From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rajendra Subject: Re: Why "high memory" in x86? Date: Tue, 01 Aug 2006 14:48:46 +0530 Message-ID: <44CF1C76.5060406@solidcore.com> References: Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-newbie-owner@vger.kernel.org List-Id: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Cc: kernelnewbies , linux-newbie@vger.kernel.org The reason for the high memory is this. o Linux divides the address space into two parts, user and kernel. o Kernel gets 1 GB of address space while user gets 3GB virtual address space. o Kernel needs to access all of the memory so ideally it needs 4 GB of virtual addresses. o But since only 1 GB (i.e. beyond 0xc000 0000) is available, so we call the rest as high memory (approx 3 GB) o The high memory is accessed using temp. page table entries that map the high memory areas in kernel address space. o The high memory region is mostly allocated to the user space programs. hope it answers the question ! regd, ~rpm Rajat Jain wrote: > Hi list, > > I recently read that the concept of "High Memory" was introduced > because certain architectures are capable of physically addressing > larger amounts of memory than they can virtually address (physical > address space > virtual address space). I also read that nowadays > "high Memory" exists only in x86. > > 1) Why is virtual memory > 896 MB on x86 designated as high memory? > AFAIK x86 has 4 GB of virtual address space (=physical address space?) > > 2) Has the "high Memory" concept got anything to do with PAE (Page > Address Extention) feature of x86? > > 3) Do any other architectures than x86 have the concept of high memory? > > TIA, > > Rajat > - > To unsubscribe from this list: send the line "unsubscribe > linux-newbie" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.linux-learn.org/faqs > - To unsubscribe from this list: send the line "unsubscribe linux-newbie" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.linux-learn.org/faqs