User Mode

  1. Code, data, and stack all have their own page frames. For the time being, allotting one page frame for the stack and enough page frames to fit the program’s code is sufficient.
  2. The binary from the GRUB module has to be copied to the page frames used for the programs code.
  3. To map the above page frames into memory, you’ll require a page directory and page tables. Because the code and data should be mapped in at 0x00000000 and increasing, and the stack should start just below the kernel, at 0xBFFFFFFB, and increase towards lower addresses, at least two page tables are required. To allow PL3 access, the U/S flag must be set.
[esp + 16]  ss      ; the stack segment selector we want for user mode
[esp + 12] esp ; the user mode stack pointer
[esp + 8] eflags ; the control flags we want to use in user mode
[esp + 4] cs ; the code segment selector
[esp + 0] eip ; the instruction pointer of user mode code to execute
OUTPUT_FORMAT("binary")    /* output flat binary */    SECTIONS
. = 0; /* relocate to address 0 */
.text ALIGN(4):
.data ALIGN(4):
.rodata ALIGN(4):




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