I cannot find records in the page table/page directory for address 0xB8000. It works correct and when I put some symbols into this memory region display shows it.
I’m in the protected mode, and paging is enabled. I’m using bochs.
cr0 is 0xe0000011 or 1110 0000 0000 0000 0000 0000 0001 0001b, cr3 is 0x1000
0xB8000 is equal 1011 1000 0000 0000 0000b, so 10111000b is index of page table, it's equal 184, index of page directory is 0, offset is zero.
Btw I have three memory descriptors (memory from 0x0 to 0xFFFFFFFF):
null: 0 code: dw 0FFFFh ; limit low dw 0 ; base low db 0 ; base middle db 10011010b ; access db 11001111b ; granularity db 0 ; base high data: dw 0FFFFh ; limit low (Same as code) dw 0 ; base low db 0 ; base middle db 10010010b ; access db 11001111b ; granularity db 0 ; base high
It is 0x2027 or 10 0000 0010 0111b so the physical address of the page table is 10b or 2. I need a content of 184th entry so I need to go to 184 * 4 + 2 = 738 = 0x2E2, but it's empty:
It's 0x2027, so the physical address of the page table is 0x2000 (not 2). The 185th entry (entry number 184) will be at offset 0x2E0 in the page table and will have the physical address 0x22E0.
Note: You could shift the page directory entry (0x2027) to the right 12 places and say "it's physical page number 2", then multiply the page number by the size of a page (or shift it left 12 places) to find the physical address of the page. It's easier/faster to just mask off the lowest 12 bits instead (e.g.
physical_address_of_page_table = page_directory_entry & 0xFFFFF000;), especially when doing it in your head with hexadecimal values (where you can just assume the last 3 digits are zeros).
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