Access to Nonlocal Names

Access to Non-local Names:
  • In some cases, when a procedure refer to variables that are not local to it, then such variables are called nonlocal variables
  • There are two types of scope rules, for the non-local names. They are
                Static scope
                Dynamic scope
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Static Scope or Lexical Scope
  • Lexical scope is also called static scope. In this type of scope, the scope is verified by examining the text of the program.
  • Examples: PASCAL, C and ADA are the languages that use the static scope rule.
  • These languages are also called block structured languages
Block
  • A block defines a new scope with a sequence of statements that contains the local data declarations. It is enclosed within the delimiters.
Example:
{
Declaration statements
……….
}
  • The beginning and end of the block are specified by the delimiter. The blocks can be in nesting fashion that means block B2completely can be inside the block B1
  • In a block structured language, scope declaration is given by static rule or most closely nested loop
  • At a program point, declarations are visible
                The declarations that are made inside the procedure
                The names of all enclosing procedures
                The declarations of names made immediately within such procedures
  • The displayed image on the screen shows the storage for the names corresponding to particular block
  • Thus, block structure storage allocation can be done by stack
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Lexical Scope for Nested Procedure
  • If a procedure is declared inside another procedure then that procedure is known as nested procedure
  • A procedure pi, can call any procedure, i.e., its direct ancestor or older siblings of its direct ancestor
                Procedure main
                Procedure P1
                Procedure P2
                Procedure P3
                Procedure P4
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Nesting Depth:
  • Lexical scope can be implemented by using nesting depth of a procedure. The procedure of calculating nesting depth is as follows:
                The main programs nesting depth is ‘1’
                When a new procedure begins, add ‘1’ to nesting depth each time
                When you exit from a nested procedure, subtract ‘1’ from depth each time
                The variable declared in specific procedure is associated with nesting depth
Static Scope or Lexical Scope
  • The lexical scope can be implemented using access link and displays.
Access Link:
  • Access links are the pointers used in the implementation of lexical scope which is obtained by using pointer to each activation record
  • If procedure p is nested within a procedure q then access link of p points to access link or most recent activation record of procedure q
Example: Consider the following piece of code and the runtime stack during execution of the program
                program test;
                var a: int;
                procedure A;
                var d: int;
                {
                                a := 1,
                }
                procedure B(i: int);
                var b : int;
                procedure C;
                var k : int;
                {
                                A;
                }
                {
                                if(i<>0) then B(i-1)
                                else C;
                }
                {
                                B(1);
                }
Displays:
  • If access links are used in the search, then the search can be slow
  • So, optimization is used to access an activation record from the direct location of the variable without any search
  • Display is a global array d of pointers to activation records, indexed by lexical nesting depth. The number of display elements can be known at compiler time
  • d[i] is an array element which points to the most recent activation of the block at nesting depth (or lexical level)
  • A nonlocal X is found in the following manner:
  • Use one array access to find the activation record containing X. if the most-closely nested declaration of X is at nesting depth I, the d[i] points to the activation record containing the location for X
  • Use relative address within the activation record
Example:
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How to maintain display information?
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  • When a procedure is called, a procedure ‘p’ at nesting depth ‘i’ is setup:
                Save value of d[i] in activation record for ‘p’
                ‘I’ set d[i] to point to new activation record
  • When a ‘p’ returns:
                Reset d[i] to display value stored
Where can display be maintained?
  • Registers
  • In statically allocated memory (data segment)
  • Store display or control stack and create a new copy on each entry


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About Data Sciences by Venu

Hi, My name is Venugopala Chary and I'm Currently working as Associate Professor in Reputed Engineerng College, Hyderabad. I have B.Tech and M.tech in regular from JNTU Hyderabad. I have 11 Years of Teaching Experience for both B.Tech and M.Tech Courses.
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