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Chapter 7

Chapter 7. Runtime Environments. Relationships between names and data objects As execution proceeds, the same name can denote different data objects Procedures, variables, etc. Execution of a procedure -> activiation of the procedure Data objects (variables) within an activitaions

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Chapter 7

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  1. Chapter 7 Runtime Environments

  2. Relationships between names and data objects • As execution proceeds, the same name can denote different data objects • Procedures, variables, etc. • Execution of a procedure -> activiation of the procedure • Data objects (variables) within an activitaions • The same procedure may have several activations alive at the same time (e.g. recursive)

  3. Procedure • Definition: a declaration associates an Idetifier with statements • The id is a procedure name • The body is statements • Optional parameters • Formal parameters (definition) • Actual parameters (arguments) • Local variables • References to global scope

  4. Activation Tree: Procedure executions • Each node represents an activation of procedure • The root represents the activation of the main program • The node for a is the parent of the node for b, if and only if control flows from a to b and • The node for a is to the left of the node b if and only if the lifetime of a occurs before the life time of b • (show examples in fig 7.2, 7.3)

  5. Control Stack for Activation Tree • Depth first traversal that starts from the root • We can use stack to keep track of live procedure activations • Can be extended to the stack storage-allocation technique (C or Pascal) • (show figure 7.4 and the current control and activation life s, q(1,9), q(1,3) and 1(2,3)

  6. The Scope of a Declaration • Declaration a syntactic construct associating information with a name • Scope rules determine which declarations of a name applies when the name appears in the program • Local • None-local • Special, static, volatile, final etc..

  7. Name Bindings • Data object is a storage location that can hold values • Name storage value • Programming language, • environment is a function that maps a name to a storage location • State is a function that maps a storage location to the value • L-value (name), l-value to r-value • Binding is the dynamic counterpart of a declaration

  8. Name Bindings

  9. Storage organization • Runtime Storage can be subdevided as follows: • The generated target code • Data object • Control stack (keep track of procedure activitation)

  10. Storage organization

  11. Activation Record • A contiguous block of storage for maintaining info during a procedure activation

  12. Storage Allocation Strategies • Static allocation lays out storage for all Data objects during compile time • Stack allocation manages the run-time storage as a stack • Heap allocation allocates and deallocates stoages as needed at runtime from heap area • Watchout for a dangling reference (garbage collections)

  13. Parameter passing • Call by value • A formal parameter is treated just like a local name. Its storage is in the activation record of the called procedure • The caller evaluates the actual parameter and place the r-value in the storage for the formals • Call by reference • Copy-Restore

  14. Parameter passing • Call by reference • If an actual parameter is a name or expression having L-value, then that l-value itself is passed • However, if it is not (e.g. a+b or 2) that has no l-value, then expression is evaluated in the new location and its address is passed.

  15. Parameter passing • Copy-Restore: Hybird between call-by-value and call-by-ref (copy in, copy out) • Actual parms evaluated, its r-value is passed and l-value of the actuals are determined • When the called procedure is done, r-value of the formals are copied back to the l-value of the actuals

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