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Posted: **Fri Apr 25, 2008 2:00 pm**

Hi I want to Identity Map all available memory.

This is how I thaught to do it:

First I will break down the highest possible memory address (Provided by GRUB) into its component parts (as if it were a virtual PAGED address), telling me how many Page Directory Entries are used and how many Page Table Entries (In the final Page Directory) are used to reach this address.

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   EndAddress = Memory Length (in bytes)
   DirectoryEntries = bits 22-32 of EndAddrees
   TableEntries = bits 12-22 of EndAddress

So the total number of Page Table Entries = ((DirectoryEntries*0x400)+TableEntries)

All of my Page Table Entries will be stored in a single array with my Page Directory situated just before it:

NOTE: PageDirectory and PageTable, both 4Kb aligned

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   unsigned long *PageDirectory = (unsigned long *)0x400000;
   unsigned long *PageTable = (unsigned long *)0x401000;

So, If i was to Fill all of the Page Table Entries with:

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   i=0, address=0;
   for(i=0;i<=((DirectoryEntries*0x400)+TableEntries);i++)  {
      PageTable[i]= address | 3;
      Address=(Address+0x1000); // 4Kb in HEX
   }

Then fill in the Page Directory with:

NOTE: See above for definition of DirectoryEntries

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   i=0;
   for(i=0;i<=DirectoryEntries;i++) {
      PageDirectory[i]=PageTable[i*0x400];
      PageDirectory[i]=PageDirectory[i] | 3;
   }
   for(i=DirectoryEntries;i<=0x400;i++) {
      PageDirectory[i]=0 | 2;
   }

Then Enable Paging with:

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   Set CR3 to address of PageDirectory
   Set PE bit in CR0

This should enable paging and have all available memory Identity Mapped...

I have tried and something is wrong.
I just can't figure out the problem

Posted: **Fri Apr 25, 2008 2:29 pm**

1) Is long 32 bits?
2) does (DirectoryEntries*0x400)+TableEntries equal 1024 * 1024
2b) DirectoryEntries and TableEntries should both equal 1024
3)

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PageDirectory[i]=PageTable[i*0x400];

needs an extra ampersand (and typecast)
4) you have a redundant for loop
5)

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i=0

do I see a missing semicolon?
6) do you have at least 12MB of memory installed?

Posted: **Sat Apr 26, 2008 4:06 am**

Note, I am not mapping thw whole address space. Only the actual available memory.

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