maximum working set size

is 512 MB of memory for VAX and 4 GB for Alpha.

Programmers can control memory management from

within an image. An image executing in a real-time pro-

cess, for example, can inhibit paging or swapping of

critical code and data.

Peripheral devices can be managed by the system or al-

located by individual processes. At least one disk must

be a system disk. Other disks can be designated as

data disks for the general use of all users logging in to

the system or for a speciﬁc group of users. The system

controls interactive terminals and one or more printers.

OpenVMS Alpha Processes and Scheduling

The basic unit of execution in OpenVMS Alpha is the

kernel thread. A kernel thread consists of individual ad-

dress space and registers known as context, and code

called an executable image. The context identiﬁes the

kernel thread and describes its current state. Each pro-

cess can have up to 16 kernel threads. Executable im-

ages consist of system programs and user programs

that have been compiled and linked.

The maximum number of concurrent processes is

16,384 per OpenVMS Alpha system.

Kernel threads receive processor time to execute their

images based on the priority of the process. Thirty-

two priorities are recognized on OpenVMS VAX, and 64

priorities are recognized on OpenVMS Alpha. Priorities

0 to 15 are for time-sharing processes and applications

that are not time critical (four is the typical default for

timesharing processes). Priorities 16 to 32 on VAX and

16 to 63 on Alpha are for real-time processes.

Each time an event such as an I/O interrupt occurs, the

system ﬁrst services the event and then passes con-

trol to the highest priority kernel thread ready to exe-

cute. The system adjusts the priorities of kernel threads

whose base priority is in the range of 0 to 15 to favor

I/O-bound and interactive processes. However, the sys-

tem does not adjust the priority of a kernel thread in the

range of 16 to 31 for VAX or 16 to 63 for Alpha.