Memory Management: Background

MEMORY MANAGEMENT: BACKGROUND

ü In general, to rum a program, it must be brought into memory.

ü Input queue – collection of processes on the disk that are waiting to be brought into memory to run the program.

ü User programs go through several steps before being run

Address binding: Mapping of instructions and data from one address to another address in memory.

Three different stages of binding:

1     Compile time: Must generate absolute code if memory location is known in prior.

2     Load time: Must generate relocatable code if memory location is not known at compile time

3        Execution time:    Need hardware support for address maps (e.g., base and limit

registers).

Logical vs. Physical Address SpaceLogical address – generated by the CPU; also referred to as “virtual address“

Physical address – address seen by the memory unit.

Logical and physical addresses are the same in compile-time and load-time address-binding schemes

 Memory-Management Unit (MMU)

ü It  is  a hardware device that  maps virtual /  Logical address to  physical address.

ü In this scheme, the relocation register‘s value is added to Logical address generated by a user process.

ü The user program deals with logical addresses; it never sees the real physical addresses

v Logical address range: 0 to max

v Physical address range:  R+0  to  R+max, where R—value in  relocation.

Dynamic relocation using relocation register

Dynamic Loading

ü Through this, the routine is not loaded until it is called.

o  Better memory-space utilization; unused routine is never loaded

o  Useful when large amounts of code are needed to handle infrequently occurring cases

o No special support from the operating system is required implemented through program design

Dynamic Linking

ü Linking postponed until execution time & is particularly useful for libraries

ü Small piece of code called stub, used to locate the appropriate memory resident library routine or function.

ü Stub replaces itself with the address of the routine, and executes the routine

ü Operating system needed to check if routine is in processes Memory addresses Shared libraries.

ü Programs linked before the new library was installed will continue using the older library.