On Sat, Apr 02, 2011 at 11:18:19PM +0530, mindentropy wrote: > On Saturday 02 Apr 2011 9:55:35 pm James Light wrote: > > > > To put the physical address into cr3 later. > > $swapper_pg_dir is not the PHYSICAL address of the PGD. > > Correct me if I am wrong but a mov $foo,%eax would move the address of foo > variable to eax right? Or is there a macro somewhere where they are adding > PAGE_OFFSET to the address so that the $swapper_pg_dir - __PAGE_OFFSET > justifies? Right, but it uses the logical address of foo which n the case of swapper_pg_dir must be converted to a physical address. When paging is not enabled, linear addresses are interpreted as physical addressess. So it has to get this logical address only one step farther into a linear address. So, this "swapper_pg_dir" is a symbol in a section of code. That section of code has an associated segment and this symbol lives at a particular location in that section and thus in that segment, and that particular location, relative to the beginning of the section is it's offset. Combine those two and you have the logical address of swapper_pg_dir. The sigil "$" is the immediate value sigil and the symbol $swapper_pg_dir thus uses the immediate value of swapper_pg_dir. The immediate value of swapper_pg_dir is the value held at the location that is logically labeled by the symbol swapper_pg_dir. It may help to remind yourself that symbol names in assembly are very similar to labels in assembly. They mark a location in the program. The program runs and while running it only uses logical addresses. This particular code is loading a linear (and w/out paging therefore physical) address. This is simply because of the design of the cr3 register and paging in x86. >From http://www.intel.com/design/processor/manuals/253668.pdf Chapter 4: Paging (Page 1) ------------------------------------------------------------------------------ Software enables paging by using the MOV to CR0 instruction to set CR0.PG. Before doing so, software should ensure that control register CR3 contains the physical address of the first paging structure that the processor will use for linear-address translation (see Section 4.2) ------------------------------------------------------------------------------ In linux2.1.66, the physical address is used directly. Just for comparison: ------------------------------------------------------------------------------ 58/* 59 * Setup paging (the tables are already set up, just switch them on) 60 */ 611: 62 movl $0x101000,%eax 63 movl %eax,%cr3 /* set the page table pointer.. */ 64 movl %cr0,%eax 65 orl $0x80000000,%eax 66 movl %eax,%cr0 /* ..and set paging (PG) bit */ 67 jmp 1f /* flush the prefetch-queue */ ------------------------------------------------------------------------------ If any of my own reasoning is wrong, I hope someone w/ more clue jumps in. ;) -James L _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
