Date: Thu, 2 Aug 2001 09:14:11 -0500 (CDT) From: mark tinguely <tinguely@web.cs.ndsu.nodak.edu> To: culverk@wam.umd.edu, riel@conectiva.com.br Cc: craiglei@pasia.com.cn, freebsd-hackers@FreeBSD.ORG, tlambert2@mindspring.com Subject: Re: How to visit physical memory above 4G? Message-ID: <200108021414.f72EEBS78044@web.cs.ndsu.nodak.edu> In-Reply-To: <Pine.GSO.4.21.0108020937090.20844-100000@rac2.wam.umd.edu>
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On 2 Aug 2001, Kenneth Wayne Culver <culverk@wam.umd.edu> said: > Also, the PIII CAN'T natively support more than 4GB of ram. If a > particular PIII motherboard supports this, then it's using some kind of > wierd chipset that allows this to happen. 4GB is the limit with a 32 bit > chip I believe; and the PIII is a 32-bit chip. Since the Pentium Pro processor, the Intel chipsets support a physical address extension (PAE) which has 4 extra addressing bits, and a third level of page table indirection called the page-directory-pointer-table base address field. The addressing use 64 bits for a memory pointer and the additional page indirection add to the overhead. The stickler is the MMU is still 32 bits. This means the PAE must segment the 64GB space into 4GB segments or 4 1GB segments. The OS must manage which pages are viewable to the process at this time. There is a third mode of addressing using 2MB pages (simular to the 4MB page addressing mode for the 32 bit addressing scheme) that will only give a process access to 4GB of memory (not segmentable to a larger space, but can address physical memory located above the 4GB address). --mark tinguely To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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