CPS110: Address translation Landon Cox February 19, 2008

 Translator: just a data structure  Tradeoffs  Flexibility (sharing, growth, virtual memory)  Size of translation data  Speed of translation Dynamic address translation User process Translator (MMU) Translator (MMU) Physical memory Physical memory Virtual address Physical address

1. Base and bounds  For each process  Single contiguous region of phys mem  Not allowed to access outside of region  Illusion own physical mem [0, bound)

1. Base and bounds Virtual memoryPhysical memory 0 Bound Base Base + Bound 0 Size of phys mem

1. Base and bounds  Translator algorithm  Only kernel can change base, bound if (virtual address > bound) { trap to kernel kernel can kill process with segmentation fault } else { physical address = virtual address + base }

2. Segmentation  Segment  Region of contiguous memory  (both virtually and physically)  Idea  Generalize base and bounds  Create a table of base and bound pairs

2. Segmentation Segment #BaseBound Segment Segment Segment 2Unused Segment Code Data segment Stack segment  Virtual address has two parts 1.Segment # (could be in high-order bits) 2.Offset (e.g. low-order bits of address) Virtual info Physical info Same for both

Virtual addresses VA={b 31,b 30,…,b 12,b 11,…,b 1,b 0 } Low-order bits (offset) High-order bits (segment number)

2. Segmentation Segment #BaseBound Segment Segment Segment 2Unused Segment Code Data segment Stack segment Virtual memory (3,fff). (Stack) (3,000) … (1,4ff). (Data) (1,000) … (0,6ff). (Code) (0,0) Physical memory 46ff. (Code segment) 4000 … 2fff. (Stack segment) 2000 … 4ff. (Data segment) 0 Segment # Offset

2. Segmentation Segment #BaseBound Segment Segment Segment 2Unused Segment Code Data segment Stack segment  Not all virtual addresses are valid  Nothing in segment 2  Nothing in segment 1 above 4ff  Valid = part of process’s address space  Accesses to invalid addresses are illegal  Hence a “segmentation fault” Virtual memory (3,fff). (Stack) (3,000) … (1,4ff). (Data) (1,000) … (0,6ff). (Code) (0,0)

2. Segmentation Segment #BaseBound Segment Segment Segment 2Unused Segment Code Data segment Stack segment  Segments can grow  (can move to new physical location)  Protection  Different protections for segments  E.g. read-only or read-write  B&B forced uniform protection Virtual memory (3,fff). (Stack) (3,000) … (1,4ff). (Data) (1,000) … (0,6ff). (Code) (0,0)

2. Segmentation Segment #BaseBound Segment Segment Segment 2Unused Segment Code Data segment Stack segment  What changes on a context switch?  Contents of segment table  Typically small (not many segments) Virtual memory (3,fff). (Stack) (3,000) … (1,4ff). (Data) (1,000) … (0,6ff). (Code) (0,0)

Segmentation pros and cons  Pros  Multiple areas of address space can grow separately  Easy to share parts of address space  (can share code segment)

Segmentation sharing Segment #BaseBound Segment Segment Segment 2Unused Segment Code Data segment Stack segment Segment #BaseBound Segment Segment Segment 2Unused Segment Code Data segment Stack segment Virtual infoPhysical info

Segmentation pros and cons  Pros  Multiple areas of address space can grow separately  Easy to share parts of address space  (can share code segment)  Cons  Complex memory allocation  (still have external fragmentation)

2. Segmentation  Do we get virtual memory?  (can an address space be larger than phys mem?)  Segments must be smaller than physical memory  Address space can contain multiple segments  Can swap segments in and out  What makes this tricky?  Performance (segments are relatively large)  Complexity (odd segment sizes  packing problem)  Solution: fixed-size segments called pages!

Course administration  Wednesday discussion section  Moved to D344 (same time)  Project 1 drawing to a close  Median score: 67% (not so hot)  Last semester at this time median was 98%  One group with 100% (another is close)  Some final tips …

Course administration  What to carefully reread  Section 4.4 of the spec (ordering stuff)  Thread implementation slides  Lock implementation slides  I see a lot of code that ignore these sources  Other hints …

Course administration /* * Allocate space for thread control block and * new stack. */ try { thread_ptr = new thread_t; thread_ptr->stack = NULL; thread_ptr->stack = new char [stack_size]; } catch(bad_alloc) { if (thread_ptr != NULL) { if (thread_ptr->stack != NULL) { delete [] thread_ptr->stack; } delete thread_ptr; } return(-1); }

Course administration  Project 2 (virtual memory) out next Thursday  In some ways easier than P1  In some ways harder than P1 (not given a solution)  Next deadline: midterm exam (February 26 th )  In-class  One sheet (two sides) of notes  No laptops or other electronic resources  Practice exam in discussion section this week

Midterm exam  What will be covered?  All threads/concurrency material  Project 1  Lectures  Homework problems

Analysis vs synthesis  No regurgitation questions  Analysis questions  Analyze system/program/design  Trace through program  Compute performance  Determine properties (deadlock, starvation, races)  Synthesis  Design/construct a new system/program/design  I like these more (e.g. learn the most doing projects)

How to study  Do homework problems  Understand Project 1  In groups  Create and answer questions I might ask  (you might get lucky; happened in past)

Syntax for programs  Real or pseudo is fine (just be clear)  STL is fine  queue.{push, pop, front, empty}  Monitors  Project 1 syntax or OO syntax  thread_lock (lock_num) or mutex.lock ()  Semaphores  OO syntax  sem1.up (), sem1.down (), sem2.up (), …  There is no “getVal” call!  Remember to specify initial value!!

Syntax for traces  Thread: line number (or range of lines) 1: checkMilk () { 2: if (noNote) { 3: leave note 4: if (noMilk){ 5: buy milk; 6: } 7: remove note; 8: } 9: } Landon: 1-2 Melissa: 1-2 Landon: 3-6 Melissa: 3-6

On-board review

3. Paging  Very similar to segmentation  Allocate memory in fixed-size chunks  Chunks are called pages  Why fixed-size pages?  Allocation is greatly simplified  Just keep a list of free physical pages  Any physical page can store any virtual page

3. Paging  Virtual addresses  Virtual page # (e.g. upper 20 bits)  Offset within the page (e.g. low 12 bits)  Each address refers to a byte of data  For 12-bit offset, how big are pages?  4KB (2^12 different offsets)

3. Paging  Translation data is a page table Virtual page #Physical page # invalid …… Invalid invalid 2^20 – 1 Why? Why no bound column?

3. Paging  Translation data is a page table Virtual page #Physical page # invalid …… Invalid invalid How does page table’s size compare to segment table’s? Page table must reside in memory. Too large for registers alone.

3. Paging  Translation process if (virtual page # is invalid) { trap to kernel } else { physical page = pagetable[virtual page].physPageNum physical address is {physical page}{offset} }

3. Paging  What happens on a context switch?  Copy out contents of entire page table  Copy new page table into translation box  (hideously slow)  Instead, use indirection  Change page table base pointer (register)  Should point to new page table  Does this look like anything else we’ve seen?  Stack pointer

3. Paging  Pages can be in memory or swapped to disk  “Swapped out” = “paged out”  How can the processor know?  Is it in physical memory or paged out to disk?  Page table entry must have more info  Resident bit

3. Paging  Resident bit in page table entry  If bit is 1 (vp is in physical memory)  Page table entry has valid translation  Use to look up physical page  If bit is 0 (vp is swapped out)  MMU causes a page fault to OS  (runs kernel’s page fault handler)  (just like how timer interrupts are handled)

3. Paging  On a page fault, what does the OS do?  OS finds free physical page  (may have to page out another virtual page)  OS does I/O to read virtual page into mem  (or zero fills it, if it’s a new page)  What happens next?  OS reruns process’s at last faulting instruction  How does it know the last instruction?  Hardware provides process’s PC to handler on fault

Valid vs. resident pages  Resident = page is in memory  Accessing non-resident pages is not an error  Access triggers page fault, OS handles it  Valid = page is legal to address  Who makes a virtual page resident (or not)?  OS memory manager (allocating physical memory)

Valid vs. resident pages  Resident = page is in memory  Accessing non-resident pages is not an error  Access triggers page fault, OS handles it  Valid = page is legal to address  Who makes a virtual page valid (or not)?  User process controls this (with limits from OS)

Valid vs. resident pages  Resident = page is in memory  Accessing non-resident pages is not an error  Access triggers page fault, OS handles it  Valid = page is legal to address  Why would a process make a page invalid?  Make the process fail-stop  Accidental/buggy references kill the process  Same for process making pages read-only  (writes to code segment are probably bugs)