CS 635

PROJECT 2		                Due Date: Noon Friday 02/28/2003

     The purpose of this project is to gain familiarity with designing a
character device-driver for Linux, and simultaneously to create a useful
tool for future study of the Linux memory-management system.

                        PROJECT STATEMENT

     Create an installable module (named 'physmem.c') which implements a
Linux device-driver for a 'read-only' character-mode device (represented 
by a device-file named '/dev/physmem'), that allows application programs
to read the computer's physical memory as if it were just one big file.  

     For the purposes of this project, you may disregard physical memory
in excess of 896 megabytes (i.e., memory physically located at addresses
equal to 0x3E000000 or higher).  For 32-bit Intel Architecture machines,
any physical memory at, or above, this address is known as 'high memory'
and is not directly mapped into the kernel's virtual address-space.  But
all physical memory located BELOW this address IS directly mapped to the 
kernel's virtual address-space, via a straightforward linear mapping:

	   physical addresses		       virtual addresses
	-----------------------		     -----------------------
	0x00000000 - 0x37FFFFFF     --->     0xC0000000 - 0xF7FFFFFF

Some symbolic constants, macros, and inline functions are defined in the
kernel header-files which facilitate your writing code that makes use of
this physical-to-virtual linear mapping: 

	#define __PAGE_OFFSET	0xC0000000
	#define PAGE_OFFSET	((unsigned long)__PAGE_OFFSET)
	#define __va(x)		((void*)(unsigned long)(x)+PAGE_OFFSET))
	#define __pa(x)		((unsigned long)(x)-PAGE_OFFSET) 

static inline void * phys_to_virt( unsigned long address )
{
	return	__va(address);
}

static inline unsigned long virt_to_phys( volatile void *address )
{
	return	__pa(address);
}

In addition, there is a kernel variable (named 'high_memory') which gets
initialized during system startup with the virtual address corresponding
to the end of the directly-mapped region of physical memory:

			void * 	high_memory;

So you can use this value to determine the size for your device-driver's
device-file, as follows:

	unsigned long 	filesize = virt_to_phys( high_memory ); 
_______________________________________________________________________
(c) Allan B. Cruse       University of San Francisco        Spring 2003