x86-bare-metal-examples/paging.S

/* https://github.com/cirosantilli/x86-bare-metal-examples# */

#include "common.h"

BEGIN
    CLEAR
    STAGE2
    PROTECTED_MODE
    SETUP_PAGING_4M

    /* Setup a test canary value. */
    movl $0x1234, 0x1000

    /* Print the canary to make sure it is really there. */
    VGA_PRINT_HEX_4 0x1000

    /* Make page 0 point to 4KiB. */
    orb $0x10, page_table + 1

    PAGING_ON

    /* THIS is what we've been working for!!!
     * Even though we mov to 0, the paging circuit reads that as physical address 0x1000,
     * so the canary value 0x1234 should be modified to 0x5678.
     **/
    movl $0x5678, 0

    /* Turn paging back off to prevent it from messing with us.
     * Remember that VGA does memory accesses, so if paging is still on,
     * we must identity map up to it, which we have, so this is not mandatory.
     * */
    PAGING_OFF

    /* Print the (hopefully) modified value 0x5678. */
    VGA_PRINT_HEX_4 0x1000
    jmp .