What is virtual memory? – Gary explains

Summary

In this video, Gary Sims from Android Authority explains the concept of virtual memory, a technology used in modern operating systems like Windows, iOS, Linux, and Android. He delves into how virtual memory works to ensure apps run smoothly by mapping virtual addresses to physical memory locations. This process prevents memory fragmentation, isolates apps from one another, and optimizes memory usage. Gary also touches on the importance of the Memory Management Unit (MMU), page tables, and the translation lookaside buffer (TLB), and how the operating system manages memory allocation and page faults, ensuring efficient performance for users.

Takeaways

• 😀 Virtual memory allows programs to think they have exclusive access to the system's memory, even though they share it with other programs.
• 😀 The Memory Management Unit (MMU) in the CPU plays a crucial role in mapping virtual addresses used by apps to actual physical memory addresses.
• 😀 Virtual memory helps prevent memory fragmentation, as programs can be split and placed in non-contiguous areas of physical memory by the operating system.
• 😀 Virtual memory systems manage memory in chunks called pages (typically 4KB), improving memory usage efficiency and reducing fragmentation.
• 😀 The operating system (OS) decides where each program's memory is located, ensuring that no two programs overwrite each other's memory.
• 😀 The Translation Lookaside Buffer (TLB) speeds up memory access by caching recent address translations, minimizing lookup times.
• 😀 When a program tries to access an invalid or unallocated memory address, a page fault occurs, which is handled by the OS or kernel.
• 😀 Virtual memory allows running multiple programs on a system without them interfering with each other, even if they are using different parts of memory.
• 😀 Memory management includes complex tasks like lazy allocation (delaying memory assignment until the program writes to it) and handling swapped-out data from disk storage.
• 😀 The process of virtual memory mapping involves converting virtual addresses into physical addresses using lookup tables, which are often stored in RAM, not the CPU.

Q & A

What is virtual memory and why is it important?

Virtual memory is a technology that allows a computer to use more memory than what is physically available by simulating extra memory space. It ensures that each program thinks it's the only one running, preventing conflicts and optimizing memory usage.

How does virtual memory address the issue of memory fragmentation?

Virtual memory resolves memory fragmentation by allowing programs to be allocated memory in non-contiguous blocks, with the operating system managing the mapping between virtual and physical memory. This eliminates gaps in memory and maximizes space usage.

What role does the Memory Management Unit (MMU) play in virtual memory?

The MMU maps the virtual addresses that a program uses to actual physical addresses in memory. It acts as the translator between the virtual memory space each application sees and the physical memory that exists in the system.

How does the operating system manage virtual memory across multiple programs?

The operating system allocates a separate virtual memory space to each program, ensuring that one program cannot overwrite another's data. It uses a lookup table to map virtual addresses to physical addresses for each process.

What is memory fragmentation and how does it occur?

Memory fragmentation happens when memory is allocated in small, non-contiguous chunks, leaving gaps between them. Over time, as programs start and stop, these gaps accumulate, making it harder to fit new programs into memory.

Why do operating systems use a page table in virtual memory?

Operating systems use a page table to store mappings between virtual memory pages and physical memory. This allows the system to manage memory more efficiently by grouping memory into fixed-size pages, typically 4KB in size.

What is the Translation Lookaside Buffer (TLB), and how does it improve performance?

The TLB is a cache that stores recently accessed memory address translations. It speeds up memory access by reducing the number of times the system has to look up address mappings in the page table.

What happens when a program tries to access a virtual address that is not mapped in the TLB?

If a program accesses a virtual address not found in the TLB, the Memory Management Unit (MMU) triggers a page fault. This causes the kernel to fetch the missing address mapping from the page table and update the TLB.

What is a page fault, and how does the system handle it?

A page fault occurs when the MMU cannot find a virtual address in the page table or TLB. The kernel handles this by either allocating the necessary memory, swapping in data from secondary storage, or terminating the program if it's an illegal access.

How does virtual memory support larger applications than the physical RAM can handle?

Virtual memory allows programs to use more memory than physically available by swapping parts of the program in and out of disk storage. This creates the illusion that the system has more memory than it actually does, enabling the execution of larger applications.