1 / 17

Memory Addressing in Linux

Memory Addressing in Linux. Logical Address machine language instruction location Linear address (virtual address) a single 32 but unsigned integer Physical address address in memory chip. Logical address translation. Segmentation Unit. Paging Unit. Logical address. Linear Address.

bob
Download Presentation

Memory Addressing in Linux

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Memory Addressing in Linux • Logical Address • machine language instruction location • Linear address • (virtual address) • a single 32 but unsigned integer • Physical address • address in memory chip

  2. Logical address translation Segmentation Unit Paging Unit Logical address Linear Address Physical Address

  3. Segmentation in Linux • Linux uses segmentation only when required by the 80x86 architecture • All processes use the same logical addresses • All segments descriptors are stored in the Global Descriptor Table • Segments used by Linux • Kernel code segment • DPL = 0 (Kernel Mode) • Type = 0xa(read and execute) • Kernel data segment • DPL = 0 • Type = 2 (read and write) • User code segment • DPL = 3 (User Mode) • Type = 0xa • User data segment • DPL = 3 • Type = 2 • Task State Segment (TSS) • Default Local Descriptor Table (LDT) • Advanced Power Management (APM) • 4 segments

  4. Paging in Linux • In 80x86 processors, paging is enabled by setting the PG flag of a control register named cr0. • When PG = 0, linear addresses are interpreted as physical addresses • When PG = 1, physical addresses are computed from the page table • The 32 bits of a linear address are divided into three fields: • Directory • most significant 10 bits • Table • intermediate 10 bits • Offset • The least significant 12 bits

  5. Paging in Linux (80x86) Directory Table Offset 4096 + + Page + Page Table cr3 Page Directory

  6. Extended Paging (Pentium) Directory Offset 4 MB Enabled by setting the PSE flag of the cr4 register + + cr3 Page Page Directory

  7. Three-Level Paging • Used in 64-bit architectures • HP Alpha • Page frames are 8kb • offset field is 13 bits • Only the least significant 43 bits of an address are used • Three-levels of page tables • 3 10-bit page tables

  8. Physical Address Extension (PAE) Paging Mechanism • Kernel cannot directly address more than 1 GB of RAM • Starting with the 80386 • 36 address pins = 64 GB • Pentium Pro • Physical Address Extension (PAE) • Set PAE flag in the cr4 control register

  9. Change to Paging Mechanism to support PAE • 64 GB split into 2^24 distinct page frames • page tables expanded from 20 to 24 bits • new level of page table called page directory pointer table (PDPT) • cr3 control register contains a 27-bit Page Directory Pointer Table base address • When mapping 4KB pages (ps flag cleared) • cr3 -> PDPT • bits 31-30: one of 4 entries in PDPT • bits 29-21: one of 512 entries in Page Directory • bits 20-12: one of 512 entries in Page Table • bits 11-0: Offset • When mapping 2MB pages (ps flag set) • cr3 -> PDPT • bits 31-30: one of 4 entries in PDPT • bits 29-21: one of 512 entries in Page Directory • bits 20-0: Offset

  10. Hardware Cache • Use the CD flag of the cr0 processor register to enable cache • Each page can have a different cache policy • Page Directory and each page table entry includes two flags • PCD (page cache disable) • PWT (page write-through) • Each 80x86 processor has its own local TLB – located in cache

  11. Loading Linux • Linux kernel is installed in RAM starting from 0x0010000 (2nd megabyte) • typical configurations give a kernel size < 2MB • BIOS uses first meg

  12. Process Page Tables • linear addresses from 0x00000000 to 0xbfffffff either user or kernel mode • linear addresses from 0xc0000000 to 0xffffffff can only be addressed in kernel mode • PAGE_OFFSET = 0xc0000000

  13. Kernel Page Tables • The kernel maintains a set of Page Tables for its own use • rooted at the Master Kernel Page Global Directory • After system initialization • This set of page tables never used directly • The highest entries are the reference model for the corresponding entries of the PGD of every regular process in the system

  14. Provisional kernel page tables • initialized statically during kernel compilation • during initialization need to be able to first 8megs are accessed directly • But were using paging • So an identity mapping is performed

  15. Fix-Mapped Linear Addresses • The 4th gigabyte is reserved for a direct mapping of physical memory • a fixed mapped linear address is a constant linear address like 0xfffffdf0 whose corresponding physical address can be set up in an arbitrary way • Linear address X maps to physical-Address X-PAGE_OFFSET

  16. Noncontiguous Memory Area Management • It is preferable to map memory areas into sets of contiguous page frames • However, if requests for memory areas are infrequent, it makes sense to use an allocation schema based on noncontiguous page frames accessed through contiguous linear addresses • Linux uses noncontiguous memory areas in several ways • Ex: to allocate data structures for active swap areas

  17. Linear addresses interval high_memory PKMAP_BASE VMALLOC_END 4 GB PAGE_OFFSET VMALLOC_START Physical memory mapping vmalloc area vmalloc area Persistent kernel mappings Fix-mapped linear addresses 8 MB 4KB high_memory = linear address that corresponds to the end of directly mapped physical memory

More Related