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(The last two alternatives are kludges used for machines with such small addresses that there is not room for both a segmentnumber and an offset)

 Segment table holds the bases and bounds for all the segments of a process.

 Show memory mapping procedure, involving table lookup + add + compare. Example: PDP-10 with high and low segments selected byhigh-order address bit.

Segmentation example: 8-bit segment number, 16-bit offset.

  • Segment table (use above picture -- all numbers in hexadecimal):
  • Code in segment 0 (addresses are virtual):
  • 0x00242:mov 0x60100,%r1
  • 0x00246:st %r1,0x30107
  • 0x0024A:b 0x20360
  • Code in segment 2:
  • 0x20360:ld [%r1+2],%r2
  • 0x20364:ld [%r2],%r3
  • ...
  • 0x203C0:ret

Advantage of segmentation: segments can be swapped and assigned to storage independently.

Problems:

  • External fragmentation: segments of many different sizes.
  • Segments may be large, have to be allocated contiguously.
  • (These problems also apply to base and bound schemes)

Example: in PDP-10's when a segment gets larger, it may have to be shuffled to make room. If things get really bad it may benecessary to compact memory.

Paging

Goal is to make allocation and swapping easier, and to reduce memory fragmentation.

  • Make all chunks of memory the same size, call them pages. Typical sizes range from 512-8k bytes.
  • For each process, a page table defines the base address of each of that process' pages along with read/only and existence bits.
  • Page number always comes directly from the address. Since page size is a power of two, no comparison or addition is necessary. Just do tablelookup and bit substitution.
  • Easy to allocate: keep a free list of available pages and grab the first one. Easy to swap since everything is the same size, which is usually thesame size as disk blocks to and from which pages are swapped.
  • Problems:
    • Internal fragmentation: page size does not match up with information size. The larger thepage, the worse this is.
    • Table space: if pages are small, the table space could be substantial. In fact, this is a problem even for normal pagesizes: consider a 32-bit address space with 1k pages. What if the whole table has to be present at once? Partial solution: keep base and bounds for pagetable, so only large processes have to have large tables.
    • Efficiency of access: it may take one overhead reference for every real memory reference (page table is so big it has to be kept in memory).

Two-level (multi-level) paging

Use two levels of mapping to make tables manageable.

Segmentation and paging

Use two levels of mapping, with logical sizes for objects, to make tables manageable.

  • Each segment contains one or more pages.
  • Segment correspond to logical units: code, data, stack. Segments vary in size and are often large. Pages are for the use of the OS; they arefixed size to make it easy to manage memory.
  • Going from paging to P+S is like going from single segment to multiple segments, except at a higher level. Instead of having a single pagetable, have many page tables with a base and bound for each. Call the material associated with each page table a segment.

Questions & Answers

What is demand
TECK Reply
the amount of a good that buyers are willing and able to purchase
Asit
what is population
Amadou Reply
The people living within a political or geographical boundary.
Ziyodilla
what happens to price and quantity when demand curves shift to the right
Asha Reply
price level goes up. quantity demand increases
Asit
example- inferior goods
Asit
demand law
Athony
Its states that higher the price the of the commodity, and lower the quantity demanded
Kosiso
I am confused but quantity demand will increase.
Asit
No. That's the law of supply
Kosiso
what happens to price and quantity when supply curve shifts left?
Asha Reply
price level will increase
Asit
quantity demand will decrease
Asit
what is inflation
Pop Reply
inflation is a general and ongoing rise in the level of prices in an entire economy.
cynthia
is the pasistance increase in the price of a country economy
Liyu
kk
Duppy
yes
Aadi
how does inflation affects the economy of a country? what is deflation?
Augustine
deflation can simply be define as the persistence decrease in price of a countrys economy
Liyu
the revenge of malthus relates "revenge" with "commodity prices". collect data for 3 commodoties and check their price evolution
Jamshi Reply
what is elasticity
dubela Reply
Elasticity is an economics concept that measures responsiveness of one variable to changes in another variable.
cynthia
right
Augustine
wooow!!
cynthia
Computer software represents
Mboledi Reply
पर्यावरण राज्यों में से किस राज्य में शिष्य शिक्षक अनुपात 30 से अधिक वाले विद्यालयों का प्रतिशत न्यूनतम होता है
plz Reply
Hey what are you trying to mean?
Kenyana
what is Asset
MUBARAK
like a banana
Ahmed
demand is the process whereby consumers are willing and able to purchase a particular product at various price over a given period of time
Samuel Reply
The law of dinimish
Frank Reply
What is the law of dinimish
Frank
What is the law of dinimish
Frank
What is the law of dinimish
Frank
opportunity cost is to forgo something for another.
jackie Reply
yes
King
what is financial market
Asheeru Reply
what is demand
Levinel Reply
Demand is an economic principle referring to a consumer's desire to purchase goods and services and willingness to pay a price for a specific good or service.
Ali
explain any three exceptions to the law of demand
Emma Reply
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Source:  OpenStax, Operating systems. OpenStax CNX. Aug 13, 2009 Download for free at http://cnx.org/content/col10785/1.2
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