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

Memory Addressing and Access

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

4-12

First Four Relative Segments

Figure 4-6 shows addressing details for the first four regions, particularly the first

region. Hexadecimal starting addresses are indicated for the regions and the first five

relative segments, 0 through 4.

Relative segment 0 is the TNS user data segment, which, for TNS processes, provides

relative addressing for process global variables and the TNS data stack. Relative

segments 1, 2, and 3 are skipped. Then, the current selectable segment begins at

relative segment 4 (hexadecimal address 00080000).

The reason for skipping relative segments 1, 2, and 3 is that these segment numbers

are reserved for backward compatibility. In the TNS architecture, these are assigned

to system data segment, current code segment, and latest user code segment,

respectively. In the NonStop S-series processors, most references to system data are

through Kseg0 addresses, and thus references to relative segment 1 are not usually

needed. Similarly, current code and user code (that is, the last-used user code

segment) also are not usually needed, because all code segments are simultaneously

available in the NonStop S-series processors. Thus these three relative segment

numbers are not normally used, but they do work—for those TNS applications that

have not been converted to remove hard-coded references.

Because there are 256 unitary segments in a region, a selectable segment that

consists of more than 252 unitary segments crosses at least one region boundary.