; MEMORY LAYOUT
; 0x0000 - 0x7000: stack
; 0x7000 - 0x7c00: globals
;   0x7000 - 0x7001: boot drive number
;   0x7001 - 0x7002: number of hard disk drives
;   0x7002 - 0x7003: 
; 0x7c00 - 0x7e00: boot sector

; BIOS puts our boot sector at 0000:7c00
org 0x7c00  
; We're (probably) in real mode
bits 16 

  ; Disable interrupts
  cli

  xor ax, ax

  mov ds, ax
  mov es, ax

  ; Put stack base at 0x7000. Stack grows high->low, so we'll grow away from our program text at
  ; 0x7c00. We reserve the range [0x7000,0x7c00) for globals.
  mov ss, ax
  mov sp, 0x7000

  ; Segment for VGA (0xb800 * 16 = 0xb8000)
  mov ax, 0xb800
  mov fs, ax

  mov di, 0x7000

  ; Store boot drive number
  mov ds:[di], dl

  ; Get drive geometry
  mov di, 0x00
  mov ah, 0x08
  int 0x13
  jb panic
  mov ds:[di + 1], dl

  ; Set VGA mode
  ; https://mendelson.org/wpdos/videomodes.txt
  mov ax, 0x0003
  int 0x10

  mov word fs:[0x0000], 0xc048
  mov word fs:[0x0002], 0xc069
  mov word fs:[0x0004], 0xc021
  hlt

panic:
  mov ax, 0x0003
  int 0x10
  mov word fs:[0x0000], 0x4f46
  mov word fs:[0x0002], 0x4f41
  mov word fs:[0x0004], 0x4f49
  mov word fs:[0x0006], 0x4f4c
  hlt

  ; TODO: enable A20

  ; TODO: load a second stage

  ; TODO: grab all the info we can from bios interrupts and deliver it to ths OS nicely
  ; e.g. in a fixed memory location

  ; TODO:
  ; - Generate GPT in justfile
  ; - Parse global parition table
  ; - Load second stage from GPT partition with a particular UUID / name like GRUB does
  ;   (it's Hah!IdontNeedEFI in GRUB)
  ;   - https://en.wikipedia.org/wiki/BIOS_boot_partition
  ; - Future work:
  ;   - Boot from UEFI
  ;   - Boot on non-GPT partitioned disk

  ; MBR bootstrap field is 440 bytes long
  %if ($ - $$) > 440
  %error "exceeded mbr bootstrap field size"
  %endif

  ; Pad to the end of the MBR bootstrap field
  times 440 - ($ - $$) db 0

  ; Unique disk ID
  dd 0x00000000
  ; Reserved
  dw 0x0000

  ; Zero the partition fields
  ; TODO: fill in the field pointing to the GPT table
  times 510 - ($ - $$) db 0

  ; Boot signature
  db 0x55
  db 0xaa