Memory Management

Address-space concept

• Protect processes from one another
• Protect the OS from user processes
• Provide efficient management of available storage

Virtual Address

• Who uses virtual address?

  • user processes
  • kernel processes
  • pretty much every piece of software

• Who uses physical address?

  • anything sitting on the bus & hardware
    • device
    • memory
    • CPU
  • The OS manages the physical address space by using the buddy system

• Why would you want to access a memory location?

  • To fetch a machine instruction
    • EIP (on an X86 machine), which contains a virtual address
  • To push EAX onto the stack
    • You need to specify a memory location to store the content of EAX
    • How do you know which memory location to write to?
      • ESP, which contains a virtual address
  • x = 123, where x is a local variable
    • EBP, which contains a virtual address

• Is there any CPU register that contains a physical address? NO

Address Translation

• Memory Management Unit (MMU)
  • attached to the bus
  • translates virtual address into physical address
  • MMU uses registers associated with the process the inner core is using
• Can be used for
  • protection/isolation
  • illusion of large memory
  • sharing
  • new abstraction

Memory Fence

• MMU uses a register that serves like a fence
  • if virtual address < fence
    • physical address == virtual address
  • else
    • MMU gives fault, traps into the OS, OS kills the program
• What if the user program won't fit in memory?
  • use overlays (piece of program)
    • programs are divided into overlays
    • only one overlay can be in physical memory at a time

Base and Bounds Registers

• OS maintains a pair of base and bounds registers for each user process

  • bounds register: address space size of the user process
  • base register: start of physical memory for the user process

• Address relative to the base register

• Virtual memory reference >= 0 and < bounds, independent of base (this is known as position independence)

• MMU registers are part of the context of a process

Generalization of Base and Bounds: Segmentation

• One pair of base and bounds registers per segment
  • code, data, heap, stack, and may be more
  • e.g. compiler compiles programs into segments

Access Control with Segmentation

• Add every bit of Read-only or Read/Write to base & bound register pair in MMU

Sharing segments

• Can simply setup base and bounds registers to share segments
• Read-only can be shared
• Read-write should have their own segments

Segmentation Fault

Cases
1. Virtual address is not within range of any base-bounds registers
2. Access is incompatible (e.g. Write to read-only)

Memory Mapped File

• the mmap() system call
• can map an entire file (or part of it) into a segment
• extra base and bound register is needed

Copy-On-Write

• A process gets a private copy of the segment after a thread in the process performs a write for the first time

Steps
1. When the process performs a write, the OS gets fault and sees that the memory segment is private, it does not generate a seg fault
2. It takes the data and makes a copy of it
3. Then changes the address to a new address of the copy, and changes from RO to RW
4. The user program tries again to write to the new data (private copy)