Kernel Level Address Spaces

In project 2, library code ran on the same stacks as user level code.
We shared an address space.

In the VAX ISA, the kernel can manipulate the page table by read/writing its virtual
memory. The hardware then needs to translate the addresses. Easier for kernel
developers, harder for harware developers.

In project 3, the library (kernel) has a separate address space. Page tables are
in virtual memory that is managed by Linux. This resembles VAX.

Addressing trick:

	-------------------------
	|   Virtual Memory	|
	|			|
	|			|
	|  |-------------|	|
	|  | pm_phys_mem |	|
	|  |-------------|	|
	|			|
	|			|
	|			|
	|			|
	|			|
	-------------------------


--------------------------------------------------------------

Protection:

Processes can't even name memory belonging to other processes,
except for shared data.

In order to accomplish this, we use some translation mechanism.
User-level processes can't see/modify the translation data,
or else they would be able to circumvent the mechanism.

So, the kernel is the only process able to modify the translation
data directly. We need to be able to track whether the running
process is the kernel or user code.

So, we need a kernel/user mode bit. But how do we stop user-level
code from modifying the mode bit?

There are two types of kernel code. 

1) Interrupts (eg: page fault, timer, yield).
In this case we onlu go to very specific parts of the kernel (ie: the handlers).
So, the hardware can aotmically change the mode bit whenever the handler starts.

2) User level invokes kernel processes. Then we got to kernel code and swap the mode bit.
We DONT stay in user-level code. Examples are calling fork(), doing I/O, rebooting the machine, etc.
Linux has ~350 system calls.

The procedure:
-- set mode bit to kernel
-- switch to kernel stack
-- push registers onto the kernel stack
-- change page table base register (PTBR) to point to kernel page table
-- jump to exception handler for the event/syscall that occured

(all done atomically by the hardware)

-----------------------------------------------------------------

Summary:

Want to hide memory

-- use address translation to hide addresses
-- restrict modifying address translation to kernel, via mode bit
-- ???

kernel gets special prvileges because it runs first and gets to set up
things as it wants.