Hi Neil.

14-Nov-03 12:00:06, Neil Heller wrote to Jasen Betts


 JB>> malloc doesn't use memory owned by dos, malloc uses memory from a
 JB>> pool that the program claims as its own when it begins execution.

 NH> Is that true for all OSes?

 No, some early 80's OSes didn't work like that. "NOMMU" variants of linux
 may work that way too.

 NH> I don't see how this is really
 NH> possible given that dynamic memory allocation can occur an unknown
 NH> number of times and can involve an unknown quantity of memory.

CPUs with memory management can allocate address space without allocating
physical ram - the process is given some address space and when it's used
the CPU detects this (it's called a page fault) and triggers the OS code
which reconfigures the memory management to allocate a page of physical
ram to cover the address that triggered the page fault.

theese pages are commonly 4 kilobytes or larger

 NH> In an OS using virtual memory management, might the OS expand the
 NH> "pool" (as you described it) to disk usage?

that's what's done when all the physical memory has been allocated
some of the least often used pieces of physical memory are copied
to disk and then are avilabe to be re-used at some other address
if addresses copied toi disk are wanted again some other piece of memory is
freed up and the data from the disk restored to the original location
(but it might be on a different piece of physical ram,

 -=> Bye <=-

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