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COMP1521 Week 7 Memory Management

This week is a bit hard and need more time to fully absorb.

Memory Manamagement

In modern machines, multiple processes can be loaded in memory. To calculate their address, each process’s base address.

Add a new process

Adding a new process where there is no one continous piece for the process, the process need to be splited into several regions on physical memory.

• each chunk of process address space has its own base
• each chunk of process address space has its own size
• each chunk of process address space has its own memory location

Address processToPhysical(pid, addr)
{
   Chunk chunks[] = getChunkInfo(pid);
   for (int i = 0; i < nChunks(pid); i++) {
      Chunk *c = &chunks[i];
      if (addr >= c->base && addr < c->base+c->size)
         break;
   }
   uint offset = addr - c->base;
   return c->mem + offset;
}

Mapping virtual address to physical address

The mapping must be done in hardware to be efficient.

If each chunck are of the same size, the mapping and mapping table would be simpler.

Address processToPhysical(pid, Vaddr)
{
   PageInfo pages[] = getPageInfo(pid);
   uint pageno = Vaddr / PageSize;  // int div
   uint offset = Vaddr % PageSize;
   return pages[pageno].mem + offset;
}

Mapping from virtual address to physical address

Computation of pageno,offset is efficient if Pagesize == 2^n

Virtual memory

We dont need to load all of the process’s pages up-front, and just need to load new process address pages into memory as needed. The strategy of dividing process memory space into fixed-size pages and on-demand loading of process pages into physical memory is called virtual memory.

Pages/frames are typically 512B .. 8KB in size.
Each frame can hold one page of process address space.

typedef struct {char status, uint frameNo, ...} PageData;

PageData *AllPageTables[maxProc];
    // one entry for each process

Address processToPhysical(pid, Vaddr)
{
   PageData *PageTable = AllPageTables[pid];
   uint pageno = PageNumberFrom(Vaddr);
   uint offset = OffsetFrom(Vaddr);
   if (PageTable[pageno].status != Loaded) {
      // load page into free frame
      // set PageTable[pageno]
   }
   uint frame = PageTable[pageno].frameNo;
   return frame * P + offset;
}