loader.asm

[BITS 16]
[ORG 0x7E00]

; Produce a map file containing all symbols and sections.
[map all ../../build/boot/loader.map]

;===============================================================================
; loader
;
; Second-stage boot loader entry point
;
; Due to the limited size of the first-stage boot loader, its only mission in
; life is to load the Second-stage boot leader. The second-stage boot loader is
; responsible for essentially preparing the system to enter into the protected
; mode.
;
; Input registers:
;
;   DL      Boot drive number
;
; Memory layout before this code starts running:
;
;   00000000 - 000003ff        1,024 bytes     Real mode IVT
;   00000400 - 000004ff          256 bytes     BIOS data area
;   00000500 - 00007bff       30,464 bytes     Free
;   00007c00 - 00007dff          512 bytes     First-stage boot loader (MBR)
;   00007e00 - 0009fbff      622,080 bytes     Free -> We are here now
;   0009fc00 - 0009ffff        1,024 bytes     Extended BIOS data area (EBDA)
;   000a0000 - 000bffff      131,072 bytes     BIOS video memory
;   000c0000 - 000fffff      262,144 bytes     ROM
;===============================================================================

; jump to start procedure
jmp start

; Include functions/constants that are useful in real mode
%include "../../include/boot/global/const.asm"
%include "../../include/boot/global/mem.asm"
%include "../../include/boot/global/gdt.asm"
%include "../../include/boot/mode/realmode.asm"
%include "../../include/boot/second_stage_loader.asm"

start:

  ; Proper initialisation of stack during BIOS bootloader
  ;   https://stackoverflow.com/a/33975465/832748
  xor ax, ax
  mov ds, ax
  mov es, ax
  mov bx, Loader.Mem.Stack.Top

  ; Turn off interrupts for SS:SP update to avoid a problem with buggy 8088 CPUs
  cli
  ; SS = 0x0000
  mov ss, ax
  ; SP = 0x7c00
  ; Set the stack starting just below where the bootloader is at 0x0:0x7c00.
  mov sp, bx
  ; Turn interrupts back on
  sti

boot:
  ; Save DriveId for later
  mov [BIOS.Drive.Id], dl

  ; Print booting message
  mov si, Realmode.SecondStage.Booting.Msg
  call display_string

  ; Get memory map
  call bios_e820_memory_map

  ; Attempt to enable the A20 line if necessary.
  call enable_A20

  ; read kernel from disk and move it to memory
  call read_kernel_from_disk

  ; read user prog from disk and move it to memory
  call read_userprog_from_disk

  ; Check whether we are running on a 64-bit processor
  call cpu_supports_64_bit_mode

  ; Prepare to enter protected mode
  call enter_protected_mode

  ; enter a endless loop. This instruction should never be reached
  jmp endless_loop


[BITS 32]

; Include Constants/Variables/routines useful in protected mode
%include "../../include/boot/mode/protectedmode.asm"


protected_mode_boot:

; Accoring to Intel 64 manual
; Section: 9.9.1 Switching to Protected Mode
;
;   9. After entering protected mode, the segment registers continue to hold the
;      contents they had in real-address mode. The JMP or CALL instruction in
;     step 4 resets the CS register. Perform one of the following operations to
;     update the contents of the remaining segment registers.
;
;     — Reload segment registers DS, SS, ES, FS, and GS.
;       If the ES, FS, and/or GS registers are not going to be used,
;       load them with a null selector.

  ; Turn off interrupts for SS:SP update to avoid a problem with buggy 8088 CPUs
  ; Also, interrupts will be disabled all the way to long mode as I don't need
  ; them in protected mode for anything. In fact, this was causing a weird
  ; race condition on QEMU that took me a few months to figure the culprit out
  cli

  ; 0x10 = 16 which points to the GDT Data Segment (and the right DPL too)
  mov ax, 0x10
  mov ds, ax
  mov es, ax
  mov fs, ax
  mov gs, ax
  mov ss, ax
  mov esp, Loader.Mem.Stack.Top


  ; fix video text printing
  call pm_retrive_video_cursor_settings

  ; display status message
  mov eax, ProtectedMode.SecondStage.Booting.Msg
  call pm_display_string

  ; copy kernel to the right location
  call pm_move_kernel

  ; Setup paging
  call pm_setup_page_tables

  ; Enter Long monde (finally!)
  call pm_enter_long_mode

  ; enter a endless loop. This instruction should never be reached
  jmp pm_endless_loop


[BITS 64]

; Include Constants/Variables/routines useful in protected mode
%include "../../include/boot/mode/longmode.asm"

;-------------------------------------------------------------------------
; Launch the 64-bit kernel
;-------------------------------------------------------------------------
long_mode_boot:
    ; It is very important that you don't enable the interrupts (unless you have set up a 64-bit IDT of course).

    ; 0x10 = 16 which points to the GDT Data Segment (and the right DPL too)
    mov ax, 0x10
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax
    mov ss, ax
    mov rsp, Kernel.New.Start.VirtualAddress

    ; display status message
    mov rax, LongMode.SecondStage.Booting.Msg
    call lm_display_string

    ; jump to memory address in which the kernel should be (fingers crossed)
    jmp Kernel.New.Start.VirtualAddress + Kernel.New.ELFTextHeader.Offset

    ; TODO: learn about which elf sections must be aligned... I'm still not convinced


; On physical devices this isn't required because the BIOS will
; pull the x number of blocks regardless of their content, however,
; if you are using QEMU and a raw image, it will strugle to Read
; the BIOS Disk Access Packet (DAP ) because the file finishes
; way earlier than the number of blocks requested.
times (Loader.File.NumberOfBlocks * 512) - ($ - $$) db 0