NonStop S-Series Server Description Manual (G06.24+)

Memory Addressing and Access

HP NonStop S-Series Server Description Manual—520331-003

4-50

Context-Bound Addresses

Because only one addressing space, Kseg2, is available for providing aliases for all the

existing nonprivileged spaces (one or more per process), absolute addressing as an

alias has a finite limitation. Therefore, an alternate means of aliasing is provided,

called context-bound addressing. This is a software facility. It permits logical

segments to be created without absolute-address aliases. Such logical segments are

known as unaliased segments and are specified with an option parameter to the

SEGMENT_ALLOCATE_ procedure. Context-bound addresses (CBAs) can represent

any location in all the process address spaces. CBAs can be used only in privileged

state.

As an example of use, an operating system process might be the current process

address space; the process address space that provides a certain service for this

process is in a foreign process address space, as illustrated in Figure 4-24. The

operating system process uses context-bound addresses to access the foreign

process address space, which performs the requested service.

A CBA consists of two separate components: a CONTEXT specifier and a context-

relative address (CRADDR). CONTEXT is hidden from the program, but it is

implemented as a low-level table address. CRADDR is a relative address, the offset

from the start of the nonprivileged space.

For all processors, CONTEXT is the starting address of an array of pointers to Vseg

tables for the foreign address space. For flat segments, this array is the region table in

the PDST. For selectable segments, this array is an appendix to the operating system

structure for that logical segment. For some situations, a degenerate CBA is used, with

a zero CONTEXT and an absolute (Kseg2) address in CRADDR.