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Welcome, everyone! Today weβre diving into run-time support, which is crucial for executing programs after theyβve been compiled.
So, is run-time support just about memory management?
Not just memory! It also involves managing how functions call each other and how data flows between parts of your program. Think of it as an engine that keeps everything running smoothly.
What kinds of things are included in run-time support?
Great question! It includes parameter passing methods, activation records, and memory allocation strategies. Letβs explore these concepts further in our next session.
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Now, let's talk about parameter passing, which allows functions to get data from their calling environment. Can anyone name a method of passing parameters?
Is 'pass by value' one of them?
Exactly! In pass by value, a copy of the data is made. However, who can tell me if this means changes to the parameter affect the original data?
No, right? Because modifications occur only in the function's local copy.
Correct! On the other hand, pass by reference means you are accessing the original data directly. This could lead to side effects, which we need to manage carefully. Remember the acronym **VRS**: Value, Reference, Safety!
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Next, letβs explore activation records, or ARs, which serve as the temporary workspace for function calls. What do you think they contain?
Maybe return values and local variables?
Absolutely! ARs also hold parameters and return addresses. Itβs structured like a neat file that organizes everything a function needs during its execution. Can anyone tell me the significance of the return address?
It tells the program where to go back after the function finishes!
Exactly! Remember that structure well; itβs key for understanding how function calls work.
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Our final topic today is memory management strategies for ARs. What are the two primary methods?
Stack allocation and static allocation?
Correct! Stack allocation is dynamic and supports recursion, while static allocation is fixed and simpler. Which method do you think is more efficient?
Stack allocation, because it adds and removes frames quickly without much overhead!
Right! But keep in mind that stack overflow is a risk with deep recursions. Well done today, class!
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This section covers how run-time support enhances program execution by managing memory allocation, orchestrating function calls, and enabling parameter passing methods. It also elaborates on the structure of activation records and different allocation strategies used during execution.
The transition of source code into an executable program involves numerous steps, with run-time support playing a crucial role in managing memory, control flow, and data exchange during program execution. It encompasses various mechanisms and conventions that enable functions to interact efficiently. This includes the parameter passing methods: Pass by Value, where data is copied, Pass by Reference, where data is accessed directly, and Pass by Name, a rare method from earlier languages. Each method has implications for performance and side effects on variable states.
Moreover, the execution context for function calls is organized through Activation Records (ARs), containing vital information such as parameter values, return addresses, and local variables. Memory management strategies like Stack Allocation and Static Allocation determine how and when these records are created and destroyed, influencing language features like recursion and efficiency. The section concludes by detailing the function call process, emphasizing the intricacies of allocating offsets and generating appropriate machine code.
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The journey of your source code from human-readable text to an executable program involves several intricate steps. Once the compiler has transformed your code into machine instructions (after lexical, syntax, and semantic analysis), the next challenge is to actually run that program effectively. This execution phase requires a sophisticated system for managing memory, orchestrating the flow of control, and facilitating data exchange between different parts of your code. This is the realm of Run-time Support.
Run-time support is crucial for executing programs correctly after they have been converted into machine instructions by the compiler. It involves organizing memory, controlling the order of operation within the program, and enabling data sharing among various elements in code. This means that when we write code, it isn't just about writing lines of instructions; a lot happens behind the scenes to ensure everything runs smoothly as intended.
Think of a play where the script is like your code, and run-time support is the stage crew. Just as the crew makes sure lights, props, and actors are all in the right place and functioning well during the performance, run-time support manages the program to ensure that all variables, functions, and control flows work perfectly during execution.
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Functions are the workhorses of programming. They allow you to encapsulate a specific task and reuse it. To perform their task, functions often need input data from the part of the code that calls them. The way this data is transferred to the function's parameters is crucial because it dictates whether the function can modify the original data or just work with a copy.
When functions are called in programming, they need to receive data to work with. The method used to give this data to the function's parameters is extremely important because it determines if the function can change the original data or if it will only be working with a duplicate of that data. There are several ways to pass parameters to functions, including 'pass by value,' 'pass by reference,' and 'pass by name.' Each of these methods has different implications on how functions interact with data.
Imagine you have a recipe that needs ingredients. If you give someone a whole bag of flour (pass by value), they can only use that bag for their baking; they canβt change the original bag you have. But if you let them borrow your favorite measuring cup (pass by reference), they can change the amount of flour in their baking, which would also change your original supply if itβs the same cup. Lastly, if you just tell them, 'Add flour until it looks right' (pass by name), they will decide how much flour to use based on the current state, which might change based on their interpretation.
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In the 'pass by value' method, when we call a function, a copy of the argument's value is created. This copy lives in its own memory space, meaning any changes made to it within the function do not affect the original variable outside the function. This method isolates the functionβs operations, ensuring that it wonβt unintentionally alter data that exists elsewhere in the program.
Consider a library loaning out a book. The library doesnβt just let you take the original book; instead, they give you a copy. You can write notes in it, highlight parts of it, or even take it apart, but the original book remains untouched on the shelf. This way, any alterations you make wonβt affect the original book.
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β Behavior and Implications:
- Local Modification Only: Any modifications made to the parameter within the function are applied only to this local copy. The original variable in the calling scope remains completely unchanged. This provides strong isolation; the function cannot inadvertently alter the caller's data.
- Safety and Predictability: This is considered the safest parameter passing mechanism because it prevents side effects on the caller's variables. It makes functions easier to reason about and debug.
When using the pass by value method, since functions work with copies of data, any changes they make will not impact the original data. This safety feature makes debugging simpler, as developers can be more confident about the integrity of the caller's variables. Since a function wonβt have unintended side effects on outside data, the predictability of program behavior is enhanced, allowing easier reasoning about how functions will operate.
Imagine a chef who specializes in desserts. If they practice making a cake but only use a model (not the actual ingredients), they can measure, taste, and adjust without ever ruining the actual cake. This practice ensures that mistakes donβt affect the final product, and itβs a safe way to learn and develop skills.
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β Overhead for Large Data: For very large data structures (like a huge array or a complex object), copying the entire structure can be computationally expensive and consume significant memory.
- Default for Many Languages: Many popular languages (e.g., C for primitive types, Java for primitive types and object references, Python for all types) use pass-by-value as their default or only mechanism.
While pass by value is beneficial for ensuring function safety, it has its drawbacks. One major disadvantage is that for large data structures, the process of copying them creates additional overhead in terms of memory usage and processing time. Consequently, developers must be mindful of performance impacts when using this parameter-passing method with large data sets. However, many programming languages use this as their standard method to maintain functional integrity.
Think of a large office that needs copies of a massive document. Making these copies requires printers and paper, which takes time and resources. If you simply sent the original document (like passing by reference), it eliminates the need for copies and saves time. In a similar way, passing large datasets by value can consume unnecessary resources.
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In the pass by reference method, the function receives a reference to the original variable rather than a copy. This means that any changes made to the parameter directly affect the original data because they refer to the same memory location. It enables direct manipulation of the data, which is useful in certain contexts but can introduce risks of unintended modifications.
Imagine having a remote control for a TV. When you use it, you are not changing the TV; instead, youβre sending commands to it. Similarly, when passing by reference, you send a link to the original item, allowing you to control how it changes without holding a duplicate.
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β Behavior and Implications:
- Direct Modification: Any changes made to the parameter inside the function directly modify the original variable in the calling scope. The parameter acts as an alias for the original variable.
Since the function works directly on the original variable, any alterations made during the function will influence the caller's data immediately. This direct modification feature is efficient but requires careful use to avoid unintentional changes that could lead to bugs or less predictable program behavior.
Think of a shared whiteboard at a meeting. When one person writes on it, everyone can see it instantly. If they make a mistake, it can affect everyone involved. This is similar to passing by reference, where one personβs changes can have immediate and widespread effects.
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β Efficiency for Large Data: This method is highly efficient for passing large data structures because only a small memory address (typically 4 or 8 bytes) is copied, not the entire structure.
- Potential for Side Effects: Because functions can modify caller's data, it requires careful programming to avoid unintended side effects.
Passing by reference is efficient particularly when dealing with large amounts of data, as only a small address needs to be shared rather than a whole dataset. However, the direct relationship between the parameter and the original variable also means programmers must write code very carefully to prevent side effects that might disrupt other parts of the program.
Imagine a family sharing a single tablet to play games. Itβs quick and easy for everyone to access, but if one person deletes a game or changes any settings, it affects the whole family. In programming, passing by reference provides instant access, but programmers must take care to avoid faults.
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Pass by name works quite differently from the previous two methods. Instead of sending a value or a reference, it sends an expression or a piece of code that will be evaluated whenever the parameter is accessed. This allows for flexibility and dynamic behavior, but it can also lead to complications, especially if the variables change between evaluations.
Consider having an assistant who takes notes for you based on your verbal instructions. Every time you ask them to write down a point, they listen to your words each time and can adapt based on what you say in the moment rather than just copying it down once. This is similar to how pass by name evaluates expressions anew each time theyβre accessed.
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β Behavior and Implications:
- Delayed Evaluation & Side Effects: This leads to a "call-by-need" or "lazy evaluation" semantic. The argument is only evaluated when it's actually used. More importantly, if the actual argument expression involves variables that change between different evaluations, the parameter's "value" will also change dynamically within the function.
With pass by name, because the argument is only computed when needed, it can result in different outcomes if the variables involved have changed since the last evaluation. This behavior adds flexibility but makes reasoning about the code more complicated, as the value might be different depending on when it is evaluated.
Imagine you're working on a puzzle that changes each time you look away. If you get a new piece of information after your last check that may impact the next steps, your understanding of how to put together the puzzle will dynamically shift with each glance. This reflects how pass by name can change the outcome based on variable states.
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Every time a function is called, it needs a dedicated block of memory to store all the information relevant to that specific invocation of the function. This block is called an Activation Record (AR), or more commonly, a Stack Frame. It's the run-time representation of a function's execution context.
When a function is called, it needs a specific space in memory to hold all its data, like local variables, parameters, and return addresses. This area is called an activation record (AR). Think of an AR as a temporary file folder that contains everything needed for the function to operate properly while it runs. Once the function is done, the AR is discarded.
Imagine a teacher that prepares a 'class folder' for each student in their class. This folder contains all the relevant notes and homework for that student's unique session. Once the class is over, the folder is either stored away or thrown out if not needed again. This scenario relates well to how activation records manage data for each function call.
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Detailed Breakdown of Typical Contents within an Activation Record:
1. Return Value Space:
- Purpose: This is an area (often a small dedicated portion or even just a register) where the function places the value it computes and returns to its caller.
2. Actual Parameters (Arguments):
- Purpose: Storage for the values (or references) that were passed to the function by its caller.
An activation record contains several important components. First, there's a space for the return value, which is where the function stores its result to send back. Next, it includes storage for the actual parameters, which are the values given to the function during a call. These parts are essential for the functionβs operation and ensure smooth communication between the function and the caller.
Think about a restaurant that takes orders. Each order is like an activation record - it has a designated space for the response (what the customer will get back) and sections for the ingredients (the parameters) that make up that meal. Without these designated areas for each order, the restaurant would be chaotic and orders might get mixed up.
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The return address is crucial because it tells the program where to go back to once the function has finished executing. When a function call is made, the return address is saved within the activation record so that, after the function completes its task, the program knows where to pick up from. This ensures the flow of control within the program remains seamless.
Imagine a phone call where you promise to call back your friend after you finish a task. You keep a note of where you left off in the conversation, so once you finish what youβre doing, you know exactly where to resume. Similarly, the return address ensures that programs know exactly where to resume after a function call.
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The control link is a pointer that connects the calling functionβs activation record to the one currently executing. This allows the function to track its caller and ensure that once its execution is done, it can return to the correct point in the program. It's essential for maintaining the integrity of the call stack as functions invoke one another.
Think about a group of friends at a restaurant. When one person gets up to use the restroom, they often look back at the table to ensure they return to the same seat they left. The control link is like their memory of their spot at the table, allowing them to find the right place again after they finish.
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The access link helps functions access variables that are defined outside their own scope, especially in cases of nested functions. By pointing to the activation record of the enclosing function, it provides access to non-local variables, making it possible for nested functions to utilize data declared in their parent functions. This is crucial for supporting closures and scopes in programming languages.
Imagine a child drawing in a workshop under the guidance of their parent. The child has access to their own materials but can also use the supplies from their parentβs workspace when needed. The access link serves a similar function, allowing the inner function to tap into the data available in its parent function.
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To ensure that the original values in the CPU registers are preserved, functions save those values when executing. This practice avoids any potential issues that could arise if the function altered the caller's register values. Once the function completes, these saved values are restored to maintain the state of the caller's execution environment.
Imagine a grocery store worker who needs a cash register for a new promotion but must return the register back to its previous state afterward. They take notes of the initial settings to ensure nothing changes in the overall system. Similarly, saved registers maintain the state consistency after function calls.
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Local variables are defined directly in an activation record and are only accessible within the scope of the function that creates them. This confinement ensures that they do not interfere with other parts of the program. Itβs crucial for maintaining order and scope when multiple functions execute simultaneously.
Picture a workshop where each project gets its own table, complete with specific tools, materials, and designs employed only for that project. Once a workshop is closed, the tables (or local variables) are cleared away, ensuring that the next project begins fresh without interference.
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Temporary storage within activation records is reserved for short-lived values that are needed only during specific calculations within a function. These could be intermediate results from expressions that don't require permanent storage but are necessary for the function's computations.
Consider an artist mixing paint. They may have a palette with several colors mixed together but only use those mixtures temporarily for the immediate artwork, intending to clean the palette afterward. Temporary storage works similarly by holding onto values that don't need to persist after the function has completed.
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The choice of how activation records are allocated significantly impacts a language's features (like recursion) and performance. The two primary strategies are Stack Allocation and Static Allocation.
There are different strategies for managing how activation records are allocated in memory. Stack allocation, often used in most modern programming languages, allows records to be created and destroyed on-the-fly, automatically supporting recursion since each function call gets its own space. In contrast, static allocation assigns a fixed memory space for records ahead of time, suitable for certain languages and contexts but limits flexibility.
Think of a filing system in an office. Stack allocation is like having a folder to store documents for each client meeting, which you can easily add or remove as needed. In contrast, static allocation resembles a library with designated spots for each book that never change; itβs efficient in organization but inflexible if new authors or genres come into play.
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Stack allocation is widely used because it efficiently manages memory for function calls using a last-in, first-out (LIFO) structure. When a function is called, an activation record is placed on the stack, and when the function returns, that record is removed. This dynamic handling allows functions to have their own memory space without programmer intervention.
This can be likened to a stack of plates in a cafeteria. The last plate put on top is the first one taken off when itβs time to serve. Similarly, each function call adds a new 'plate' (activation record) to the stack, which is removed once the function completes.
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β Advantages:
- Automatic Lifespan Management: Memory is automatically allocated and deallocated as functions are called and return.
- Direct Support for Recursion: Each recursive call to a function gets its own distinct activation record on the stack.
Stack allocation makes it simple for programmers because memory for local variables is handled automatically. It also supports recursion well since every separate function call can keep its own activation record, preventing data interference among calls. Each call can use local variables without conflicting with others.
Consider a construction project where each new task is handled in its own area. The tasks can be completed simultaneously without affecting each other because theyβre all in separate places. This mirrors how each function call uses its own space, maintaining order and visibility.
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β Disadvantages:
- Fixed-Size Variables: The compiler needs to know the size of local variables and parameters at compile time to correctly allocate space in the AR.
- Stack Overflow: If a program calls functions too deeply, the stack can grow beyond its allocated memory limit.
While stack allocation is helpful, it has some limitations. For instance, the exact size of local variables must be known beforehand, limiting flexibility. Stack overflow can also occur when a program makes too many nested function calls, leading to the stack exceeding its size limit.
Think of a truck that can only carry a certain amount of cargo. If you keep trying to load it until it overflows, you risk damaging the truck or losing important goods. In programming, deep function calls can lead to similar overflow issues, which can crash the program.
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Static allocation retains memory storage for variables throughout the programβs lifetime. It means that at the start, specific memory addresses are designated for variables, which donβt change during runtime. This method is commonly used for global variables and static local variables that need to maintain their values between function calls.
Imagine a storage unit that rents space to companies for their files permanently. They can access their files anytime without worrying about losing them during transactions. Thatβs how static allocation provides consistent memory availability for variables.
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β Advantages:
- Simplicity of Management: No complex runtime memory allocation/deallocation logic is needed.
- Persistent Data: Static variables retain their values across function calls.
Static allocation's biggest advantages include ease of management, since memory locations are predetermined and don't change. Additionally, static variables are incredibly useful as they can preserve their values even after function execution, leading to possible efficiency in working with data.
Think about a toy collection. If you have a specific shelf that stores all your toys, you know where every item is without searching each time you want to play with them. This clarity mirrors how static allocation organizes variables for easy access.
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β Disadvantages:
- No Recursion Support: If a function's AR is statically allocated, calling it recursively would mean all recursive calls try to use the exact same memory space, leading to incorrect behavior.
One of the main drawbacks of static allocation is that it cannot adequately support recursion. If multiple recursive calls were made, they would compete for the same space in memory, which could overwrite each other's data and cause incorrect execution outcomes.
Imagine a teacher who uses a single workbook for multiple students. If a student writes in the workbook while another is attempting to use it at the same time, they could easily mess up each otherβs notes. This reflects how static allocation can lead to chaos in recursive scenarios.
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The act of calling a function and returning from it isn't a single, simple machine instruction. It's a carefully orchestrated sequence of operations, generated by the compiler, that sets up the environment for the called function and cleans it up afterward.
Function calls and returns involve a series of intricate steps rather than a direct command. The compiler generates code to manage everything, from setting up necessary memory space to controlling the flow of function execution and ensuring a smooth return to the caller. This orchestration defines how functions interact within the programming environment.
Think of a cabaret ballet performance. Each dancer has a specific entry, performance, and exit sequence. The choreographer needs to plan those movements precisely to deliver a seamless show. Similarly, the compiler ensures that all parts of a function are coordinated so the overall program runs smoothly.
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When an activation record is created, each variable within it is given a specific offset, or distance, from a reference point called the frame pointer (FP). This helps the program know where to find each variable when itβs needed throughout the functionβs execution. The compiler does this automatically during the code generation process.
Consider a treasure chest with compartments, each assigned to store certain items. A map is drawn indicating exactly where each type of item can be found (the offset), ensuring that when it's time to retrieve something, you can do it efficiently without digging through everything. The offsets function similarly by pointing to where each piece of data is stored in memory.
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The call sequence consists of steps executed by the caller to ensure everything's in place before transferring control to the called function. This includes pushing arguments onto the stack, saving the return address, and preparing any necessary links for dynamic and static frameworks. Each of these actions ensures that the function can execute properly and return control to the caller at the end.
Envision a waiter taking order notes before the kitchen starts cooking. They write down each element (ingredients) and the table number (return address) so once the meal is prepared, the right dish goes to the right table. This collection of actions mirrors how the call sequence prepares the function call.
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b. Function Prologue (Executed by the Called Function): This is the entry sequence. It sets up the called function's own execution environment.
The function prologue establishes the environment necessary for the function to execute. It sets the frame pointer and allocates space for local variables needed during the functionβs execution. This preparatory work is crucial to ensuring everything is ready for operations once control is handed over.
Think of a workshop where a craftsperson gets all their tools, materials, and workspace tidy before starting a job. They need to set everything up properly to ensure that their work goes smoothly without interruptions. Analogously, a function prologue does that same groundwork for a function.
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c. Function Epilogue (Executed by the Called Function): This is the exit sequence. It cleans up the called function's environment before returning.
The function epilogue ensures that all processes necessary to safely terminate the function are completed. This includes placing the return value in the designated spot, restoring any saved registers, and deallocating the space used for local variables. The epilogue guarantees that when control transfers back to the caller, everything is left in a predictable state.
Imagine a chef cleaning up the kitchen after finishing a meal. They put away ingredients, clean dishes, and prepare the workspace for the next cooking session. Similarly, the function epilogue wraps up the function's work for the next steps in the program.
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d. Return Sequence (Executed by the Caller): This is the cleanup code. It finalizes the call on the caller's side.
The return sequence involves the code executed by the caller after the callee has completed execution. This may involve cleaning up parameters pushed onto the stack and retrieving any necessary return values. It ensures that the caller is ready to continue executing correctly, and that any resources utilized during the call are properly managed.
Picture a theater audience leaving after a show. As the lights come back up, ushers make sure the aisles are cleared and any lost items are returned, allowing people to exit smoothly without chaos. The return sequence does similar housekeeping, ensuring everything is in order for the program to proceed.
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Key Concepts
Run-time Support: Critical for managing execution in programs after compilation.
Parameter Passing: Methods include pass by value, reference, and name, impacting data accessibility and safety.
Activation Records: Temporary structures that facilitate function call contexts, containing essential data like local variables.
Memory Management: Strategies like stack and static allocation determine how functions utilize memory during execution.
See how the concepts apply in real-world scenarios to understand their practical implications.
In pass by value, modifying a copied parameter does not alter the original variable in the caller scope.
In pass by reference, the original variable value can be changed directly by the function due to memory aliasing.
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
In the stack, they store the facts, activation records push and lax.
Imagine a ship (function) that sets sail (calls another function). The activation record is its map, guiding it back after exploring all routes.
For parameter passing, remember Value, Reference, Safety for understanding the methods.
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Review the Definitions for terms.
Term: Runtime Support
Definition:
Mechanisms and data structures that assist in the execution of programs post-compilation.
Term: Activation Record (AR)
Definition:
A memory structure that contains information related to a function's execution context.
Term: Pass by Value
Definition:
A method where a copy of the data is passed to a function, preventing original data modification.
Term: Pass by Reference
Definition:
A method where a function receives a reference to the actual data, allowing for direct modification.
Term: Static Allocation
Definition:
A method of memory allocation where data is assigned fixed memory at compile-time.
Term: Stack Allocation
Definition:
A method where memory for function calls is managed in a last-in, first-out (LIFO) structure at run-time.