HP 3000 III Series Manual page 58

System/CPU Overview
and caused new additions to the stack as indicated. Next, Pro-
cedure B called Procedure C and caused the final stack picture as
shown.
As the program progresses, Procedure C will end and, after saving
its answer in a convenient place for Procedure B to access, issue
an EXIT instruction. Then, all the other stack additions due to
Procedure C will be eliminated
by moving Sand Q back and Pro-
cedure B will continue its computations on its own data stack.
In the same manner, Procedure B will end, save its data, and
exit. This eliminates the data stack for Procedure B. Finally,
Procedure A will exit and return the net answer to the new TOS on
th e ma in te mp
0
r a r y s t or a ge are a .
Each time control is returned from the called procedure to the
caller's procedure (w ithin the code segment), the stack registers
also return to the caller's data area. Thus, the stack marker
chain virtually eliminates system overhead in keeping track of
nested procedures. For example, the simple return sequence pre-
viously discussed (C-to-B.-to-A-to rna in p rogr am) is not impera-
tive. Procedure C could have been called again before the return
to the main program was complete or other procedures could have
been called. Regardless, the return for both code and data will
always remain perfectly in step; from the called to the caller.
Note that the area between DB and First Q in figure 2-11 is the
global data area. The locations in this area are reserved by the
process for variables which the process has declared to be global
for all procedures called by that process. That is, any proce-
dure using this particular data segment may reference the vari-
ables in this area. The individual locations in the global area
can contain an actual value or an indirect address pointing to
some other location that either" contains the value or is the
start of an array. Since DB-relative addressing (paragraph 2-49)
is restricted to a maximum of DB+255, only the first 256 loca-
tions of this area can be addressed directly. These areas are
called the primary global data area. If the number of entries
exceed 256, indirect addressing (paragraph 2-50) must be used.
These locations are called the secondary global data area.
When the operating system has completed assigning space for the
global variables, it points
Q
at the next succeeding location
(First
Q,
figure 2-11). This is the actual start of the data
stack. Since there is not data on the stack, S also points to
this location. As the executing code segment proceeds to obtain,
manipulate, and generate data for the st&ck, S moves away from
Q,
always indicating the top of such data. At some time during the
execution of the code segment, it is assumed that Procedure A is
called. Usually, a set of procedure parameters accompany the
call and these parameters are placed on the stack just prior to
the issuance of the PCAL instruction. These are actual param-
eters to be substituted for formal parameters in the procedure
and are referenced by Q addressing. (Refer to paragraph 2-49.)
2 -26