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scatter gather DMA

Discussion in 'Electronic Basics' started by prav, Jun 18, 2004.

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  1. Tim Smith

    Tim Smith Guest

    Actually, it's address mapping that lets everything be loaded at the same
    address. You can have that without having virtual memory.
     
  2. Tim Smith

    Tim Smith Guest

    The real tragedy is the 80286. Consider protected mode. In protected mode,
    instead of loading a segment value into CS, DS, SS, or ES, you loaded a
    selector, which looked like this:

    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    | INDEX | PROT | G |
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

    (I may have PROT and G switched). INDEX is an index into either the local
    descriptor table or the global descriptor table. PROT is some protection
    information. G tells whether INDEX refers to the local descriptor table or
    global descriptor table.

    Suppose instead they had made one simple change, and arranged the bits in a
    selector like this:

    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    | PROT | G | INDEX |
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

    What a difference that would have made to software! If the OS then set up
    the process address space as a set of non-overlapping 65536 byte segments,
    mapped via consecutive LDT entries, the process could treat a
    selector/offset pair as a simple 32-bit pointer with 3 magic-bits at the
    top. Pointer arithmetic would work. There would be no special case every
    64K.

    Basically, it would be like a paged architecture with a 64K page size, with
    the oddity that the pages do not need to be aligned on page boundaries in
    physical memory (an oddity that would only be of concern to the OS).
     
  3. John Larkin

    John Larkin Guest


    Yes, I believe I said that.

    John
     
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