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| Only low 16 bits of address seem to matter in protected mode https://forum.osdev.org/viewtopic.php?f=1&t=31729 |
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| Author: | kurmasz [ Tue May 02, 2017 12:39 pm ] |
| Post subject: | Only low 16 bits of address seem to matter in protected mode |
When my C code in protected mode writes to address 0xa204, the value in the variable at 0x10a0e4 is also changed. It's as if only the low 16 bits are relevant. Is there something I'm overlooking about how protected mode and or the GDT works? I understand why this would happen in real mode; but, I don't see why it would happen in protected mode. At the moment I have a very simple boot loader that follows the example of x86 bare metal (https://github.com/cirosantilli/x86-bare-metal-examples) and OSDev's Bare Bones to load the rest of the code, enter protected mode, and calls the C function that is the kernel entry point. I also have a few functions that can write using VGA text mode. This is the "OS" code: Code: #include "icos.h" #include "vga_text.h" unsigned d1 = 0x57575757; // initialized and put in .data section unsigned not_initialized; // uninitialized and put in .bss section void bss_test() { not_initialized = 0x11223344; // Set up the VGA output vga_text_section_t head, body; vgat_initialize_head_body(&head, &body, 5); //print the addresses and values of the global data. vgat_write_unsigned_hex(&body, (unsigned)&d1, " <= &d1\n"); vgat_write_unsigned_hex(&body, (unsigned)¬_initialized, " <= &ni\n"); vgat_write_unsigned_hex(&body, not_initialized, " <= Original ni value\n"); // Take the address of not_initialized, discard all but the low 16 bits, // then use that new value as a pointer. unsigned* pni = ¬_initialized; unsigned* pni2 = (unsigned*) ((unsigned) pni &0xffff); vgat_write_unsigned_hex(&body, (unsigned) pni2, " <='fake' pointer\n"); *pni2 = 0x55667788; // Modifying the "fake"/"truncated" pointer modifies not_initialized vgat_write_unsigned_hex(&body, not_initialized, " <= updated ni value\n"); } Running this code produces the following output Code: 0x9814 <= &d1
0x10a204 <= &ni 0x11223344 <= Original ni value 0xa204 <= 'fake' pointer 0x55667788 <= updated ni value |
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| Author: | eryjus [ Tue May 02, 2017 12:48 pm ] |
| Post subject: | Re: Only low 16 bits of address seem to matter in protected |
You will need to post your GDT code as well. The GDT has the ability to "shift" the starting value of the selector addresses. |
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| Author: | Brendan [ Tue May 02, 2017 12:52 pm ] |
| Post subject: | Re: Only low 16 bits of address seem to matter in protected |
Hi, kurmasz wrote: Running this code produces the following output Code: 0x9814 <= &d1 0x10a204 <= &ni 0x11223344 <= Original ni value 0xa204 <= 'fake' pointer 0x55667788 <= updated ni value Looks like you forgot to enable A20... Cheers, Brendan |
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| Author: | eryjus [ Tue May 02, 2017 1:00 pm ] |
| Post subject: | Re: Only low 16 bits of address seem to matter in protected |
Brendan wrote: Looks like you forgot to enable A20... Missed that thought. |
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| Author: | kurmasz [ Tue May 02, 2017 2:18 pm ] |
| Post subject: | Re: Only low 16 bits of address seem to matter in protected |
That was it: I hadn't set the A20 line. |
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