Only low 16 bits of address seem to matter in protected mode

[kurmasz]

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:

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#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)&not_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 = &not_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

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0x9814 <= &d1
0x10a204 <= &ni
0x11223344 <= Original ni value
0xa204 <= 'fake' pointer
0x55667788 <= updated ni value

[eryjus]

You will need to post your GDT code as well. The GDT has the ability to "shift" the starting value of the selector addresses.

[Brendan]

Hi,

Running this code produces the following output

Code: Select all

0x9814 <= &d1
0x10a204 <= &ni
0x11223344 <= Original ni value
0xa204 <= 'fake' pointer
0x55667788 <= updated ni value

Looks like you forgot to enable A20...


Cheers,

Brendan

[eryjus]

Looks like you forgot to enable A20...

Missed that thought.

[kurmasz]

That was it: I hadn't set the A20 line.