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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
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
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
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.
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