Memory Management in Modern OSes: Paging and Caches

Modern operating systems use two ideas to manage memory: paging and caches. Paging divides the program’s memory into small blocks called pages and maps them to physical memory. Caches sit closer to the CPU and keep recently used data ready. Together, paging and caches help keep programs safe, fast, and responsive.

Paging basics are simple in concept. A process sees a virtual address space, split into pages. The OS stores a page table that translates each page number to a physical frame in RAM. Each page table entry carries the frame number plus flags such as read/write access and whether the page is allowed for the current process. The hardware uses a translation lookaside buffer, or TLB, to speed up these translations. When the CPU accesses data, the TLB check is quick; if the data is not there, a longer page table walk happens, and the translation is filled in. If the data is not in RAM, a page fault occurs. The operating system then loads the needed page from disk, updates the table, and restarts the access.

As programs and data grow, OSes use multi-level page tables. A single giant table would be enormous, so levels help locate the right entry more efficiently. The TLB remains the fast path, but a miss means a walk through the levels, which adds latency. This balance between fast translations and larger address spaces is a core part of memory performance.

CPU caches add another layer. L1, L2, and sometimes L3 caches store copies of instructions and data in fixed-size lines near the core. A memory request first checks the cache: a hit returns data quickly, while a miss fetches from a higher cache level or memory. Caches generally work with physical addresses, but some designs use virtual addresses for speed, which then requires careful handling when paging changes mappings.

The interaction between paging and caching matters for performance. After a translation, the data path may go through the cache. If a page is swapped out, a page fault blocks the access until the page is brought back into RAM. Good locality—using data you were just using—helps both paging and caching stay efficient. Modern operating systems tune replacement policies and prefetching to keep the most active pages in memory and the most useful data in caches.

In practice, developers notice faster apps when memory access shows strong locality. Understanding paging and caches helps explain why some programs feel snappy on one machine but slower on another, especially under heavy load or with large data sets.

Key Takeaways

• Paging creates virtual memory and enforces process isolation.
• Caches speed memory access and depend on locality of reference.
• The TLB accelerates address translation; page faults bring data from disk.
• Multi-level page tables save memory, but a TLB miss can add latency.
• The synergy of paging and caching shapes overall system performance.