Paged Virtual Memory Definition Accuracy

The Page size is fixed by the Offset maxima which sets the upper limit of a Page frame in physical memory. In a Page Table, the Offset bits are used to store process control information as it has no significance in the virtual context.

Now, we know enough to perform an address-translation example. Let’s imagine the process with that tiny address space (64 bytes) is per- ... Va0 the lowest-order bit. Because we know the page size (16 bytes), we · can further divide the virtual address as follows: ... Thus, we have a 2-bit virtual page number (VPN). The remaining bits tell · us which byte of the page we are interested in, 4 bits in this case; we call ... Instead, we store the page table for each process in memory ...

For each process, a page map defines the base address of each of that process' pages along with read-only and "present" bits. Page map stored in contiguous memory (with base register in hardware). Translation process: page number always comes directly from the address.

Since there are 4K bytes in a cache block, the offset field must contain 12 bits (212 = 4K). The remaining 20 bits are page number bits. Thus a logical address is decomposed as shown below. Virtual memory address translation uses a page table. In the diagram to the left, a page table is represented ...

Remember, accesses to the disk are much, much slower than main memory—so the OS tries to intelligently place frequently-accessed data in memory. The goal ends up very much like CPU caches: it exploits temporal and spatial locality to maximize the number of accesses that go to main memory, not to disk. The strategy for implementing swapping is to mark paged-out memory as invalid in the page table. Remember that, when the CPU tries to access any virtual address, it must first consult the page table to perform protection checks (and to do the address translation).

This physical page number, concatenated with the page offset from the virtual address, is the physical address. Hardware uses various optimizations to speed up this process, notably using the Translation Lookaside Buffer. Address translation is designed so that the hardware guarantees that · Only kernel-mode code can change the page table. All memory accesses are guarded by the page table.

Use pages to partition segments into fixed size chunks · ◆Makes segments easier to manage within physical memory · » Segments become “pageable” – rather than moving segments · into and out of memory, just move page portions of segment · ◆Need to allocate page table entries ...

Since the segments are defined by the compiler, the segment table can be fixed at link time. The table contains one entry for each segment. The entry contains the base and limit for the segment. The base is the physical address of the beginning of the segment in memory. The limit is the length of the segment, so that the last physical address within the segment is specified by the base plus the bound (base + bound). Since each program enjoys its own virtual address space, each process has it's own segment table.

Previous discussions have centered on process memory, which can be conveniently broken up into page-sized chunks, and the only fragmentation that occurs is the average half-page lost to internal fragmentation for each process ( segment.