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DOS programmes in virtual memory

Discussion in 'A+' started by Aslana, Sep 27, 2005.

  1. Aslana

    Aslana Bit Poster

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    Hi. I was just wondering :unsure Does anybody know why Windows 98 has to run DOS programmes in a virtual memory.
    I would be very grateful for your answers. :helpread
     
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  2. tripwire45
    Honorary Member

    tripwire45 Zettabyte Poster

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    Try this:
    Source: http://www.ecs.soton.ac.uk/~hkvt99r/memory.html
     
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  3. Boycie
    Honorary Member

    Boycie Senior Beer Tester

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    Nice explanation Trip. :)
     
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  4. Aslana

    Aslana Bit Poster

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    Thankyou Tripwire. That is exactly the answer i was looking for :biggrin
    We had been taught this at college, but i don't remember making a note of it :rolleyes:
     
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  5. tripwire45
    Honorary Member

    tripwire45 Zettabyte Poster

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    I've said it before and I'll say it again...Google is your friend. :biggrin

    If I didn't have search engines, I wouldn't know anything. :tongue
     
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  6. Aslana

    Aslana Bit Poster

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    Very true trip! Though i think this site is really great as well :cheers
     
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  7. Clyde

    Clyde Megabyte Poster

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    [pedant]
    to be pedantic ...

    its not virtual memory, but memory protection.

    Virtual memory refers to disk space being used as additional RAM.

    [/pedant]

    :biggrin
     
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  8. snoopy51

    snoopy51 Bit Poster

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    Are you sure?

    I thought that is known as page file(s)! :twisted:
     
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  9. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    Absolutely correct Clyde, you are not being pedantic at all.

    Virtual memory is the page file, otherwise known as the swap file. Applications are loaded into RAM (Random Access Memory) and run from there, not from the paging file. If the RAM is running out of space, the computer will swap some chunks of code out to a file on the hard drive (swap/page file) and pull it back into RAM again when it is needed. Hence more RAM less paging = faster performance.
     
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  10. ffreeloader

    ffreeloader Terabyte Poster

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    Dang, snoopy. You've been around IT for all these years and you still can't keep this straight? :twisted:

    On a more serious note. I think the OP was referring to real mode vs. protected mode and memory spaces, not virtual memory a.k.a. paging files. I didn't read Trip's post in entirety so I don't know if it covered real mode and protected mode which were basically virtual machines in todays langauge.
     
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  11. Bluerinse
    Honorary Member

    Bluerinse Exabyte Poster

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    Well Aslana, if you read these posts especially what Freddy has said here and piece it all together, you should have your perfect answer 8)
     
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  12. Veteran's son

    Veteran's son Megabyte Poster

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    Thanks for posting the message, trip!
    You have lots of great computer information! :respct
     
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  13. Veteran's son

    Veteran's son Megabyte Poster

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    Good explanation, ffreeloader! :)
     
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  14. snoopy51

    snoopy51 Bit Poster

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    Are you sure virtual memory is the page file or swap file?
    Here is the simple diagram:

    [​IMG]

    Please note virtual memory can be in physical memory or swap out on page file.
    Source: http://en.wikipedia.org/wiki/Virtual_memory

    Please note that both Red, Green, Gray and Blue are all virtual memory not just the the Blue! They all used some hardware or OS technique to translate virtual address to physical address!

    BTW, page or swap file can be on devices other than hard drive. Here is one Linux implemenation using video card memory:

    http://hedera.linuxnews.pl/_news/2002/09/03/_long/1445.html

    And Windows Vista is toying with USB memory!

    http://www.tomshardware.com/hardnews/20050922_172416.html
     
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  15. ffreeloader

    ffreeloader Terabyte Poster

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    You are correct in your explanation, snoopy. I doubt that too many people will ever really need to know that virtual memory can be blocks of imaginary ram set aside by developers or the kernel though. I've read a couple of articles on it but they were directed at developers/programmers. I've never seen the explanation used in any articles pointed at admins. Well, I have to qualify that a little too. I've seen it in a Henderson "Guru" book pointed at senior SQL Server DBA's in describing how SQL server uses memory. It's pretty esoteric knowledge.
     
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  16. Clyde

    Clyde Megabyte Poster

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    just a little more on this

    from the wikipedia article

    "In technical terms, virtual memory allows software to run in a memory address space whose size and addressing are not necessarily tied to the computer's physical memory."

    Commonly, this is interpreted to mean the use of disk space as additional simulated RAM storage space. The disk paging file is commonly referred to as 'virtual memory' (which it is) The impression being that the disk paging file is the only virtual memory in the system. More accurately (thanks snoopy51 *G*) virtual memory is an addressing scheme that allows the use of real and disk paged memory to increase the available storage space. Though we commonly refer to paged disk memory as virtual memory

    Regarding DOS programs I think aslana was referring to a 'virtual machine' which simulates a DOS based memory model in a protected memory space. This allows older s/w to work safely within windows. I'm sure I've left stuff out - its been a while since I worried about this stuff *g*

    hope that helps...
     
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