The document discusses concepts related to sequence control and subprograms in programming languages. It covers conditional statements, loops, exception handling, subprogram definition and activation, and subprogram environments. Key points include implicit and explicit sequence control using statements, precedence and associativity rules for expressions, stack-based implementation of subprogram calls, and static versus dynamic scoping of identifiers through referencing environments.

1. By-
Garima Jain

2.  Sequence control with expressions
 Conditional Statements, Loops
 Exception Handling
 Subprogram definition and activation
 Simple and Recursive Subprogram
 Subprogram Environment

3. Control of the order of execution of the operations
both primitive and user defined.
Implicit : determined by the order of the statements
in the source program or by the built-in execution
model
Explicit : the programmer uses statements to change
the order of execution (e.g. uses If statement)

4. Expressions: How data are manipulated using
precedence rules and parentheses.
Statements: conditional and iteration statements change
the sequential execution.
Declarative programming: an execution
model that does not depend on the order of the
statements in the source program.
Subprograms: transfer control from one program to
another.

5. What is the sequence of performing the operations?
How is the sequence defined, and how is it represented?
Functional composition : Basic sequence-control
mechanism:
Given an operation with its operands, the operands may
be:
· Constants
· Data objects
· Other operations

6. Example 1: 3 * (var1 + 5)
operation - multiplication, operator: *, arity - 2
operand 1: constant (3)
operand 2: operation addition
operand1: data object (var1)
operand 2: constant (5)

7. Example 2: 3* var1 +5
Question: is the example equivalent to the above one?
Example 3: 3 + var1 +5
Question: is this equivalent to (3 + var1) + 5,
or to 3 + (var1 + 5) ?

8. Precedence concerns the order of applying
operations
Associativity deals with the order of operations of
same precedence.
Precedence and associativity are defined when the
language is defined - within the semantic rules for
expressions.

9. Linear representation of the expression tree:
Prefix notation
· Postfix notation
· Infix notation
Prefix and postfix notations are parentheses-free.

10.  Machine code sequence
 Tree structures - software simulation
 Prefix or postfix form - requires stack, executed by an
interpreter.

11. Eager evaluation - evaluate all operands before
applying operators.
Lazy evaluation

12. Side effects - some operations may change operands of
other operations.
Error conditions - may depend on the evaluation
strategy (eager or lazy evaluation)
Boolean expressions - results may differ depending on
the evaluation strategy.

13. if expression then statement1 else
statement2
if expression then statement1
a choice among many alternatives
nested if statements
case statements
Implementation: jump and branch machine
instructions, jump table implementation for case
statements

14. Simple repetition (for loop)
Specifies a count of the number
of times to execute a loop:
perform statement K times;
for loop -
Examples:
for I=1 to 10 do statement;
for(I=0;I<10; I++) statement;

15. while expression do statement;
Evaluate expression and if true execute statement, then
repeat process.
repeat statement until expression;
Execute statement and then evaluate expression.
Repeat if expression is not true.
C++ for loop functionally is equivalent to repetition
while condition holds

18.  Multiple exit loops
 Exceptional conditions
 Do-while-do structure
Solutions vary with languages, e.g. in C++ - break
statement, assert for exceptions.

19. Exception Handlers are subprograms that are not
invoked by explicit calls
Special situations, called exceptions:
 Error conditions
 Unpredictable conditions
 Tracing and monitoring

20. Exception handlers typically contain only:
• A set of declarations of local variables
• A sequence of executable statements
Exception Handlers can be
- predefined in the language
- programmer defined

21. Languages provide methods for raising (throwing) and
testing for exceptions.
try {
statement1;
statement2;
…
if badCondition throw ExceptionName;
}
catch ExceptionName
{ ……….// do something for exception…….}

22. Operating system exceptions - raised directly
by hardware interrupts.
Programmer defined -
the translator inserts code to handle the
exceptions.

23. Subprogram Control :
interaction among subprograms
how subprograms pass data among themselves

24. Simple subprogram call return
Copy rule view of subprograms:
the effect of a call statement is the same as if the
subprogram were copied and inserted into the
main program.

25. • Subprograms cannot be recursive
• Explicit call statements are required
• Subprograms must execute completely at each call
• Immediate transfer of control at point of call
• Single execution sequence

26. CALL
RETURN

27. Execution of subprograms
Subprogram definition.
Subprogram activation.

28. The definition is translated into a template, used
to create an activation each time a subprogram is
called.

29. a code segment (the invariant part) -
executable code and constants,
an activation record (the dynamic part) -
local data, parameters.
created a new each time the subprogram is called,
destroyed when the subprogram returns.

30. • Current-instruction pointer – CIP
address of the next statement to be executed
• Current-environment pointer – CEP
pointer to the activation record.

32.  An activation record is created
 Current CIP and CEP are saved in the created
activation record as return point
 CEP is assigned the address of the activation
record.
 CIP gets the address of the first instruction in the
code segment
 The execution continues from the address in CIP

33.  The old values of CIP and CEP are retrieved.
 The execution continues from the address in
CIP
Restrictions of the model:
at most one activation of any subprogram

34. Allocate storage for a single activation record statically
as an extension of the code segment.
Used in FORTRAN and COBOL.
The activation record is not destroyed - only reinitialized
for each subprogram execution.
Hardware support - CIP is the program counter,
CEP is not used, simple jump executed on return.

35. The simplest run-time storage management technique
call statements : push CIP and CEP
return statements : pop CIP and CEP off of the stack.
Used in most C implementations
LISP: uses the stack as an environment.

36. Specification
Syntactically - no difference
Semantically - multiple activations of the
same subprogram exist simultaneously at
some point in the execution.
E.G. the first recursive call creates a second
activation within the lifetime of the first activation.

37. Stack-based -
CIP and CEP are stored in stack, forming a
dynamic chain of links.
A new activation record is created for each call
and destroyed on return.
The lifetimes of the activation records cannot
overlap - they are nested.

38. Data control features determine the accessibility of data at
different points during program execution.
Central problem:
the meaning of variable names, i.e. the correspondence
between names and memory locations.

39. Two ways to make a data object available as an operand
for an operation
Direct transmission
Referencing through a named data object

40. A data object computed at one point as the result of
an operation may be directly transmitted to another
operation as an operand
Example: x = y + 2*z;
The result of multiplication is transmitted directly as
an operand of the addition operation

41. A data object may be given a name when it is
created, the name may then be used to designate it
as an operand of an operation.

42. Variables
Formal parameters
Subprograms
Defined types
Defined constants
Labels
Exception names
Primitive operations
Literal constants

43. Association: binding identifiers to particular data
objects and subprograms
Referencing environment: the set of identifier
associations for a given subprogram.
Referencing operations during program execution:
determine the particular data object or subprogram
associated with an identifier

44. Subprogram Environment
The set of associations created on entry to a subprogram formal
parameters, local variables, and subprograms defined only within that
subprogram.
Non-local referencing environment
The set of associations for identifiers
• used within a subprogram
• not created on entry to it
Global referencing environment:
associations created at the start of execution of the main program, available
to be used in a subprogram.
Predefined referencing environments:
predefined associations in the language definition.