Operating System Chapter 8: Memory management services

Absolute address

Relative address bound to

Limit register

Range of the physical addresses of a process.

Page table

Table containing page indices and offsets.

Logical address space

Set of all logical addresses of a program.

Pure code

Syn. "Reentrant code".

TLB miss

When the page number is not in the TLB, and a reference to that page must be made.

Physical address

Address as seen by the memory unit.

Logical address

Address generated by the CPU.

Relocatable address

Adress relative from the beginning of the program, assigned by the compiler.

Relocation register

Base register used by the MMU, to relocate the user code to the corresponding physical address space.

Execution-time address binding

Binding delayed until run-time, so that the process can move in memory. Special hardware needed, and logical and physical address spaces differ.

Paging

Breaking physical memory into fixed-sized blocks (frames) and logical memory into pages. No external fragmentation.

Memory stall

CPU has to wait because not all data from the main memory is available.

Relocatable code

Code of which it is not known at compile time where it will reside in memory. Change of starting address requires the code to reload.

Reentrant code

Code that does not change during execution, can be shared (usually through shared pages).

Absolute code

Code that will be in a fixed place in memory, generated at compile time. Complete recompilation needed if the starting address changes.

Frame table

Contains all physical frames, if they are free or not, and which process is allocated to it.

Static linking

Dynamically linked libraries - Copying of the library code at compile time.

Dynamic linking

Dynamically linked libraries - Linking occurs at execution-time, using stubs that tell the loader how to load the library and which one to load.

Stub

Dynamically linked libraries - Reference telling how to load a system library using dynamic linking.

Translation-look-aside buffer (TLB)

Fast-access buffer containing a part of the page table, with the physical address. Avoids two memory accesses (one to the page table and one to the byte).

CPU-register

Fast-access memory on CPU, accessible within one CPU-cycle.

Load-time address binding

Final binding is delayed until load time. Same logical and physical address space.

Best-fit

Find the smallest hole that can satisfy the process-load request. Produces the smallest leftover hole.

Worst-fit

Finds the biggest possible hole, creating the biggest leftover hole.

50-percent rule

For every N allocated blocks, another 0,5N blocks will be wasted due to external fragmentation.

Internal fragmentation

Free, unused memory within a fixed size block.

Compile-time address binding

Generation of absolute code (fixed location in main memory). Same logical and physical address space.

Hole

Group of free partitions.

Clustered page table

Hash page table for 64-address bit spaces, with each entry mapping several pages.

Forward-mapped paging

Hierarchical paging, starting from the outer page table inward.

Page number

Index in a page table.

Relocatable link loader

Loads the routine into memory and updates the program's address tables

Segmentation

Logical address space is a collection of segments, with a name and a length.

Virtual address

Logical address that differs from the physical address.

Binding

Mapping an address space to another.

Sparse memory space

Memory space with a lot of non-contiguous memory references.

Variable-partition method

OS keeps a table of free partitions, using different allocation algorithms.

Transient OS code

OS services that come and go as needed.

Page offset

Offset in the page base address.

Segment table

Ordered table maintained by the hardware, containing the segment base and segment limit.

Hierarchical paging

Page table can page a page table, to limit the size of the page table.

Hashed page table

Page table that can handle address spaces larger than 32-bits, with the hash value being the virtual page number.

Page-table base register (PTBR)

Pointer to page tables stored in main memory.

Memory address register

Register in the memory unit where the physical address will be loaded into.

Page-table length register (PTLR)

Register indicating the size of the page table.

Physical address space

Set of all addresses corresponding to the logical addresses.

Compaction

Shuffling the blocks to make a big hole. Solves external fragmentation, resource and time intensive.

Base register

Smallest legal physical address of a process.

First-fit

Stop looking for holes as soon as one is found big enough to satisfy the request.

Backing store

Storage devices where processes are temporarily swapped to from the main memory.

Ready queue

Swapping - Queue maintained by the system of all the processes in the backing store.

Dynamically linked library

System library linked to user programs during either compile-time or by using stubs.

Swapping

Temporarily moving processes in and out of the main memory to a backing store, to increase the degree of multiprogramming.

Two-level paging

The page table is also paged, limiting the size of the page table.

Dynamic storage allocation problem

The problem of how to satisfy a request of size n from a list of free holes.

Swap time

Time it takes to swap a process back in and context switch, fairly high.

Memory management unit (MMU)

Unit containing a relocation register, for dynamic relocation of user code.

External fragmentation

Unused contiguous blocks, because the free block(s) could not accommodate the loading of a bigger process.

Page sharing

Using common pages as reentrant code, or for interprocess communication.

Multiple-partition method

When a partition is free, a waiting program is loaded into it.