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Thanks a lot for the response. Will u be able to provide me a link that will be able to help in making this PAGE-OFFSET and related issues more clear. Also, it seems to be whats going on here is high-end expertise level stuff. I am new to Driver Writing even though I was working on linux before this. So, will u be able to suggest me a list where in I can post my basic issues about Memory management issues, Time Control issues and Driver Control, so on. Or else is it ok if I post them here. Thanks for that again. Regards, Mukund jampala -----Original Message----- From: eric@ebiederm.dsl.xmission.com [mailto:eric@ebiederm.dsl.xmission.com] On Behalf Of Eric W. Biederman Sent: Monday, August 23, 2004 7:59 PM To: Mukund JB. Cc: linux-kernel@vger.kernel.org Subject: Re: Support for HIGHMEM ...can any one explain my Q&A .. "Mukund JB." <mukundjb@esntechnologies.co.in> writes: > Hi all, > Sorry if I asked any thing that basic level stuff? > This is the paasge I found in the net while surfing for details about > HIGHMEM stuff. > > During 2.3 kernel development (I think), "HIGHMEM" support was added. > Normally, the kernel can only address (4GB-PAGE_OFFSET)/PAGE_SIZE pages > of RAM, since all physical pages must be mapped to kernel addresses > between PAGE_OFFSET and 4GB. (So if PAGE_OFFSET is 3GB, only 1GB of > physical RAM can be used - not even that, in practice, due to fixed > kernel mappings and so forth.) The HIGHMEM patches allow the kernel to > use more than 1G of memory by mapping the additional pages into the high > part of the kernel address space just below 4GB as necessary. They also > allow high-memory pages to be mapped into user process address space. > > > ***) Does the above passage mean that PAGE_OFFSET is the starting > address of my RAM ? No. PAGE_OFFSET is the virtual address at which physical address 0 is mapped into an arch/i386 kernel. > I understood so from the below line > "since all physical pages must be mapped to kernel addresses between > PAGE_OFFSET and 4GB". > > ***) Does it mean that the lowest portion of the 4GB os nothing but RAM > ? No. 0-PAGE_OFFSET is the virtual address space provided to user space. So it has an arbitrary mapping to virtual memory. Eric - To unsubscribe from this list: send the line "unsubscribe linux-kernel" 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.tux.org/lkml/ | ||||||||||||
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