CIS250 OPERATING SYSTEMS Memory Management Since we share memory, we need to manage it Memory manager only sees the address A program counter value indicates.

CIS250

OPERATING SYSTEMS

Memory Management

• Since we share memory, we need to manage it

• Memory manager only sees the address

• A program counter value indicates which memory address the CPU should fetch

• The instruction is decoded, operated and might be re-saved to memory

Chapter 9- Memory Management• Background

– an array of bytes containing an address– The CPU gets the instruction from memory based on

the program counter value– The memory unit sees the data as a stream of addresses

• The program is compiled and converted into an executable

• Program is loaded into memory and associated with a process; the process accesses the data and instructions from memory

• Process is swapped in and out

Binding

• Setting the actual address used by the relocator

• The sequence of addressing by a program– Address Binding– Dynamic Loading– Dynamic Linking– Overlays

Address Binding

• Binding the instructions and data to memory addresses

• The source program memory addresses are symbolic 0-x– The compiler binds this to relocatable addresses– The loader will bind relocatable to absolute

addresses 0-x• 14 bytes from the beginning of the module 74014

• The binding of instructions and data to memory can be done at:– compile time - when you know at compile time where

program will be in memory an absolute code is generated (.COM in DOS)

– load time - don’t know address at compile time, compiler generates a relocatable code; an offset in a program

– execution time - combined with special hardware, when a program is moved during exe

Dynamic Loading

• Routine is not loaded until it is called; good memory utilization– example: main program is loaded into memory; but

function is not loaded until it is called

• The relocatable linking loader will load and update the program address table

• Advantage: unused programs don’t take up memory (error rtns); done programatically, no extra support from the O/S

Dynamic Linking

• Opposite of static. With static linking, language libraries are combined into the binary program image (takes up more space)

• With dynamic, linking is postponed. A stub tells the linker (via an address) to to locate the library routine

• The O/S must allow multiple processes to access the same memory address

Overlays• Segments of routines

• Keep only the data and instructions in memory that are needed. A overlays B in mem

• Good for when your total program size > than the memory allocated

• No extra support from the O/S; programmatically ensure that overlays don’t interfere

• Used on systems with low resources

Address Space

• Logical - address generated by the CPU

• Physical - the address loaded into the memory address register; the set of all physical addresses corresponding to the logical address. The user program never sees this address.

• For compile and load time address binding, the logical and physical addresses are the same

• With executable and address binding, the logical (virtual) and physical differ. This is central to proper memory management.

• MMU - memory management unit - maps from virtual to physical address. The base register is the relocatable register. This value is added to each user process generated address. Ie 0 + 14000 (offset)

Swapping

• Copying a process from memory to disk to free up memory space for other processes

Swapping

• A process is swapped out of memory and placed in the backing store. (a backing store is a fast disk with a ready queue)– If binding is done at assembly or load, the

process must be placed in the same location– If binding done at execution, the process can be

placed anywhere.

• Context switch time - the time it takes to swap one process out, swap another in

• the transfer time depends on the amount of memory swapped. It should be low relative to the CPU time. The swap space is separate from the the file system.

• When you swap, make sure the process is idle. Otherwise an operation (I/O) may try to access the memory of the new process

• In WIN3.1, the user controls. In NT, MMU features: the scheduler can decide on the process.

• The relocation register - the base register is used to give the lowest physical address for a process. Add this to the user process to get the actual address.

• Main memory supports the O/S and the user programs

• Partitioned - the O/S and user; O/S usually loaded low (near the interrupt vector)

Contiguous Allocation

• Single Partition - to protect the O/S from the user; to protect the user programs from each other. Relocation and limit register. MMU maps dynamically and is loaded as part of the context switch.

• Multiple Partition - divide memory into fixed-sized partitions - each with one process. The degree of multi-programming = the number of partitions. IBM360 used.

• Fragmentation– External

– Internal

Fixed Partitioning• Batch processing

• The O/S keeps track of what memory is available - hole. When a process needs memory, the O/S searches for the holes.

• FCFS and RR examples.

Fixed Partitioning• Algorithms

– First fit - the first hole that’s big enough; fastest– Best fit - the smallest hole that’s big enough.

Good for storage utilization, but you search the entire list if not sorted by size.

– Worst fit - allocate the largest hole; searches entie list unless sorted; produces largest leftover hole - can be allocated to other processes.

Contiguous Allocation

• Single Partition

• Multiple Partition

• Fragmentation– External

– Internal

Paging• Method

• Page Table Structure

• Multilevel Paging

• Inverted Page Table

• Shared Pages

Segmentation

• Method

• Hardware

• Implementation of Segment Tables

• Protection and Sharing

• Fragmentation

Chapter 10 - Virtual Memory

Virtual Memory

• Background

• Definition

Demand Paging

• Definition

• Process

• Hardware support– Page Table– Secondary memory

Demand Paging Performance

• Page fault

• Effective access time computation

Page Replacement

• Process

• Algorithms– FIFO– Optimal– LRU

LRU Approximation

• Additional-Reference-Bits

• Second-Chance

• Enhanced-Second-Chance

• Counting Algorithms

• Page Buffering

• Allocation of Frames

• Thrashing

• Other Considerations

• Demand Segmentation