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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.

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Memory Addressing in Linux

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  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

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