Memory Management Operating Systems Course Fall 2017 Erfan Sharafzadeh Iran University of Science and Technology The Abstraction: Address Spaces Interlude: Memory API Agenda Mechanism: Address Translation Segmentation Open Sses: The Ey Pi (Rem . Ara-Dusu d Ane C. Ara-Dusu) 2 The Abstraction: Address Spaces Early Systems Now 3 Virtualizing the Memory: Goals Transparency Efficiency The OS should implement virtual 1. Time memory in a way that is invisible to 2. Space the running program. OS will need hardware support! Protection Deliver the property of isolation among processes. 4 EVERY ADDRESS YOU SEE IS VIRTUAL location of code : 0x1095afe50 location of heap : 0x1096008c0 location of stack : 0x7fff691aea64 5 Memory API Stack H ➔ To what extent are they controlled by the OS or language runtime? ➔ What is their scope? ➔ What determines the size of each of them? ➔ What makes one faster? Take a look: https://stackoverflow.com/questions/79923/what-and-where-are-the-stack-and-heap 6 Linux Memory Management ● The malloc() Call ● The free() Call ● Some Errors: ○ Segmentation fault! => YOU DID SOMETHING WRONG WITH MEMORY YOU FOOLISH PROGRAMMER AND I AM ANGRY. ○ Forgetting To Free Memory ○ Freeing Memory Before You Are Done With It => Dangling Pointer! ● Underlying OS Support ○ brk() ○ mmap() 7 Mechanism: Address Translation ● manage memory: keeping track of which locations are free and which are in use, and judiciously intervening to maintain control over how memory is used. ● Once again the goal of all of this work is to create a beautiful illusion: that the program has its own private memory, where its own code and data reside. Behind that virtual reality lies the ugly physical truth: that many programs are actually sharing memory at the same time, as the CPU (or CPUs) switches between running one program and the next. 8 Three Assumptions ● User’s address space must be placed contiguously in physical memory ● The size of the address space is not too big; specifically, that it is less than the size of physical memory. ● Each address space is exactly the same size. 9 An Example void func() { int x = 3000; // thanks, Perry. x = x + 3; // this is the line of code we are interested in } 128: movl 0x0(%ebx), %eax ;load 0+ebx into eax 132: addl $0x03, %eax ;add 3 to eax register 135: movl %eax, 0x0(%ebx) ;store eax back to mem 10 An Example: Process Address space • Fetch instruction at address 128 • Execute this instruction (load from address 15 KB) • Fetch instruction at address 132 • Execute this instruction (no memory reference) • Fetch the instruction at address 135 • Execute this instruction (store to address 15 KB) 11 An Example: Physical Memory dynamic relocation: using base and bounds registers: physical address = virtual address + base Bounds register help with protection All of these need hardware support Exception handling in OS 12 An Example: Hardware Support 13 Put it together: 1 14 Put it together: 2 15 Put it together: 3 16 Segmentation What segmentation allows the OS to do is to place each one of (CODE, STACK & HEAP) segments in different parts of physical memory, and thus avoid filling physical memory with unused virtual address space. 17 Segmentation ASIDE: THE SEGMENTATION FAULT! ● arises from a memory access on a segmented machine to an illegal address. ● Humorously, the term persists, even on machines with no support for segmentation at all. ● Or not so humorously, if you can’t figure why your code keeps faulting. 18 Segmentation ● A more complete hardware support: ● External fragmentation ○ Fitting algorithms: best-fit, worst-fit, first-fit, buddy 19 What’s left... ● Free space management ● Paging ● TLB 20.