Mach

Main idea

• virtual memory management
• maintain all VM state in a Machine-independent module

• Treat hardware pages tables /TLBs as caches of machine independent info

• Difference?

  • Can allocate any region in VAS(large , sparce)
  • Children can inferit regions for sharing (copy-on-write)
  • Memory mapped files
  • User-level pagers and backing store

data strc

• Resident page table
  • Keep track of every physical page
  • Linked by
    • Memory object using the page
• Address maps
• Memory object
• pmap

resident page table

Memory obj. List

The memory obj

Represents A contiguous block of virtual memory ( a file) Tasks access by mapping obj into their address space Contains any resident pages and port for backing store. Physical….

The address map

|First | Current | Last| I \ | \ v v Could map code segment could map stack segment (inheritance = share) (inheritance = copy)

Each process has this address map

Inheritance

Code segment is shared between parent and child Child inherits a copy of data segment of

Pages in the data segment can either be

• copied to maintain UNIX fork() semantics
• shared if we want to create a thread instead of a regular UNIX process

shadow objects

Page replacement

• Similar to that of VAX VMS
• Global FIFO pool replacing resident sets of all programs
  • Much easier to tune
  • Does not support real-time processes
  • Can use external pages

Page fault

External pagers