NonStop S-Series Server Description Manual (G06.27+)

Memory Addressing and Access

HP NonStop S-Series Server Description Manual—520331-004

4-4

Organization of the Process Address Space

As Figure 4-2 indicates, all of the nonprivileged space and half of the privileged space

are subdivided for addressing purposes into three substructures. (The first half of the

privileged space, shaded dark, does not use this same organization, as explained

further in the next two topics.)

For those areas of the process address space that use the organization shown in

Figure 4-2, the major organizational division is the region. There are 64 regions in the

nonprivileged space and 32 regions in the higher-address half of the privileged space.

The entity called a region exists primarily to reduce the amount of memory consumed

by the addressing tables. That is, segment tables need not exist for unallocated

regions.

Although there are 64 regions are available in the nonprivileged space, processes

typically use less than half of them.

Although regions can be numbered decimally (such as 0 through 63, shown in the

figure for the nonprivileged space), the more useful reference is the hexadecimal

starting address. Thus, for example, “the 5E region” is the one that starts at

hexadecmial address 5E000000. Following this convention, “the 50 region” is spoken

of as the “five-zero region,” rather than “region fifty,” to avoid the implication of decimal

numbering.

A region is 32 megabytes, or 256 unitary segments. Thus in the entire nonprivileged

space there are 16,384 unitary segments (64 times 256). The adjective “unitary” is

used to distinguish this special use of the term “segment” from the primary use,

specifically “logical segment,” an application programming entity. The logical (or

“extended”) segment consists of one or more unitary segments. The unitary segment

is mostly a memory-management entity. The term is sometimes abbreviated to

“unitseg.”

A unitary segment is 128 kilobytes, or eight pages.

A page is 16,384 bytes of information. When stored in physical memory, a page

occupies one 16,384-byte physical frame of storage space.