Allocation of Frames

• Now, first major problem mentioned in Section 1.4.1 will be discussed. How do we allocate the fixed amount of free memory among the various processes?
• If we have 93 free frames and two processes, how many frames does each process get?
• The simplest case is the single-user system.
  • Consider a single-user system with 128 KB of memory composed of pages 1 KB in size. This system has 128 frames.
  • The OS may take 35 K8, leaving 93 frames for the user process.
• Under pure demand paging, all 93 frames would initially be put on the free-frame list.
  • When a user process started execution, it would generate a sequence of page faults.
  • The first 93 page faults would all get free frames from the free-frame list.
  • When the free-frame list was exhausted, a page-replacement algorithm would be used to select one of the 93 in-memory pages to be replaced with the 94th, and so on.
  • When the process terminated, the 93 frames would once again be placed on the free-frame list.
• There are many variations on this simple strategy. We can require that the OS allocate all its buffer and table space from the free-frame list.
• When this space is not in use by the OS, it can be used to support user paging. The user process is allocated any free frame.