Memory Management Memory Manager Requirements Minimize executable memory

Memory Management

Memory Manager • Requirements – Minimize executable memory access time – Maximize executable memory size – Executable memory must be cost-effective • Today’s memory manager: – Allocates primary memory to processes – Maps process address space to primary memory – Minimizes access time using cost-effective memory configuration – May use static or dynamic techniques

Address Space vs Primary Memory Process Address Space Mapped to object other than memory Hardware Primary Memory

Building the Address Space Source code Library code Secondary memory C Reloc Object code Other objects Link Edit Primary memory • Compile time: Translate elements • Link time: Combine elements Loader Process address • Load time: space • Allocate primary memory • Adjust addresses in address space • Copy address space from secondary to primary memory

Primary & Secondary Memory CPU • CPU can load/store • Ctl Unit executes code from this memory • Transient storage Load • Access using I/O operations • Persistent storage Primary Memory (Executable Memory) e. g. RAM Secondary Memory e. g. Disk or Tape Information can be loaded statically or dynamically

Static Memory Allocation Unused In Use pi Issue: Need a mechanism/policy for loading pi’s address space into primary memory Operating System Process 3 Process 0 Process 2 Process 1

Fixed-Partition Memory Mechanism Operating System pi needs ni units ni pi Region 0 N 0 Region 1 N 1 Region 2 N 2 Region 3 N 3

Fixed-Partition Memory -- Best-Fit Operating System Internal Fragmentation Region 0 N 0 Region 1 N 1 pi 2 Region N 2 Region 3 N 3 • Loader must adjust every address in the absolute module when placed in memory

Fixed-Partition Memory -- Worst-Fit Operating System pi Region 0 N 0 Region 1 N 1 Region 2 N 2 Region 3 N 3

Fixed-Partition Memory -- First-Fit Operating System pi Region 0 N 0 Region 1 N 1 Region 2 N 2 Region 3 N 3

Fixed-Partition Memory -- Next-Fit Operating System Region 0 N 0 pi 1 Region N 1 Pi+1 Region 2 N 2 Region 3 N 3

Variable Partition Memory Mechanism Operating System Process 0 Process 1 Process 6 Process 2 Process 3 Process 4 Process 5 Process 4 Loader adjusts every address in every absolute module when placed in memory Process 4 • External fragmentation • Compaction moves program in memory

Cost of Moving Programs load R 1, 0 x 02010 3 F 013010 Program loaded at 0 x 01000 3 F 016010 Program loaded at 0 x 04000 • Must run loader over program again! Consider dynamic techniques

Dynamic Memory Allocation • Could use dynamically allocated memory • Process wants to change the size of its address space – Smaller Creates an external fragment – Larger May have to move/relocate the program • Allocate “holes” in memory according to – Best- /Worst- / First- /Next-fit

Special Case: Swapping • Special case of dynamic memory allocation • Suppose there is high demand for executable memory • Equitable policy might be to time-multiplex processes into the memory (also space-mux) • Means that process can have its address space unloaded when it still needs memory – Usually only happens when it is blocked

Dynamic Address Relocation CPU Relative Address 0 x 02010 Relocation Register 0 x 10000 load R 1, 0 x 02010 + 0 x 12010 MAR • Program loaded at 0 x 10000 Relocation Register = 0 x 10000 • Program loaded at 0 x 04000 Relocation Register = 0 x 04000 We never have to change the load module addresses!

Runtime Bound Checking CPU Relative Address + Relocation Register Limit Register • Bound checking is inexpensive to add • Provides excellent memory protection < MAR Interrupt

L 1 Cache Memory L 2 Cache Memory “Main” Memory Optical Memory Sequentially Accessed Memory Faster access Primary (Executable) CPU Registers Rotating Magnetic Memory Secondary Larger storage Memory Hierarchies – Dynamic Loading

Exploiting the Hierarchy • Upward moves are (usually) copy operations – Require allocation in upper memory – Image exists in both higher & lower memories • Updates are first applied to upper memory • Downward move is (usually) destructive – Destroy image in upper memory – Update image in lower memory • Place frequently-used info high, infrequently-used info low in the hierarchy • Reconfigure as process changes phases

Memory Mgmt Strategies • Fixed-Partition used only in batch systems • Variable-Partition used everywhere (except in virtual memory) • Swapping systems – Popularized in timesharing – Relies on dynamic address relocation – Now dated • Dynamic Loading (Virtual Memory) – Exploit the memory hierarchy – Paging -- mainstream in contemporary systems – Segmentation -- the future

NT Memory-mapped Files Ordinary file • Open the file • Create a section object (that maps file) • Identify point in address space to place the file Section object Secondary memory Executable memory Memory mapped files