On Mon, 2007-07-02 at 10:02 +0530, Rajat Jain wrote: > Hi, > > > Does the kernel ever switch to use the physical address? > > No. > > > Let me give an example (to make sure i understand things correctly) > > > > 1 - A process in the user space is running (in virtual memory) > > > > 2- Process requests something from kernel and issues an > > interrupt (lets say that process wants to read from a file). > > > > 3- The 'int' assembly instruction switches from the user space > > to kernel space (switch from ring 5 to ring 0, and stack pointer > > changes to point kernel stack) > > > > 4- The process is now running in the kernel space and still with > > the SAME VIRTUAL MEMORY(?) as the caller process. > > (not sure if this is correct) > > That is correct. The virtual address space is the same. BUT now the kernel is restricted to (directly) use only the virtual memory beyond 3GB (PAGE_OFFSET). > > > 5- Now the kernel has to issue an I/O operation to make the hdd > > copy the information to memory. > > > > I have a question at this point. If everything i said is correct > > and all. How does the kernel get the information from hdd controller > > and move it to user space? > > Does it switch to physcal memory? Does it find a page which is empty, > > pass address of that page to hdd controller, and mark that page in > > the VM table of current process? > > The kernel does not use physical addresses at point. The closest it gets to that is using bus addresses for DMA. The kernel simply allocates a buffer and inserts a request into the request queue of the device. The block device driver then gets the data from device (may use DMA) and thus buffer is returned. > > Thanks, > > Rajat > > PS: Please use plain text instead of HTML mail. Thank you so much for the information Rajat. It helped me alot. Sincerely yours. -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
