Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!zaphod.mps.ohio-state.edu!rpi!bu.edu!polygen!jerry
From: jerry@polygen.uucp (Jerry Shekhel)
Newsgroups: comp.sys.amiga.advocacy
Subject: Re: The Amiga's Future
Message-ID: <1153@stewart.UUCP>
Date: 24 Jun 91 18:43:39 GMT
References: <#g1H3+$o@cs.psu.edu> <12901@uwm.edu> <22308@cbmvax.commodore.com> <1991Jun11.134256.9465@cs.mcgill.ca> <1134@stewart.UUCP> <59@ryptyde.UUCP> <1146@stewart.UUCP> <89@ryptyde.UUCP>
Reply-To: jerry@stewart.UUCP (Jerry Shekhel)
Organization: Polygen Corporation, Waltham, MA
Lines: 38

In article <89@ryptyde.UUCP> dant@ryptyde.UUCP (Daniel Tracy) writes:
>
>I understand that, on the 286, the segment registers (in 16-bit protected
>mode) are used as pointers to a table of 24-bit addresses, and that's how
>it addresses 16MB. However, I don't understand the underlying difference
>between accessing physical memory and virtual memory. That is, why one
>should be less limited than the other. Maybe I should take a class on
>CPU design and not bug you guys about it. I also don't understand how
>it can address 16384 segments if it only has 6 segment registers!
>(each one-bit, right? Or is that where my mistake is?)
>

Each segment register contains a 13-bit index into a table of segment
descriptors.  Each segment descriptor is a data structure that contains
the 24-bit base address, protection bits, an in-memory/swapped bit, a 16-bit
segment length, and a couple of other things.  Each task has access to a
Local Descriptor Table which contains up to 8192 segment descriptors
(task-isolated memory), and a Global Descriptor Table, which also contains
up to 8192 segment descriptors (systemwide shared memory).  That's how you
get a virtual memory space of 16384 64K segments (1 GB).  Remember that two
segments can be mapped to the same 64K block within the 16MB of physically-
addressable RAM, as long as only one is loaded into memory at a time and the
rest are swapped to disk.

So, a segment register is not one-bit; it's 16-bit.  Each contains:

	-- a 13-bit offset into a descriptor table
	-- a bit indicating whether to use the Local or Global Descriptor Table
	-- two bits identifying the requested priviledge level
--
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| JERRY J. SHEKHEL  | POLYGEN CORPORATION  | When I was young, I had to walk |
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