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

TNS Execution Modes

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

6-34

Defining the Procedure’s Data

As previously explained, the first half of the user data segment contains the process’s

global variables and TNS user data stack. The left part of Figure 6-20 illustrates the

overall organization of the user data segment.

Variables in the global data area are addressable by any instruction in the TNS

procedure. The addressing base of the global area is defined as G[0], and individual

locations in this area are referenced with G-relative addresses. The primary global

area exists within the first 256 words of the segment. This area is accessed with direct

addressing. It can contain indirect addresses to anywhere in the segment, but they

often point into an area of secondary global data beginning at a location immediately

following the last primary variable. The secondary global data area can be much larger

than the primary area.

The user data stack (or TNS stack) is where the manipulation of memory data by TNS

instructions takes place. When a procedure is called into execution, that invocation is

allocated its own temporary storage area on the stack, called a stack frame. This

stack frame, which consists of memory data that the procedure is currently using, is

known only to that invocation of the procedure and is logically separate from the stack

frames of other procedures.

At any moment many frames may be in existence, one immediately following another.

The first frame on the stack is that of the program’s main procedure. The base of the

stack frame is defined by the 16-bit L register. The L (for Local) register contains the

G-relative address base of the procedure’s local data and is defined as L[0]. Elements

following (and preceding) L[0] can be addressed with positive or negative offsets.

The current top-of-stack location is defined by the 16-bit S register, which contains

the G-relative address of the last word currently defined in the user data stack. This

address is defined as S[0], and it serves as an addressing base for a top-of-stack area

called the sublocal area. This sublocal area consists of up to 32 word locations

including and preceding S[0]; it is typically used by subprocedures. Data in the

sublocal area is known only to the currently executing subprocedure.

During the execution of a procedure, the address in the S register advances as

elements are moved from the register stack to the top of the user data stack, and

recedes as elements are moved from the top of the user data stack to the register

stack.