© 2004, D. J. Foreman 1 Memory Management

© 2004, D. J. Foreman 2 Building a Module -1  Compiler ■ generates references for function addresses may be inside current file may be external - in a separate compile ■ addresses are 'marked' in compiler output - the "object" file  Linker or Link Editor ■ Resolves addresses (references) ■ Makes them relative to start of module ■ Saves relocatable module

© 2004, D. J. Foreman 3 Building a Module -2  Loader ■ Builds a table of addresses (ref table) References Definitions ■ replaces every relative address with sum of actual load address + relative address ■ gives control to the module ■ original relocatable file is still on disk  This is called "static binding" of the logical address to the physical address

© 2004, D. J. Foreman 4 Building a Module -3  Dynamic memory - malloc, etc ■ Linker examines all calls to malloc ■ Adds up requests (as much as possible) ■ Adds space for stack data  Loader assigns space for both ■ Heap space allocated from one end of block ■ Stack space allocated from other end ■ Grow toward middle  Ask O/S for more, when space is gone

© 2004, D. J. Foreman 5 Strategies  Fixed-size partition ■ Divide all RAM into N blocks ■ Program size must be < block size ■ Internal fragments exist - cannot be used  Variable-size partition ■ Blocks defined as needed Must keep track of blocks External fragmentation occurs and gets worse – Smaller holes left, as processes fill old holes – Favors smaller and smaller processes Eventually must move processes

© 2004, D. J. Foreman 6 Managing fragments  Move programs - compaction  Allocation strategies ■ Best fit - search for block with closest size ■ Worst fit - find biggest block - leaves largest hole when program terminates ■ First fit - use first hole in list, from front ■ Next fit - circular list, just use next entry in list  Part of block points to next ■ Helps manage 'garbage collection' - the process of building big blocks from little ones

© 2004, D. J. Foreman 7 Current Strategies  All use variable-sized partitions  Most use fixed sized blocks within the partition - will be called pages  Simplifies list mgmt  Other considerations: ■ Program segments don't (usually) change size ■ If pgm seg changes size, pgm must be moved ■ Loader must re-bind - increases overhead

© 2004, D. J. Foreman 8 Dynamic Binding  All addresses are relative to "0"  Adjust all addresses as in static binding  Repeat when moving program  Can be done with h/w relocation register  Simple relocation done easily  h/w adjusts every memory reference  Relocation value added as needed

© 2004, D. J. Foreman 9 Improvements  Multiple relocation registers ■ Code segment register - during fetch cycle ■ Stack segment register - for stack instructions ■ Data segment register - during execute cycle  Automatic relocation-register management ■ Compiler generates code to update register when references cross segment boundary ■ No auto-help for Assembler programs Programs with data segments > register capability Programs that change segment registers

© 2004, D. J. Foreman 10 Making it work  Trusted s/w for register manipulation ■ Privileged instructions ■ Compiler generates a segment trap ■ NOT a violation trap ■ O/S changes the relocation registers ■ O/S changes the Program Counter Also privileged ■ Program proceeds

© 2004, D. J. Foreman 11 Isolation  Need to ensure access protection  "Limit register" added ■ Contains length of segment ■ Compute: segment register + relative address ■ If relative address <= limit then proceed ■ Else segment fault (protection error)

© 2004, D. J. Foreman 12 Swapping  Swapping removes the entire address space  Cost analysis: ■ If a process has S memory space units ■ And a disk block holds D memory units ■ K=Ceil (S/D) disk writes required to save ■ And K reads to restore it later = 2*K overhead ■ If P i is blocked for T time, cost = T*S ■ T must be > X (assume 1 I/O = 1 Time unit)

© 2004, D. J. Foreman 13 Another strategy  Swapping removes an entire process  What if we only remove specific blocks?  Must recognize spatial locality  Sometimes called locality of reference  And temporal locality  We can remove/restore selected pages

© 2004, D. J. Foreman 14 Virtual Memory Mgmt  Responsibilities: ■ Determine locality ■ Keep referenced pages in memory  Problems: ■ Partitioning ■ Scatter-loading ■ Re-binding ■ Architecture ■ Construction ■ Actual

© 2004, D. J. Foreman 15 Shared Memory  Multiprocessors need to share  Increases bus load, reduces speed  Speedup via cache, but new problems: ■ Stale data (coherency) ■ Expense  Interconnection networks: ■ More speed ■ More expense Must detect changes everywhere Must force cache update synch