boot/boot0.s

; MEMORY LAYOUT
; R = reserved, U = usable
; --------------------------------------------------------------------
; R | 0x000000 - 0x000400: real-mode interrupt vector table
; R | 0x000400 - 0x000500: bios data area
; U | 0x000500 - 0x007c00: stack
; U | 0x007c00 - 0x007e00: boot sector
; U | 0x007e00 - 0x080000: conventional usable memory
; R | 0x080000 - 0x0a0000: extended bios data area (maximum possible size)
; R | 0x0a0000 - 0x0c0000: video memory
; R | 0x0c0000 - 0x0c8000: video bios
; R | 0x0c8000 - 0x0f0000: bios expansiosn
; R | 0x0f0000 - 0x100000: motherboard bios

; BASE STACK FRAME
; --------------------------------------------------------------------
; bp - 0x02: drive number
; bp - 0x04: sectors per track
; bp - 0x06: number of heads
; bp - 0x08: partition array starting LBA
; bp - 0x0a: number of GPT partitions (saturated to 16-bits)
; bp - 0x0c: sector stride when loading GPT entries

; 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 the stack base at 0x7c00.
  ; Stack grows high->low, so we'll grow away from our program text at 0x7c00.
  mov ss, ax
  mov bp, 0x7c00
  mov sp, bp

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

  ; Store boot drive number
  xor dh, dh
  push dx

  ; Get drive geometry
  mov di, 0x00
  mov ah, 0x08
  int 0x13
  jc .panic
  ; Load sectors per track into cx & spill
  and cl, 0x3f
  xor ch, ch
  push cx
  ; Load number of heads into bx & spill
  movzx bx, dh
  inc bx
  push bx

  ; Load LBA 1.
  cmp cl, 1  ; Division of 1 by nonzero can be done with cmp
  sete al    ; temp = LBA / (sectors per track)
  setne cl   ; sector - 1 = LBA % (sectors per track)
  inc cl
  xor ah, ah ; Zero-extend temp
  xor dx, dx ; div by 16-bit register divides dx:ax, so we zero dx
  div bx     ; ah = mod (head), al = div (cylinder)
  
  mov dh, ah ; Head
  mov ch, al ; Cylinder
             ; We already have sector in cl
  mov ah, 2  ; Read disk
  mov al, 1  ; Load one sector
  mov bx, 0x7e00
  mov dl, [bp - 0x2] ; Drive number
  int 0x13
  jc .panic

  ; head = temp % number of heads
  ; cylinder = temp / number of heads

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

  mov cx, 8
  mov si, .gpt_magic
  mov di, 0x7e00
  repe cmpsb
  jne .panic

  ; Ensure the 8-byte GPT starting LBA fits in 16 bits
  mov di, 0x7e00 ; The rep increments di so we need to reset it
  xor bx, bx
  mov ax, [di + 0x4a]
  or bx, ax
  mov ax, [di + 0x4c]
  or bx, ax
  mov ax, [di + 0x4e]
  or bx, ax
  jnz .panic
  ; Load and spill the GPT starting LBA
  mov ax, [di + 0x48]
  push ax

  ; Load number of partitions
  mov ax, [di + 0x50]
  mov bx, [di + 0x52]
  or bx, bx
  jz .gpt_n_partitions_loaded
  mov ax, 0xff ; Number of partitions overflows 16 bits
.gpt_n_partitions_loaded:
  push ax

  ; Load GPT entry size
  mov ax, [di + 0x54]
  mov bx, [di + 0x56]
  mov cx, ax
  mov dx, bx
  ; Find number of sectors per entry
  shr ax, 9
  shr bx, 9
  or bx, bx
  jnz .panic ; Sectors per entry overflows 16 bits
  mov bx, ax
  cmp bx, 1
  jnz .gpt_sector_stride_loaded
  inc bx ; Sector stride must be at least one or we'll load the same sector forever!
.gpt_sector_stride_loaded:
  push bx

  ; ax stores sectors per entry
  ; FIXME: if sectors per entry > 1, then entry size must be a multiple of sector size

  mov al, [0x7e00]
  mov byte fs:[0x0000], al
  mov al, [0x7e01]
  mov byte fs:[0x0002], al
  mov al, [0x7e02]
  mov byte fs:[0x0004], al
  
  ; mov word fs:[0x0000], 0xc048
  ; mov word fs:[0x0002], 0xc069
  
  hlt

.panic:
  mov ax, 0x0003
  int 0x10
  mov ax, 0xb800
  mov ds, ax
  mov word [0x0000], 0x4f46
  mov word [0x0002], 0x4f41
  mov word [0x0004], 0x4f49
  mov word [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

.gpt_magic:
  db "EFI PART"

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