Mail Archives: djgpp/1997/01/03/17:48:29
On 2 Jan 97 at 1:46, Mark Habersack wrote:
> > >1. in i386 Intel introduced a new level of indirection in memory
> > >addressing - paging. Memory in i386 can be divided into units as
> > >small as 4KB (PAGES) while in iAPX286 the smallest swappable (in
> > >virtual memory sense) was a SEGMENT (64KB).
> >
> > That's not quite getting the point, Mark. The 286 segments were
> > limited to a maximum size of 64Kb, but could be any size. The idea
> > was that you swapped things in and out at a high level; whole DLLs
> > and things like that lived in individual segments. The 386 flat
> > address space just arbitarily carves itself into 4k bits for
> > management, they don't mean anything structurally.
> What I mean is that 286-based virtual memory manager was somehow
> forced to swap data between memory and disk in 64KB chunks. It was,
> of course, possible to exchange exactly that many bytes the given
> segment contained, but the overhead involved was unacceptable. The
> overhead was related to the different sizes of the segment sin
> question. If the VMM were to pay attention to the sizes, it'd have to
> maintain some structure (probably sort of FAT) to find segments in
> the swapfile. Even though searching through the structure wouldn't be
> that time consuming, the operations involved in rearranging data
> layout inside of the swapfile would be unacceptable. Imagine such a
> situation:
[snip]
Yes, but this system excelled when static program modules were
swapped in and out - things like segments in EXE files, like Windoze
used to (my great grandfather tells me ;-), since these were constant
sized things from fixed positions on disk. They could be demand
loaded and dropped out when they were getting stale...
> > What about AS and BS? :-)
> ;-))
Another Segment and... and... erm... Bigger Segment???
AS is like ES, used for /trinary/ string operations (XOR these two
strings to this string) and BS is used for arrays and stuff (lots of
BS:EAX+EBX*sqrt(ECX*pi)^3 addressing modes...)
(grin)
ABW
--
Governments are merely protection rackets with good images.
Alaric B. Williams Internet : alaric AT abwillms DOT demon DOT co DOT uk
http://www.abwillms.demon.co.uk/
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