HP 3000 III Series Manual page 48

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Sys tern/CPU Overv iew
Table 2-4. Reserved
Low
Main Memory Locations
I-------.-~----r----·----~-·~--·------·-
I
Memory Location
I
Contents
I I
Code segment Table Base
Code Segment Table Ex tens ion
Da ta Se gment Table Base
Process Control Block Base
Current Process Control Block
Interrupt Stack Base
In t err up t Sta c k Lim it
Inte r r upt Mask
Reserved
Device Reference and Ex ternal
Inte rrupt Table
System Global Table (Pointers
to resident tables, etc.)
I-Nofe:Th~symbor-precedTng
a
number "indicates
I
an octal value.
I
------_.~--_._-~_._-~---------
2-26. DATA SEGMENT TABLE. The DST contains a list of the various
data segments currently in use by the operating system and user
programs. These segments include I/O buffers, system and user
process stacks, and extra data segments. The DST length is de-
te rm ined at sys tern genera tion time and it conta ins a four-word
entry for each data segment in the format shown in figure 2-7.
The actual number of ent.r ies in use at anyone time is var iable,
limited only by the length of the table. Entries are dynamically
alloca ted by the opera ting sys tern as programs are loaded and un-
loaded or as special capability processes request or release ad-
ditional data segments.
2-27. Code Segment Linkage
During the execution of one user process, there will usually be
several code segments in memory and a single data segment. As~
sume that the current process presently has two code segments in
memory as shown in figure 2-8. The purpose of figure 2-8 is to
illustrate how the sys tern keeps track of where code segments are
and to show how references can be made from one segment to anoth-
er. Although figure 2-8 illustrates hardware, it is the respon-
sibility of the MPE operating system to control the tables shown.
The CST Pointer is permanently resident in Location 0 and it
contains an absolute address pointing to the starting location of
the CST (1), figure 2 -8. The CST te lIs where each code segment
(present or absent) is loca ted. I f the segment is a progr am
segment, Location 1 is used. Each entry in the CST has a unique
number (code segment number) that identifies the particular seg-
2-16