User's Manual

EXTENDED INSTRUCTION SET
The Formal Definition Of The ENTER Instruction For All Cases Is Given By The Following Listing. LEVEL Denotes The Value Of The
Second Operand.
Push
BP
Set a temporary value FRAME_PTR : ~ SP
If LEVEL> 0 then
Repeat
(LEVEL -1) times:
BP:~ BP-2
Push the word pOinted to by BP
End repeat
Push FRAME_PTR
End
If
BP : ~ FRAME_PTR
SP : ~ SP - first operand.
Figure 4-1. Formal Definition of the ENTER Instruction
The main procedure (with other procedures nested within) operates at the highest lexical level, level 1.
The first procedure it calls operates at the next deeper lexical level, level 2. A level 2 procedure can
access the variables of the main program which are at fixed locations specified
by the compiler. In the
case of
levell, ENTER allocates only the requested dynamic storage on the stack because there is no
previous display to copy.
A program operating at a higher lexical level calling a program at a lower lexical level requires that
the called procedure should have access to the variables of the calling program.
ENTER provides this
access through a display that provides address ability to the calling program's stack frame.
A procedure calling another procedure at the same lexical level implies that they are parallel
proce-
dures and that the called procedure should not have access to the variables of the calling procedure.
In this case,
ENTER copies only that portion of the display from the calling procedure which refers to
previously nested procedures operating at higher lexical levels. The new stack frame does not include
the pointer for addressing the calling procedure's stack frame.
ENTER treats a reentrant procedure as a procedure calling another procedure at the same lexical
level. In this case, each succeeding iteration of the reentrant procedure can address only
its own varia-
bles and the variables of the calling procedures at higher lexical levels. A reentrant procedure can
always address its
own variables; it does not require pointers to the stack frames of previous iterations.
By copying only the stack frame pointers of procedures at higher lexical levels, ENTER makes sure
that procedures access only those variables of higher lexical levels, not those
at parallel lexical levels
(see figure 4-2). Figures 4-2a through 4-2d demonstrate the actions of the
ENTER instruction if the
modules shown
in figure 4-1 were to call one another in alphabetic order.
Block-structured high-level languages can use the lexical levels defined by
ENTER to control access
to the variables of previously nested procedures. For example, if
PROCEDURE A calls
PROCEDURE B which, in turn, calls PROCEDURE C, then PROCEDURE C will have access to
the variables of
MAIN and PROCEDURE A, but not PROCEDURE B because they operate at the
same lexical level. Following
is the complete definition of the variable access for figure 4-2.
1. MAIN PROGRAM has variables at fixed locations.
2. PROCEDURE A can access only the fixed variables of MAIN.
3. PROCEDURE B can access only the variables of PROCEDURE A and MAIN. PROCEDURE
B cannot access the variables of PROCEDURE C or PROCEDURE D.
4-3