syntax help

Andrew Haley aph@redhat.com
Sun Jan 4 14:42:00 GMT 2009


Philip Herron wrote:
> Hey Guys
> 
> This might be kind of silly question but i don't understand this code
> segment:
> 
> ====================================================
> //our first page table comes right after the page directory
> unsigned int *first_page_table = page_directory + 0x1000;
> 
> // holds the physical address where we want to start mapping these pages
> to.
> // in this case, we want to map these pages to the very beginning of memory.
> unsigned int address = 0; 
> unsigned int i;
> 
> //we will fill all 1024 entries, mapping 4 megabytes
> for(i = 0; i < 1024; i++)
> {
>     first_page_table[i] = address | 3; // attributes: supervisor level, read/write, present.
>     address = address + 4096; //advance the address to the next page boundary
> }
> ==================================================
> 
> This is taken of: http://wiki.osdev.org/Setting_Up_Paging
> Been working at a small os project of my own: http://gitorious.org/projects/zepher
> 
> For something in my spare time, starting to love gnu as. Anyways the thing i don't understand is:
> 
> first_page_table[i] = address | 3;
> 
> I don't know what this does. first_page_table was it not just an integer? Or does this go trough each 
> byte in the int and set rights? with the '|' like value address and rights 3 or something. I haven't really seen this 
> syntax so sorry if it sounds really stupid!

An int is four bytes: if an int is at address 0x1000, the next int is
at address 0x1004, and so on.  Ints are almost always aligned, so the
lowest two bits of an int* are almost always zero.  Knowing that they
are zero, we can use these lowest two bits for something else, such as
flags.

> One more thing is there some documentation on the __atribute__ system in gcc i have set some structs to be packed, 
> is there some more documentation to see what other modes gcc supports for data types?

Have you found the gcc manual?

Andrew.



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